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Mislocalizations of touch to the locus of a fake hand Austen, Erin Leigh 2003

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MISLOCALIZATIONS OF TOUCH TO THE LOCUS OF A F A K E H A N D by ERIN LEIGH AUSTEN B.A., Saint Francis Xavier University, 1996 M.A., University of British Columbia, 1999 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Psychology; Cognitive Systems) We accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA June 2003 © Erin Leigh Austen, 2003  In  presenting  degree  this  at the  thesis  in  University of  freely available for reference copying  of  this  department publication  or of  thesis by  this  partial  British Columbia, and study.  his  or  her  1V(<^«,U>Q|^  DE-6 (2/88)  3o 2oo3 1  the  I agree  requirements that the  I further agree  representatives.  may be It  is  thesis for financial gain shall not be  The University of British Columbia Vancouver, Canada  Date CXo~e.  of  for scholarly purposes  permission.  Department of  fulfilment  >  for  an  advanced  Library shall make it  that permission for extensive granted  by the  understood  that  allowed without  head  of  my  copying  or  my written  Abstract Space-relevant tactile localizations may be influenced by non-tactile sources of information. This is evident i n the fake hand effect wherein observers mislocalize tactile targets delivered to their unseen hand towards a visible fake hand that is positioned next to a pair of distractor lights. The aim of the present study was to use the fake hand effect to explore the factors that influence tactile localizations. First, the effect was quantified by comparing tactile localizations i n the presence of the fake hand to those when the observer's hand instead occupied the normal fake hand location. The effect was quantitatively weaker than one w o u l d expect if the tactile targets were mislocalized to the locus of the fake hand (Experiment 1). Surprisingly, the fake hand effect d i d not depend on direct vision of the fake hand (Experiments 1 & 2), nor was it enhanced by tactile information that was congruent with the fake hand (Experiment 3). The effect, however, was sensitive to the consistency between the orientation of the fake hand and the observer's hand such that it disappeared when the two were inconsistent (Experiment 4). It was also sensitive to the mapping between the location of the digit stimulated and the type of foot response required (Experiment 5). These results have important implications for the flexibility of one's body schema.  Table of Contents Abstract  ii  Table of Contents  iii  List of Tables..  v  List of Figures  vi  Introduction  1  Overview of Experiments 1-5  13  Experiment 1  16  Methods  19  Results  25  Discussion  30  Experiment 2  33  Methods  35  Results  37  Discussion  41  Experiment3  43  Methods  .....  43  Results  44  Discussion  46  Experiment 4  48  Methods  49  Results  50  Discussion  53  Experiment 5  54  Methods  57 iii  Results...  57  Discussion  62  General Discussion  64  I m p l i c a t i o n s of the Present Results  68  O u t s t a n d i n g Questions a n d F u t u r e Directions  71  References  77  Appendix  87  List of Tables Table 1: Descriptive information for each Experiment Table 2: Breakdown of trial types i n Experiments 1-5  List of Figures  Figure 1: Sensory homunculus depicting the devotion of specific cortical areas within the somatosensory cortex to different parts of the human body. From Sensation and Perception (5 ed.) (p. 231), by S. Coren, L . M . Ward, th  J.T. Enns, 1999, N e w York: Harcourt Bruce. Copyright 1950 by Macmillan Publishing Co., renewed 1978 by Theodore Rasmussen.  Permission  granted  2  Figure 2. Two-point tactile discrimination thresholds for different regions of the human body. From Sensation and Perception (5 ed.) (p. 234), by S. Coren, th  L . M . Ward, J.T. Enns, 1999, N e w York: Harcourt Bruce. Permission granted  3  Figure 3. A n illustration of the experimental setup used by Botvinick and Cohen (1998)  9  Figure 4. A schematic of the experimental setup used by Pavani, Spence a n d Driver (2000). (A) N o Fake H a n d Condition (B) Fake H a n d Condition (C) Example of Congruent target-distractor elevations (D) Example of Incongruent target-distractor elevations. Note that the congruent and incongruent trial types apply also to the Fake H a n d condition though they are only pictured here for the N o Fake H a n d condition  11  Figure 5. Mean CEs (in ms) for Experiment 1 as a function of Condition (No Fake H a n d , Fake H a n d , Real H a n d Baseline) and Target-Distractor Separation (None, Small, Large) for (A) Visible and (B) H i d d e n limbs  21  Figure 6. Mean CEs (in ms) for Experiment 2 as a function of Fake H a n d Condition (No Fake Hands, Both Visible, Both H i d d e n , One H i d d e n -  vi  Touch on H i d d e n Side, One H i d d e n - Touch on Visible Side) for (A) Same and (B) Opposite target-distractor presentations  38  Figure 7. Mean C E (in ms) for Experiment 3 as a function of Condition (No Fake Hands, Fake Hands), Observer Gloves (Plastic, None) for (A) Same and (B) Opposite side target-distractor presentations  45  Figure 8. Mean C E (in ms) for Experiment 4 as a function of Fake H a n d Condition (No Fake H a n d , Inconsistent, Consistent) and Observer H a n d Orientation (Prone, Supine) for (A) Same and (B) Opposite targetdistractor presentations  51  Figure 9. Mean C E (in ms) for a top-heel & bottom-toe digit-response mapping as a function of Fake H a n d Condition (No Fake H a n d , Inconsistent, Consistent) and Observer H a n d Orientation (Prone, Supine) for (A) Same and (B) Opposite target-distractor presentations  60  vii  Introduction The human tactile system is often confronted w i t h an important spatial ambiguity to resolve when a tactile stimulus is delivered to the surface of the skin. In addition to determining the location of the tactile event on the body's surface, it is also often important to know the location of the tactile event w i t h respect to the body's external surround. Consider, for example, a tactile stimulus delivered to the hand. First, one must correctly recognize where the stimulus was delivered on the body (e.g., to the hand). Then, i n order to k n o w where the event occurred i n space, one must figure out where the hand is positioned i n space (e.g., at one's side). This thesis w i l l examine several factors that may influence the accuracy and efficiency of tactile localization. The tactile system provides important body-relevant information. W h e n touched, the mechanoreceptors under the skin are activated and a message signaling the skin stimulation is sent from the spinal cord and the thalamus to the somatosensory cortex region of the brain (Roberts & W i n g , 2001). Within this region are groups of neurons that are devoted to particular parts of the body; the spatial layout of these neurons i n the brain roughly corresponds to the spatial layout of their receptive fields under the skin's surface. In order to identify where i n the somatosensory cortex each part of the body is represented, Penfield and Rasmussen (1950) carefully stimulated small regions of this area of the cortex in patients undergoing brain surgery and then watched for and recorded the resulting bodily reactions exhibited by them. In this way, Penfield and Rasmussen were able to construct a topographic map linking specific areas of the body to corresponding areas within the cortex (see Figure 1).  1  Figure 1: Sensory homunculus depicting the devotion of specific cortical areas w i t h i n the somatosensory cortex to different parts of the human body. From Sensation and Perception (5 ed.) (p. 231), by S. Coren, L.M. Ward, J.T. Enns, 1999, N e w York: Harcourt Bruce. Copyright 1950 by Macmillan Publishing Co., renewed 1978 by Theodore Rasmussen. Permission granted. tf(  Areas of the body that have a greater functional significance, such as the hand and face, are represented by relatively large regions of the somatosensory cortex while other areas like the shoulder are represented by much smaller regions (Roberts & W i n g , 2001). The spatial sensitivity of the tactile sensory system is not uniform, rather, the areas of the body that are w e l l represented cortically are the areas that are most sensitive to touch and have a greater number of mechanoreceptors under their skin surface (Cholewiak & Collins, 1991). This fact becomes clear when one looks at the two-point discrimination thresholds for different regions of the body (Sherrick & Cholewiak, 1986, see  Figure 2). To obtain these thresholds, either one or two tactile points are applied to the observer's skin (Cholewiak & Collins, 1991). The observer is then asked to report whether they felt a single point or two separate points. In the case where two points are presented, the separation between the two is varied. The threshold is defined as the smallest distance between the two points for which observers are able to report the dual presence. In a region like the thumb (greater neuronal representation), observers are able to identify the presence of two tactile stimuli at relatively small spatial separations. In contrast, i n a region like the shoulder (less neuronal representation), the two tactile stimuli must be presented relatively farther apart before the two stimuli are distinguishable from a single point. Tactile recognition accuracy, like localization accuracy, also varies by body loci (Loomis & Lederman, 1986). SO 45 40 E  o  V  Shoulder .  «- Forearm  35 30 25 20  Z  Upper arm *  Forehead  15  Palm  10 5  Figure 2. Two-point tactile discrimination thresholds for different regions of the human body. From Sensation and Perception (5 ed.) (p. 234), by S. Coren, L.M. Ward, J.T. Enns, 1999, N e w York: Harcourt Bruce. Permission granted. th  3  Although this tactile system is relatively adept at providing body-relevant information, it is far less capable of providing space-relevant information on its own. Consider once again the example of localizing touch to the hand. Note that the perception of touch is the same whether the hand is held at one's side, or is held above the head. The tactile system alone cannot easily distinguish between these two possibilities. In other words simply k n o w i n g that the hand was touched is not enough information for precise localization of the stimulus. H o w then does the brain solve this fundamental localization problem? In addition to the information gained from the tactile sensory system, there are sources of space-relevant information available from the visual and proprioceptive sensory systems (Driver & Spence, 1998; Warren & Rossano, 1991). While vision permits observers to orient their gaze towards the locus of the tactile stimulus, proprioception permits observers to determine where their limbs are i n space by relying on information from pressure sensors to determine the position of the joints, the amount of tension i n a particular muscle or tendon, or changes i n muscle length (Roberts & W i n g , 2001). Information from these three sources (tactile, visual, proprioceptive) is thus combined i n some way i n order to solve the tactile localization problem. Since the sensory channels often provide redundant information, combining that information is often without incident. However, when spatial or temporal discrepancies or ambiguities exist i n the multisensory information received by the brain, interesting sensory illusions can arise. In movie theatres, for example, sound is typically perceived at the location of the movie screen rather than at the true location of the speakers. In this example, the available visual information biases the perceived location of the auditory information. 4  This effect is commonly referred to as the ventriloquist illusion (e.g., Bertelson & Aschersleben, 1998; H o w a r d & Templeton, 1966). A comparable illusion, referred to as the M c G u r k effect, is one i n which observed lip movements influence speech perception (e.g., M c G u r k & MacDonald, 1976). Observers who listened to the sound 'ba', for example, but who saw lip movements for the sound 'ga', actually perceived the sound 'da', a combination of the heard and seen sounds. Consider the case of mystery houses or 'magnetic hills', where people standing within the 'mystery house' appear to be standing on a dramatic tilt rather than upright, and cars on the 'magnetic h i l l ' appear to defy gravity and roll uphill when i n neutral gear (Shimamura & Prinzmetal, 1999). Popular tourist attractions, these areas are clear examples of h o w vision can significantly bias proprioception. These sites work because unbeknownst to the eye, the houses and stretches of road are shifted by about a 25-degree angle from the true horizontal. Since all visual cues to the true horizontal are hidden from the eyes, things look normal, and the brain takes its cues from the available horizontal features i n the environment. Thus, rather than relying on signals from proprioception to indicate upright, for example, people tend to rely on vision, w i t h the result that everything within the house, including other people, is seen as tilted rather than upright. Visual perception itself is susceptible to bias from information i n other sensory modalities. Sekuler, Sekuler and L a u (1997), for example, reported an experiment where the presentation of a sound biased observers' perception of visual motion. That is, the brief onset of a task-irrelevant sound, presented at the point of impact of two identical disks i n motion, biased observers to perceive the 5  ambiguously moving disks as bouncing off of one another rather than passing through. In contrast, when the sound was absent, or was presented at a different point in time than the point of impact, observers were more likely to report that the disks passed through one another. Shams, Kamitani, and Shimojo (2000) reported another sensory illusion wherein the number of auditory beeps that observers heard influenced how many visual flashes they saw. For instance, when a single visual flash was accompanied by multiple auditory beeps, observers reported seeing multiple flashes. In the case where ambiguous or inconsistent information is available through vision, touch and proprioception, the modality that biases the perception of information i n the others is typically the one that is more precise, salient, or more appropriate for the task (Welch et al., 1979; see Welch & Warren, 1980), or the modality to which attention is mainly directed (e.g., Colavita & Weisberg, 1979; Kelso, Cook, Olson, & Epstein, 1975; Posner, Nissen & Klein, 1976). For example, vision is k n o w n to be more precise, i n most cases, for spatial localization than either proprioception or audition. Thus, when making a spatial localization judgment, observers w i l l tend to rely on vision. This can be the case even when vision is blurred (Fishkin, Pishkin, Stahl,1975), or when only a minimal view is provided (e.g., fingertip rather than hand for limb localization; Warren & Schmitt, 1978). There are several empirical examples of visual biases of the perception of touch and proprioception (Hay, Pick & Ikeda, 1963; Mon-Williams et al., 1999; Rock & Harris, 1967). Rock and Harris (1967), for example, reported that when observers view their arm as they point to a central visual target that is displaced by prism goggles, they shift their target-pointing responses i n the direction 6  induced by the goggles. When the goggles are removed, the target-pointing responses are shifted i n the opposite direction. These pointing errors are specific to the target hand - the hand used to point when the goggles are on (i.e., they are not obtained w i t h the non-target hand). This latter result suggests that the goggles d i d not simply change the perceived location of the visual image, which w o u l d have similarly affected the target and non-target hand. Rather, the felt position of the target arm must have been altered by the task. Nielsen (1963) reported an experiment wherein observers were given instructions to first view a straight line on a piece of paper through a small viewing hole, and then to trace the given line at a pace set by a metronome. A s far as the observers knew, the view provided was always of their o w n limb. In reality, however, it was sometimes the experimenter's hand that they saw. During the initial phase of the experiment, the experimenter matched the observer's motion (i.e., accurately traced the straight line). In the second phase, however, the experimenter began by following the straight line, but then curved to the right. In response to what they saw the limb do, observers made compensatory limb movements i n the opposite direction. Subjective reports collected from observers indicated that they also felt as though their hand was drifting to the right and so tried to correct for this apparent change in location by moving the hand leftward. Results such as these indicate that observers typically rely on additional information from vision and proprioception i n order to determine the spacerelevant location of tactile information. This makes them vulnerable to tactile and proprioceptive illusions when the visual information is inconsistent w i t h the other two sources (e.g., Botvinick & Cohen, 1998; Pavani, Spence & Driver, 2000). 7  Botvinick and Cohen (1998) reported an experiment wherein observers mislocalized brush strokes applied to their out-of-sight hand onto a visible and spatially shifted fake hand. The authors had observers fixate on a fake hand positioned on a tabletop, while the observer's o w n left hand was occluded from his or her view (see Figure 3). During the exposure phase, the experimenter used two small paintbrushes to stroke both the observer's hidden hand and the visible fake hand synchronously. After 10 minutes or so, most observers reported feeling the tactile sensation on their hand as though it arose from the location of the fake hand, a n d / o r they reported that they felt as though their o w n hand was at the location of the fake hand. In an effort to objectify this experience, the experimenters asked observers to close their eyes, keep their left hand motionless on the table, and to move the index finger of their right hand along the bottom of the table until they believed it to be aligned w i t h the index finger of their left hand. Displacement measures between the two fingers were then taken (both before and after the exposure phase described above). Results revealed larger displacements of the two fingers i n the direction of the fake hand following longer exposure periods. Thus, seeing the fake hand influenced the perceived location of the real hand as well as the tactile sensations delivered to it.  1  Importantly, the authors demonstrated that the synchronicity between the brush strokes on the real and fake hands was critical for obtaining sizable displacement errors. 1  8  Figure 3. A n illustration of the experimental setup used by Botvinick and Cohen (1998).  As intriguing as the illusion described above is, however, there are a couple of limitations with the interpretation of Botvinick and Cohen's (1998) reported data. One, observers were directly asked to report the perceived location of their adapted hand. As a result, it is difficult to know whether observers mislocalized the brush strokes on their hand towards the fake hand, or whether they simply provided responses that they believed were expected of them. To minimize the possibility of such observer response biases, a more indirect measure of tactile and limb localization is needed. Secondly, observers were asked to provide subjective reports of whether the visual information provided (e.g., fake hand) altered perception of their own limb position. The phrasing or ordering of the questions in the brief questionnaire may have biased observers to respond in one way or another, thereby tainting interpretation of the  9  results. T o a v o i d this, the tactile localization a n d l i m b p o s i t i o n measures o u g h t to be objective rather t h a n subjective. P a v a n i , Spence a n d D r i v e r (2000) recently r e p o r t e d results s i m i l a r to those of B o t v i n i c k a n d C o h e n (1998), b u t u s i n g m e t h o d s that c i r c u m v e n t e d some of the p r o b l e m s described above. That is, their k e y measure of tactile a n d l i m b mislocalizations w a s i n d i r e c t (i.e., observers w e r e n o t asked d i r e c t l y a b o u t the perceived l o c a t i o n of either), a n d objective (i.e., d i d n o t r e l y o n subjective impressions r e p o r t e d b y observers). The t w o k e y c o n d i t i o n s r e p o r t e d b y P a v a n i et al. (2000) w i l l be referred to here as the N o Fake H a n d c o n d i t i o n (Figure 4 A ) a n d the Fake H a n d c o n d i t i o n (Figure 4B). I n b o t h c o n d i t i o n s , the a u t h o r s h a d observers place their h a n d s beneath a s m a l l stand, such t h a t t h e y w e r e out-ofsight, a n d h o l d a f o a m cube i n each h a n d so that a tactile v i b r a t o r w a s p o s i t i o n e d u n d e r their f i n g e r a n d one u n d e r their t h u m b (see Figure 4 A ) . The task of observers w a s to use a f o o t response to indicate, as q u i c k l y as possible, the elevation of the target tactile v i b r a t i o n (top or b o t t o m ) . A m a t c h i n g p a i r of f o a m cubes, w h i c h h e l d v i s u a l distractors i n the place of the tactile v i b r a t o r s , w a s p o s i t i o n e d i n v i e w of observers o n the stand just above the cubes i n the observer's h a n d s b e l o w . O n e of the v i s u a l distractors w a s presented s i m u l t a n e o u s l y w i t h the tactile target. The elevation of the v i s u a l distractor c o u l d either be c o n g r u e n t (Figure 4C) or i n c o n g r u e n t (Figure 4 D ) w i t h respect to that of the tactile v i b r a t i o n . I n the Fake H a n d c o n d i t i o n , a fake h a n d w a s p o s i t i o n e d above each of the observer's h a n d s a n d w a s a r r a n g e d so as to appear to ' h o l d ' the distractor lights (see Figure 4B).  10  Figure 4. A schematic of the experimental setup used by Pavani, Spence and Driver (2000). (A) No Fake Hand Condition (B) Fake Hand Condition (C) Example of Congruent target-distractor elevations (D) Example of Incongruent target-distractor elevations. Note that the congruent and incongruent trial types apply also to the Fake Hand condition though they are only pictured here for the N o Fake Hand condition.  In general, observers were faster and more accurate i n localizing the target tactile vibration on congruent trials (Pavani et a l , 2000). By computing a difference score between response times on the two trial types (i.e., Incongruent RT minus Congruent RT) the authors were able to index this advantage or Congruency Effect (CE). In the N o Fake Hand condition, the C E reported was about 90 ms. Interestingly, this C E jumped to 145 ms i n the Fake H a n d condition. That is, the C E increased significantly when the fake hands were positioned next to the distractor lights and were aligned with the observer's hands below.  11  Further, when the fake hands were misaligned with the observer's hands by rotating them 90°, the C E dropped back to 85 ms. According to Pavani et al., the distractor lights captured the location of the tactile vibration, and this capture effect was enhanced by the presence of the aligned fake hands. The larger C E i n the Fake H a n d condition w i l l be referred to here as the fake hand effect. Pavani et al. (2000) report that "the visual information was as if the rubber hands were the participants' own. Hence, the distractor lights lay close to the position specified by vision for tactile stimulation, even though they remained above the true location of the tactile stimulators and unseen real hands (as specified proprioceptively)" (p. 353). Although they d i d not say so directly, the authors allude to the idea that if observers had the experience of having their hands at the location of the fake hands, the apparent location of the tactile vibrations w o u l d be closer to the distractor lights, and the apparent targetdistractor proximity w o u l d enhance the effects of the distractors on the perception of the tactile targets. One limitation to the interpretation of Pavani et al.'s (2000) findings, however, is that the authors d i d not include a baseline measure that could be used to objectively determine whether, i n the presence of the fake hands, observers behave as though the tactile vibrations are felt at the location specified by the fake hands. That is, i n order to establish whether this is the case, the authors w o u l d need to know how observers behave when the tactile vibrations are actually presented at that location. The initial goal of the present research was to objectively measure the perceived location of the tactile stimuli i n the fake hand effect by introducing a condition wherein the observers held the tactile vibrations i n the location 12  normally occupied by the fake hand. From this, it was possible to index the magnitude of the effect. The effect was then used as the foundation for addressing a number of questions about the factors that contribute to spacerelevant tactile localizations. It was possible, for example, to systematically vary the available visual information, and then measure the fake hand effect to determine the extent to which the visual information influenced tactile localizations. It should be noted that since the study of visual-tactile integration is still relatively new, it is difficult to put the fake hand effect i n the context of existing theories and predict the outcome of the manipulations introduced here. Thus, for each question addressed i n the present research, a variety of possible outcomes were entertained rather than making any specific predictions. Overview of Experiments 1-5 H o w compelling is the fake hand effect? D o observers actually feel the tactile vibrations at the location occupied by the fake hand? This question was explored i n Experiment 1 by comparing the magnitude of the C E across three key conditions: A ) when the fake hand was at the level of the distractor lights (Fake H a n d condition), B) when the observer's o w n hand was at the level of the distractor lights (Real H a n d Baseline condition), and C) when the observer's o w n hand was below the level of the distractor lights (No Fake H a n d condition). If the fake hand effect is so compelling that observer's feel as though the tactile vibrations arise from the location of the fake hand, then C E s i n the Fake H a n d condition should be similar to those i n the Real H a n d Baseline condition and larger than those i n the N o Fake H a n d condition. Three additional questions of interest regarding the fake hand effect were explored i n Experiment 1. First, i n order to determine whether the pattern of 13  responses i n the Fake H a n d condition was relatively more similar to those i n the Real H a n d Baseline or the N o Fake H a n d condition, the spatial separation between the tactile vibration and the distractor light was manipulated. Second, to determine whether the magnitude of the C E in the N o Fake H a n d condition was influenced by limb visibility, the visibility of the observer's hand was manipulated. Third, to determine whether it was necessary to directly see the fake hand to obtain the fake hand effect, it was compared under conditions of full vision of the fake hand (the standard) with that of partial vision of the fake hand where the fake hand was covered w i t h a cloth and therefore could not be seen directly by the participant. There were two novel findings in Experiment 1. One, although the fake hand effect reported by Pavani et al (2000) was measurable, it was also clearly less compelling than one w o u l d think given the subjective impressions that Pavani et al. collected from observers. That is, the mean C E obtained when the participant's hand was i n the location normally occupied by the fake hand was larger than that obtained when the fake hand was present. T w o , vision of the fake hand was not necessary to produce the effect, rather, it was sufficient to have knowledge of the hand's presence reinforced by partial visual cues. This finding was replicated i n Experiment 2 even when the cover used to occlude the fake hand was a box, and therefore no longer permitted the hand's shape to be seen. Experiment 2, however, also revealed an important limitation to the above conclusion; the fake hand effect persists when both fake hands are hidden, but it is weaker on the side of a hidden fake hand if that hand is i n the context of a second fake hand that is visible at a non-target location.  14  Experiment 3 tested whether tactile sensory experiences that were consistent w i t h the fake hand effect made the effect more compelling (i.e., increased CEs i n the Fake H a n d condition relative to the N o Fake H a n d condition). Observers wore soft plastic kitchen gloves on their hands that were identical to those used for the fake hands. Even w i t h the feel of plastic on their hands, however, the effect remained the same size as that observed i n Experiments 1 and 2. This means that the fake hand effect is neither strengthened nor weakened by the presence or absence of consistent tactile information. In Experiment 4, the rotational posture of the fake hands was manipulated in order to be either consistent with the posture of the observer's hand (plausible) or inconsistent (implausible). In both cases, the fake hands were aligned w i t h the observer's hands immediately below. W h e n the posture of the fake hands was inconsistent w i t h that of the observer's hands, the fake hand effect disappeared. This means that the fake hand effect depends on postural compatibility between the fake hands and those of the participant. In Experiments 1-4, the mapping between the location of the tactile vibration (top digit vs. bottom digit) and the response of the participant (toe lift vs. heel lift) was assigned i n an arbitrary manner. Tactile vibrations localized to the top digit were always assigned to the toe lift and those localized to the bottom digit were assigned to the heel lift. Experiment 5 tested whether the fake hand effect depends i n any way on the combinations of the location of the target tactile stimulus (top digit vs. bottom digit), the rotational posture of the observer's hand (prone vs. supine), and the part of the foot used to indicate the response (toe lift vs. heel lift). The data indicated that the effect was weaker 15  overall for a top-heel and bottom-toe digit-response mapping. W h e n this particular mapping was combined w i t h a supine observer hand orientation, the effect both weaker and i n the reverse direction . This latter case indicates that the 2  fake hand effect is not only sensitive to visual posture (i.e., the posture of the fake and the real hand must be compatible), but it is also sensitive to posture when particular response mappings are assigned. In particular, when the observer's hand is supine, the finger does not map well onto the toe and the thumb does not map well onto the heel. This is especially true when the fake hand is prone (i.e., fake hand posture is rotationally inconsistent with the observer's hand). It is as though, under these conditions, the observer is compelled to imagine their limb being rotated into the fake hand. This result has important implications for a variety of human-machine interactions. Experiment 1 The goal of Experiment 1 was to evaluate recent claims that tactile stimuli delivered to a hidden limb may be mislocalized to a visible fake hand positioned next to a pair of distractor lights (e.g., Botvinick & Cohen, 1998; Pavani et al., 2000). The behavioral index of these mislocalizations was a larger Congruency Effect (CE) in the presence of the fake hand relative to the C E i n its absence. The logic for using this index was that if the target tactile vibrations are mislocalized to the fake hands and thus towards the distractor lights, the apparent proximity between the target vibration and distractor lights w i l l lead to an increased C E  The term 'reversed' is meant to distinguish the effect i n this case where CEs are negative and i n the direction of being larger for the Inconsistent Fake H a n d condition from the standard effect where CEs are positive and larger i n the Consistent Fake H a n d condition.  2  16  (e.g., slower RTs on Incongruent trials, faster RTs on Congruent trials, or a combination of both). There were four main questions addressed w i t h i n Experiment 1: Are tactile vibrations mislocalized to the fake hands? To make such a judgment, one w o u l d first need to establish a baseline C E measure i n order to know what magnitude of C E to expect if the target tactile vibrations were at the location normally occupied by the fake hand digits. To do this, a Real H a n d Baseline condition was included i n the present study wherein the observer's o w n hand, and thus the target tactile vibrations, were positioned in the normal fake hand position. It was expected that if the tactile vibrations are mislocalized i n the presence of the fake hands (Fake H a n d condition), then their apparent location should be at the same location as the tactile vibrations i n the Real H a n d Baseline, and thus, the resulting CEs should be the same magnitude in the two conditions. That is, the magnitude of CEs i n the Fake H a n d condition should be comparable to CEs i n the Real H a n d Baseline condition where the tactile vibrations are actually at the location that is specified by vision of the fake hand i n the Fake H a n d condition. This is in contrast to the C E s i n the N o Fake H a n d condition where the tactile vibrators are vertically separated from the distractor lights. A r e the dynamics similar for a fake versus real hand? Regardless of the absolute magnitude of CEs i n the Fake H a n d versus Real H a n d Baseline conditions, it is equally important to know whether similar patterns of response are generated in the two conditions following a given manipulation. For instance, previous research indicates that tactile target processing is disrupted more by near than by far distractors (Driver & 17  Grossenbacher, 1996; Macaluso, Frith & Driver, 2000), even w h e n other factors are controlled (e.g. distance of distractor from eye or ear; e.g., Driver & Spence, 1994; Spence, Ranson, & Driver, 2000). By measuring the CEs at multiple targetdistractor horizontal separations and comparing the response patterns across conditions, it is possible to determine whether the pattern generated i n the presence of the fake hand (Fake H a n d condition) is more similar to the case where the observer's o w n hand is at the location specified by vision (i.e., Real H a n d Baseline condition), or at the location specified by proprioception but below that of vision (i.e., N o Fake H a n d condition). Evidence for the former w o u l d support the notion that observers behave, at least to some degree, as though the visible fake hand were their own. A r e CEs influenced by viewing the limb i n the N o Fake H a n d condition? In the N o Fake H a n d condition of the Pavani et al. (2000) study, the observer's o w n hands were hidden from view, and only the distractor lights and the central feedback cube were visible. Previous research suggests that vision of a limb can significantly influence the perception of unseen tactile stimuli delivered to the limb (di Pellegrino, & Frassinetti, F., 2000; Kennett, TaylorClarke, & Haggard, 2001; Ladavas, Fame, Zeloni, & d i Pellegrino, 2000; Pierson et al., 1991), even when the view is given by a monitor and observers see their o w n limb indirectly (Tipper et al., 1998; 2001). It is possible that CEs i n the N o Fake H a n d condition w o u l d increase if observers were permitted to see their limbs. O n the other hand, there is some evidence to suggest that the view that observers had of the location just above the hidden limbs might be sufficient to benefit target localization (Newport, Hindle, & Jackson, 2001), and thus seeing the limb might not alter CEs. Yet another possible pattern of results is that CEs 18  w o u l d decrease if the observer's hand were visible. This might be the case if the vertical separation between targets and distractors is more pronounced when the hand is visible (i.e., spatial separation w o u l d reduce the effectiveness of the distractors). To address the question of whether limb visibility influences the magnitude of the C E in the present experiment, the observer's limbs i n both the N o Fake H a n d and the Real H a n d Baseline conditions were visible on half of the trials, and hidden from view on the remaining trials. Is partial vision of the fake hand sufficient for the effect? Thus far, the fake hand effect has only been tested under conditions where the fake hand is i n complete view of observers. The question remains then as to whether the fake hand effect w o u l d persist when the observers k n o w that the fake hand is present, but are no longer permitted to see it. To test this, the visibility of the fake hand was manipulated such that on half of the trials i n which it was present it was visible, and on the remaining trials it was hidden from view under a black cloth. It was expected that if partial vision of the fake hand is sufficient for the effect, CEs should be of the same magnitude whether the fake hand is hidden or visible. In contrast, if a completely visible fake hand is critical for the effect, then relatively large CEs should only be obtained when the fake hand is visible, but not when it is hidden.  Methods Participants: Forty-two undergraduate observers from the University of British Columbia Psychology subject pool participated i n a 45-minute experimental session i n return for partial course credit.  A l l had normal or  corrected-to-normal vision. A l l observers were naive to the purpose of the  19  e x p e r i m e n t a n d w e r e f u l l y debriefed u p o n c o m p l e t i o n . F u r t h e r p a r t i c i p a n t i n f o r m a t i o n is p r o v i d e d i n Table 1. Procedure: The task of observers w a s to m a k e a speeded forced-choice tactile localization j u d g m e n t . The target w a s a tactile v i b r a t i o n a p p l i e d to either the f i n g e r or t h u m b of the observer's r i g h t h a n d . L o c a t i o n responses w e r e m a d e v i a t w o foot-pedals, one p o s i t i o n e d u n d e r the toes of the r i g h t f o o t , a n d the other u n d e r the heel. A t rest, b o t h pedals w e r e depressed . T o indicate t h a t the tactile 3  v i b r a t i o n location w a s at the finger, observers released the p e d a l u n d e r their toes b r i e f l y a n d t h e n rested. To indicate that the tactile v i b r a t i o n l o c a t i o n w a s at the t h u m b , they b r i e f l y released the p e d a l u n d e r their heel, a n d t h e n rested. Observers w e r e i n s t r u c t e d to i g n o r e the s i m u l t a n e o u s onset of a distractor L E D w h i l e m a i n t a i n i n g center f i x a t i o n a n d a t t e n d i n g to the target tactile v i b r a t i o n . Observers t o o k short breaks b e t w e e n each e x p e r i m e n t a l b l o c k of trials. A p p a r a t u s : Figure 5 includes illustrations of the basic e x p e r i m e n t a l setup for each of the three c o n d i t i o n s , i n c l u d i n g samples of the v i s i b i l i t y m a n i p u l a t i o n : visible (Figure 5 A ) a n d h i d d e n (Figure 5B). A l s o i n c l u d e d i n F i g u r e 5 is a n example of each of the three target-distractor separations (none, s m a l l , a n d large). A c h i n rest w a s centered o n a table. I t w a s used to stabalize the p o s i t i o n of the observer's h e a d at a distance of 47 c m f r o m a r e d central f i x a t i o n L E D . C a r d b o a r d b o u n d a r i e s w e r e constructed a n d secured o n the left a n d r i g h t sides  This t o o k little-to-no effort o n the observer's part. The mere w e i g h t of the observer's f o o t at rest w a s all that w a s r e q u i r e d to depress the pedals. S i m i l a r l y , little effort w a s needed to release the p e d a l . The toes or heel h a d to be raised just e n o u g h so that the p e d a l w a s n o longer c o m p l e t e l y depressed. N o t e t h a t this d i d n o t r e q u i r e the toes or heel to clear the p e d a l . 3  20  of the chin rest, and were used to maintain the position of the observer's forearms. Visible Target-Distractor Separation •  None  S3 Small  CO  •  j ! 150  Large  $100  No Fake Hand  Fake Hand  Real Hand Baseline  Condition Hidden Target-Distractor Separation •  200  None  53 Small •  150  Large  S 100 T  c CO  £  50 -  H No Fake Hand  Fake Hand  Real Hand Baseline  Condition  Figure 5. Mean CEs (in ms) for Experiment 1 as a function of Condition (No Fake Hand, Fake Hand, Real Hand Baseline) and Target-Distractor Separation (None, Small, Large) for (A) Visible and (B) Hidden limbs. 21  The t w o target tactile v i b r a t o r s w e r e O t i c o n - A bone c o n d u c t i o n v i b r a t o r s of 100 O h m s . T h e y w e r e p o s i t i o n e d i n the u p p e r a n d l o w e r left h a n d corners of a f o a m cube. A T E N M A f u n c t i o n generator w a s used to d e l i v e r each tactile target. T o m a s k the s o u n d of the v i b r a t o r s , w h i t e noise w a s p l a y e d t h r o u g h a p a i r of headphones w o r n b y observers. T w o y e l l o w distractor L E D s w e r e p o s i t i o n e d i n the u p p e r a n d l o w e r r i g h t h a n d corners of a second f o a m cube, w h i c h w a s placed o n t o p of a t a l l strip of f o a m r u n n i n g h o r i z o n t a l l y f r o m the back of one set of f o r e a r m b o u n d a r i e s to the other. The fake h a n d , used at d i f f e r e n t p o i n t s d u r i n g the e x p e r i m e n t , w a s constructed u s i n g a r i g h t - h a n d e d p i n k soft plastic d i s h w a s h i n g g l o v e s t u f f e d w i t h c o t t o n b a t t i n g . W h e n the fake h a n d w a s present, i t rested o n a sheet of black c o r k b o a r d that w a s p o s i t i o n e d over t o p of the f o r e a r m b o u n d a r i e s . N o t e that w h e n e v e r the fake h a n d w a s present, this m e a n t t h a t the observer's o w n h a n d s w e r e u n d e r the c o r k b o a r d a n d thus o u t of sight. Observers m a d e their responses b y releasing one of t w o footpedals as q u i c k l y a n d as accurately as possible. W h e n t h e y localized the tactile v i b r a t i o n to their finger observers w e r e i n s t r u c t e d to release the p e d a l u n d e r the toes, a n d w h e n they localized the tactile v i b r a t i o n to their t h u m b t h e y w e r e i n s t r u c t e d to release the p e d a l u n d e r the heel. S t i m u l i : To p r o d u c e a v i s u a l distractor, one of the t w o L E D s w a s flashed three times f o r a d u r a t i o n of 50 ms each t i m e , w i t h each flash separated b y a 50 ms ISI. T o p r o d u c e the tactile target, three 50 m s 200-Hz sine-wave signals separated b y 50 m s ISIs w e r e sent f r o m one of the t w o tactile v i b r a t o r s . The target a n d distractor w e r e a l w a y s presented s i m u l t a n e o u s l y . Target a n d  22  distractor locations were random with the constraint that each location was selected from equally often. Whenever observers made an error i n response (including a failure to provide a response), a yellow L E D positioned two centimeters below the fixation L E D was flashed six times for 50 ms each time (50 ms between each flash). Design: Each observer completed two practice blocks of 15 trials at the beginning of the experiment. In the first practice block, only tactile stimuli were presented (i.e., no distractor lights). The second practice block consisted of both visual and tactile stimuli. Following the practice blocks, observers participated in six experimental blocks of 96 trials (576 trials i n total). Correct response times (RT) and mean percentage errors were recorded. Observers were randomly assigned to one of three conditions: N o Fake H a n d (n = 14), Fake H a n d (n = 14), or Real H a n d Baseline (n = 14). In all three conditions, the observer held the vibrator cube between the index finger and thumb of their right hand. The N o Fake H a n d and Fake H a n d conditions were similar to those tested by Pavani et al. (2000) wherein the observer's right hand was always below the position of the distractor cube. In the Fake H a n d condition, a sheet of corkboard was spread over the top of the forearm boundaries thus hiding the observer's hands below, and the fake hand was positioned on top of the corkboard on the right hand side. Note that this positioning of the fake hand meant that it was always aligned vertically w i t h the observer's hand below, and it was aligned horizontally w i t h the distractor cube. A foam cube was positioned between the index finger and thumb of the fake hand to enhance the similarities w i t h the observer's hand (refer to the Fake H a n d condition pictured i n Figure 5). O n trials where the distractor cube was 23  p o s i t i o n e d beside the fake h a n d , the fake i n d e x f i n g e r rested n e x t to the u p p e r distractor l i g h t a n d the fake t h u m b next to the l o w e r distractor l i g h t . The Real H a n d Baseline c o n d i t i o n w a s the k e y c o m p a r a t i v e c o n d i t i o n i n t r o d u c e d i n the present s t u d y . I t w a s s i m i l a r to the N o Fake H a n d c o n d i t i o n w i t h the one exception t h a t the h e i g h t of the observer's h a n d w a s raised to the level of the distractor cube b y h a v i n g observers rest their a r m o n a t h i c k s t r i p of f o a m . Three w i t h i n - o b s e r v e r factors w e r e m a n i p u l a t e d : C o n g r u e n c y of targetdistractor elevations, h o r i z o n t a l Separation b e t w e e n the distractor cube a n d target cube, a n d the V i s i b i l i t y of the observer's h a n d or of the fake h a n d ( w h e n present). Each of these factors is described i n t u r n b e l o w . Target-distractor elevations w e r e either c o n g r u e n t or i n c o n g r u e n t ( C o n g r u e n c y factor). C o n g r u e n t trials, i n this case, consisted of a n u p p e r L E D p a i r e d w i t h a t o p - f i n g e r v i b r a t i o n , or a l o w e r L E D p a i r e d w i t h a b o t t o m - t h u m b v i b r a t i o n . I n c o n g r u e n t trials, i n contrast, consisted of an u p p e r L E D p a i r e d w i t h a b o t t o m - t h u m b v i b r a t i o n or a l o w e r L E D p a i r e d w i t h a t o p - f i n g e r v i b r a t i o n . Each target-distractor p a i r o c c u r r e d e q u a l l y often i n each b l o c k . The h o r i z o n t a l separation b e t w e e n the L E D distractor cube a n d the target tactile v i b r a t i o n cube w a s m a n i p u l a t e d b y m o v i n g the distractor cube i n a l e f t w a r d d i r e c t i o n a w a y f r o m the tactile v i b r a t i o n cube. There w e r e three possible separations: none (0 c m apart, distractor cube at r i g h t ) , s m a l l (15 c m apart, distractor cube at center), or large (30 c m apart, distractor cube at far left). N o t e that w i t h i n a n y c o n d i t i o n , the p o s i t i o n of the target tactile v i b r a t i o n cube r e m a i n e d constant i n that i t r e m a i n e d o n the r i g h t side. T h u s , the distractor cube w a s a l w a y s to the left of the target cube, a n d o n l y the p o s i t i o n of the distractor cube w a s altered. This m e a n t that w h e n the fake h a n d w a s present, i t r e m a i n e d 24  on the right side above the location of the observer's hand below, and the distractor lights were either beside the fake hand, a small distance away, or a larger distance away. The visibility of the observer's limbs i n the N o Fake H a n d and Real H a n d Baseline conditions, and the visibility of the fake hand i n the Fake H a n d condition were manipulated such that they were visible on half of the trials, and hidden on the remaining trials. The sheet of corkboard was used to hide the observer's hand in the N o Fake H a n d condition, while a black cloth was used to hide the fake hand and the observer's hand i n the remaining two conditions. The two within-observer factors, Separation (3 levels) and Visibility (2 levels), were blocked resulting i n 6 blocks i n total. The order of blocks was randomized. Condition (3 levels) was a between-observer factor. A breakdown of the trial types is presented i n Table 2.  Results O n each trial, target localization response times and mean percentage errors were recorded. It should be noted that for all of the experiments that follow (1-5), observers were excluded if either their average RTs were longer than 1200 ms, or they had fewer than 75% correct responses i n any experimental condition . 4  Only trials where observers made correct localization responses were included i n the RT analyses. The RT Congruency Effect (CE) was calculated for each condition by finding the difference between the mean RT on Incongruent  Data was collected on a total of 137 observers throughout experiments 1-5. Of those, 12 had to be excluded for not meeting the RT or Accuracy criteria described above.  4  25  trials m i n u s the m e a n RT o n C o n g r u e n t trials. The analyses a n d discussion w e r e m a i n l y focused o n the CE results f o r t w o reasons. First, the CE measure is a convenient s u m m a r y measure as its m a g n i t u d e is d i r e c t l y i n d i c a t i v e of the extent to w h i c h the onset of the L E D s interfered w i t h the speeded responses to the tactile v i b r a t i o n s . That is, the larger the C E , the m o r e effective the L E D as a distractor. Second, the relative m a g n i t u d e of the CE is i n d i r e c t l y i n d i c a t i v e of the p e r c e i v e d separation b e t w e e n the tactile v i b r a t i o n s a n d the distractor l i g h t s , w h e r e the larger the CE, the smaller the perceived target-distractor separation. The m e a n percentage errors w e r e also a n a l y z e d as CEs ( I n c o n g r u e n t Errors - C o n g r u e n t Errors) a n d this p a t t e r n of data w a s c o m p a r e d t o t h a t of the CEs f o r RTs. I f either there w e r e n o significant effects observed i n the E r r o r CE data, or i f the same p a t t e r n of data w a s f o u n d across c o n d i t i o n s i n b o t h RT CEs a n d E r r o r CEs (e.g., the larger RT CEs p a i r e d w i t h the larger error CEs), t h e n speed-accuracy tradeoffs w e r e e l i m i n a t e d as a concern. I n the event t h a t o p p o s i n g patterns w e r e observed (e.g., the larger RT CEs p a i r e d w i t h the smaller error CEs), speed-accuracy tradeoffs h a d to be considered as a p o s s i b i l i t y . I n a d d i t i o n to these m a i n analyses of interest u s i n g the CEs, i d e n t i c a l analyses w e r e c o m p u t e d u s i n g the m e a n Correct RT a n d m e a n Percentage E r r o r data. This w a s done to ensure that the p a t t e r n of results observed f r o m the CEs, w h i c h are difference scores, w a s s i m i l a r to that for the means. These latter analyses are presented for each E x p e r i m e n t i n an A p p e n d i x . Since the results w e r e a l w a y s consistent w i t h the RT a n d E r r o r CE analyses, t h e y w i l l n o t be discussed f u r t h e r . B o t h the RT CEs a n d E r r o r CEs w e r e subjected to analyses of variance ( A N O V A ) i n v o l v i n g w i t h i n - o b s e r v e r factors of Separation ( N o n e , S m a l l , Large) 26  a n d V i s i b i l i t y (Visible, H i d d e n ) , a n d the between-observer factor of C o n d i t i o n ( N o Fake H a n d , Fake H a n d , Real H a n d Baseline). Significant t h r e e - w a y interactions w e r e f o l l o w e d u p u s i n g Simple I n t e r a c t i o n Effects, w h i l e significant t w o - w a y interactions w e r e f o l l o w e d u p u s i n g Simple Effects testing. Significant m a i n effects w e r e f o l l o w e d u p u s i n g Least Significant Difference (LSD) tests. A s p i c t u r e d i n Figure 5, there w e r e several notable f i n d i n g s i n the present s t u d y . First, w h e n the l i m b s w e r e visible a n d there w a s n o separation b e t w e e n targets a n d distractors (see the s o l i d black bars i n F i g u r e 5 A ) , there w e r e significant differences i n the m a g n i t u d e of the CEs across c o n d i t i o n s , w i t h the CE b e i n g the largest i n the Real H a n d Baseline c o n d i t i o n , a n d smallest i n the N o Fake H a n d c o n d i t i o n . This suggests that a l t h o u g h tactile targets w e r e b e i n g m i s l o c a l i z e d to the fake h a n d w h e n it w a s present, the fake h a n d effect w a s n o t of equal s t r e n g t h to the presence of the observers' o w n l i m b s i n the same location. For the s m a l l a n d large target-distractor separations, the CEs w e r e of s i m i l a r m a g n i t u d e across conditions. Second, the CEs t e n d e d to decrease w i t h increasing target-distractor separation for a l l c o n d i t i o n s except the N o Fake H a n d c o n d i t i o n . The fact that s i m i l a r patterns w e r e observed i n the Fake H a n d a n d Real H a n d Baseline c o n d i t i o n s suggests that there are some similarities b e t w e e n the fake h a n d a n d a real h a n d despite differences i n the absolute m a g n i t u d e of CEs. T h i r d , h i d i n g the l i m b s f r o m v i e w h a d v e r y little i m p a c t o n the m a g n i t u d e of the CEs relative to w h e n the l i m b was visible (compare F i g u r e 5B t o 5 A ) . There w a s one exception to this. W h e n there w a s n o target-distractor separation (see s o l i d black bars i n Figure 5), CEs w e r e larger i n the N o Fake H a n d c o n d i t i o n w h e n the h a n d w a s h i d d e n (Figure 5B) t h a n w h e n i t w a s v i s i b l e (Figure 5 A ) . 27  This finding suggests that the visual cues to the vertical separation between targets and distractors when the observer's hands were visible i n the N o Fake Hands condition may reduce the influence of the distractor lights on tactile localization. When the RT CEs were used as the dependent variable, both the main effect of Condition [F (2, 39] = 4.29, p < .05, M S E = 8325], and that of Separation [F (2, 78) = 19.13, p < .001, M S E = 1562] were significant. These main effects were tempered by a significant two-way interaction of Condition x Separation [F (4, 78) = 5.95, p < .01, M S E = 4259], and a significant three-way interaction of Condition x Separation x Visibility [F (4, 78) = 2.59, p < .05, M S E = 1400]. The three-way interaction was first broken d o w n into its component Condition x Separation interaction at each level of Visibility. When the limbs were visible (see Figure 5A), the Condition x Separation interaction was significant, F (4, 78) = 7.99, p < .01, M S E = 2984. This interaction was followed up by first testing the main effect of Condition at each level of separation. The main effect of Condition was significant only when there was no target-distractor separation [F (2,39) = 12.75, p < .001, M S E = 5371], where the C E was largest for the Real H a n d Baseline condition (189 ms), smaller for the Fake H a n d condition (128 ms), and smallest for the N o Fake H a n d condition (50 ms), all paired comparisons were significant at p < .05. Secondly, the main effect of Separation was tested for each of the three conditions. For the Real H a n d Baseline condition, the main effect and all paired comparisons were significant, all p's < .05. CEs were largest when there was no target-distractor separation (189 ms) and decreased for the small separation (95 ms) and decreased further for the large separation (44 ms). For the Fake H a n d condition, the main effect of 28  separation was significant, F (2, 26) = 5.53, p < .05, M S E = 3667. Least significant difference testing revealed that CEs tended to be larger when there was no separation (128 ms) than when there was either a small separation (82 ms; p < .06) or a large separation (52 cm; p < .05), but the comparison between CEs at the latter two separations d i d not reach significance (p > .05). Finally, the main effect of Separation approached significance for the N o Fake H a n d condition, p < .06. Follow-up tests indicated that CEs tended to be larger at the small separation (85 ms) than when there was no separation (50 ms), p < .05. The comparisons with the large separation (56 ms) d i d not reach significance, p's > .05. When the analysis was limited to the data for trials where the limbs were hidden (see Figure 5B), the interaction between Condition and Separation d i d not reach significance, p > .10. The overall three-way interaction was also broken d o w n by computing the simple interaction of Condition x Visibility at each level of Separation i n order to determine the effect of covering the hand on CEs within each condition. This interaction approached significance when there was no target-distractor separation [F (2,39) = 3.05, p < .06, M S E = 2108], but d i d not reach significance at either the small or large target-distractor separations, F's < 1. Follow-up tests for the former revealed that the effect of Visibility was significant only for the N o Fake H a n d condition where CEs were larger when the hand was hidden (81 ms) than when it was visible (50 ms), F (1,13) = 4.54, p < .055, M S E = 1494. Remaining p's > .05. When the overall analysis as that reported above was computed using mean Error CEs as the dependent variable, only the main effect of Separation was significant, F (2, 78) = 6.98, p < .01, M S E = 41. Least significant difference 29  testing revealed that CEs were larger when there was no target-distractor separation (7%) than either the small (4%) or large (4%) separations (p's < .01), which were not significantly different from one another, p >.20. The C E s that were significant were i n the same direction as the RT CEs suggesting that speedaccuracy tradeoffs were not a concern (e.g., larger RT CEs and Error CEs when there was no target-distractor separation). Discussion There were four main findings i n the present study. The implications of each are discussed i n turn below. Larger CEs i n the Real H a n d Baseline versus Fake H a n d condition: This finding implies that the experience of the fake hand effect is not identical to the experience observers have when the tactile vibrations are presented at the location of the fake hand. That is, the felt location of touch appears to be different when vision alone indicates that the tactile vibrations are beside the distractor lights (Fake H a n d condition) compared to when vision and proprioception indicate the spatial alignment between targets and distractors (Real H a n d Baseline condition). This is an important new finding, as it provides an objective indication that the fake hand effect may be less compelling than the observer's subjective impressions w o u l d lead one to believe. It is, however, consistent w i t h reports from the earlier multimodal discrepancy literature that indicates that the visual bias of proprioception is rarely 100%, rather, it tends to average around 75% (see Welch & Warren, 1980). CEs decrease w i t h increases i n target-distractor separation: This pattern was observed i n both the Fake H a n d and Real H a n d Baseline conditions but not i n the N o Fake H a n d condition. This is consistent w i t h the 30  idea that the fake h a n d is s i m i l a r to a real h a n d for observers (note t h a t this can be the case even t h o u g h the absolute CE data s h o w s that responses i n the presence of the fake h a n d are w e a k e r t h a n those o b t a i n e d w i t h the real h a n d ) . It is interesting to note that i n these t w o c o n d i t i o n s CEs w e r e largest o v e r a l l w h e n there w a s n o target-distractor separation despite the fact t h a t at the s m a l l separation, the distractor lights w e r e located at the center of gaze. I f seeing the distractors w e r e the k e y factor, one w o u l d expect that it w o u l d be at this location that the l i g h t s w o u l d have the greatest i m p a c t o n target l o c a l i z a t i o n responses. T h a t they h a d their greatest i m p a c t w h e n t h e y w e r e n e x t to the target v i b r a t i o n s a n d a w a y f r o m the center of gaze indicates t h a t i t is the spatial p r o x i m i t y b e t w e e n the targets a n d distractors that is of m o s t i m p o r t a n c e . This target-distractor separation effect is consistent w i t h earlier reports suggesting that the greater the discrepancy i n i n f o r m a t i o n w i t h i n the v i s u a l a n d p r o p r i o c e p t i v e sources, the less evidence f o r v i s u a l bias ( O v e r , 1966; see W e l c h & W a r r e n , 1980). The fact t h a t the above separation p a t t e r n w a s n o t observed i n the N o Fake H a n d c o n d i t i o n indicates t h a t the vertical separation b e t w e e n targets a n d distractors is e n o u g h to w e a k e n the m a g n i t u d e of the CE. I n fact, i t w a s o n l y i n this c o n d i t i o n that CEs t e n d e d to be h i g h e r at the center of gaze (small separation) t h a n w h e n there w a s n o separation b e t w e e n targets a n d distractors. F u r t h e r , since the separation effect w a s present i n the Fake H a n d c o n d i t i o n , i t suggests that the fake h a n d m u s t have successfully b r i d g e d the v e r t i c a l targetdistractor separation that w a s apparent i n the N o Fake H a n d c o n d i t i o n .  31  Little-to-no effect of limb visibility on CEs: The only condition i n which there was an effect of visibility was the N o Fake H a n d condition wherein CEs were smaller when the hand was visible relative to when it was hidden. Since this effect of visibility was limited to the N o Fake H a n d condition, it seems likely that making the observer's hand visible in this case emphasized the vertical separation between the tactile vibrations and the distractor lights thereby reducing the C E . Partial vision of the fake hand is sufficient for the effect: A s noted above, hiding the fake hand i n the Fake H a n d condition d i d not significantly change the magnitude of the CEs. That is, the CEs tended to be larger i n the Fake H a n d condition than in the N o Fake H a n d condition whether the hands were visible or not. These results are somewhat of a surprise as they suggest that directly seeing the hand is not critical for tactile mislocalizations, and knowing that the fake hand is present, and having partial visual information to reinforce that, is sufficient to influence responses. The experimental design used i n Experiment 1 was a modified version of that used by Pavani et al. (2000). The modifications were necessary i n order to incorporate the target-distractor separation manipulation used. One drawback to modifying the Pavani et al. (2000) design, however, is that direct comparisons are more difficult to make between the results of their experiment and the present one. For this reason, we reverted back to the Pavani et al.'s original design i n Experiment 2 such that tactile vibrations were presented to either the observer's right or left hands, and distractor lights were either presented to the right or left sides of fixation.  32  In Experiment 2, a more stringent test of whether partial vision of the fake hand's presence is sufficient for the effect was applied by introducing two different manipulations. One, a box cover as opposed to a cloth cover was used to hide the hand, the difference being that the former eliminated hand shape information while the latter d i d not. It was expected that if a placeholder reminder of the fake hand's presence is sufficient for the effect, then hiding the hand under the box w i l l have no effect on the magnitude of the CEs. In contrast, if partial vision of the fake hand's presence is not sufficient, but the accessibility of hand shape information contributed to the results i n Experiment 1, it was expected that CEs w o u l d be smaller when hidden under the box cover than when visible. Additionally, Experiment 2 was designed to test whether partial vision of the fake hand w o u l d continue to be sufficient for inducing the effect if there were a second fake hand that was uncovered and within the observer's view. One possibility is that the effect w i l l be present for both the hidden and visible fake hand. Another, more interesting possibility, is that the available visual limb information w i l l compete with the hidden limb information, and the effect w i l l be influenced i n some way by this competition.  Experiment 2 There were two main goals of Experiment 2. The first was to determine whether observers w o u l d continue to mislocalize tactile targets when both fake hands were hidden under box-covers that eliminated hand shape information. Three possible results were considered at the outset. One possibility is that once the shape of the fake hand is no longer visible, observers w i l l behave as though the fake hand were absent (i.e., CEs for the Fake H a n d condition w i l l be similar  33  to those i n the N o Fake H a n d condition). This result w o u l d indicate that seeing the features of the hand (e.g., shape) is an important part i n generating mislocalizations, and that the conclusions drawn i n Experiment 1 that partial vision of the fake hand is sufficient for mislocalizations w o u l d have to be modified. A second possibility is that observers w i l l continue to mislocalize tactile targets to the fake hand, but w i l l do so to a lesser degree than when the fake hand is visible (i.e., CEs w i l l be smaller for the hidden versus visible fake hand but larger than CEs when the fake hand is absent). A result such as this w o u l d indicate that feature information about the hand contributes to the mislocalizations (i.e., CEs are larger when shape information is available), but is not necessary for them. A third possibility is that simply knowing that the fake hand is present is sufficient for mislocalizations (i.e., CEs are the same magnitude for a hidden and visible fake hand). This is the more interesting possibility, as it w o u l d highlight that the effect is not a purely visual or stimulus-driven one but rather it may be elicited by top-down information and imagery based on partial vision. The second goal of the'present study is to determine whether mislocalizations to a hidden fake hand change as a function of the visible presence of a second fake hand. It could be the case that the effect is obtained as before w i t h both the hidden and visible hand. Another possibility, however, is that the effect is weaker on the side of the hidden fake hand than on the side of the visible fake hand. This latter outcome w o u l d indicate that the more detailed visual information carries more weight than does partial visual information when both are available. 34  The design of Experiment 2 was similar to that of Pavani, Spence and Driver (2000). Tactile targets were now held i n both the left and right hands, and in the Fake H a n d condition, a fake hand was positioned above each of the observer's hands. In the N o Fake H a n d condition, the fake hands were absent and only the distractor lights and central feedback cube were visible. W i t h this design, it was possible to present tactile targets and visual distractors to either the right or left side of space. Most importantly, it was possible to manipulate the visibility of both fake hands, or the visibility of just a single fake hand. In this latter case hiding only a single fake hand meant that the other fake hand was visible. This effectively places the hidden and visible hand i n direct competition w i t h one another. This manipulation provided an opportunity to-test the reliability of the finding that a partial visual reminder of the fake hand's presence is sufficient for tactile mislocalizations. Methods  Participants: Thirteen undergraduate students at the University of British Columbia participated i n the one-hour experimental session for partial course credit. Personal descriptive information is provided i n Table 1. A l l observers reported normal or corrected-to-normal vision. Observers were naive to the purpose of the experiment and were fully debriefed upon completion. Apparatus: Similar to that used i n Experiment 1. Observers laid their arms on the surface of a table (within arm boundaries on either side), and underneath a smaller stand. For this experiment, they held a foam cube i n each hand. Each foam cube contained a pair of tactile vibrators as described i n Experiment 1. O n the surface of the smaller stand, two foam cubes were positioned so that one was aligned above each tactile foam cube held below. 35  These visible cubes held pairs of distractor lights, again these were as described in Experiment 1. In four of the five conditions, a pair of fake hands (constructed as before from a pair of soft pink kitchen gloves stuffed w i t h cotton batting), were positioned so as to appear to 'hold' the distractor lights on their respective sides. To hide either or both of the fake hands, a box cover was placed over the top of the hand. In doing so, hand shape information was eliminated. O n the remaining block, the fake hands were absent. The five conditions were completed i n random order. Stimuli: Same as that used i n Experiment 1, w i t h the exception that there were now four possible target locations and four possible distractor light locations. Locations were randomly selected w i t h the constraint that all locations were selected from equally often, and that there were an equal number of congruent and incongruent trials, and an equal number of same and opposite side target-distractor presentations. Design: Observers participated in three training sessions of 15 trials each, followed by five experimental blocks of 96 trials. A l l factors were run as withinobserver factors. Observers participated i n five randomly ordered Fake H a n d Conditions: N o Fake H a n d , Both Visible, Both H i d d e n , Left H a n d H i d d e n , and Right H a n d H i d d e n . Note that in all conditions, the observer's hands were always underneath the small stand, below the level of the distractor lights. In the N o Fake H a n d condition, there were no fake hands on the stand, only the distractor light cubes and the feedback cube i n the center were visible. In the remaining conditions, both fake hands were present and were positioned so as to appear to 'hold' the distractor lights. In the Both Visible condition, both fake hands were i n view, while i n the Both H i d d e n condition, both fake hands were 36  h i d d e n beneath b o x covers. I n the Left H a n d H i d d e n a n d R i g h t H a n d H i d d e n c o n d i t i o n s , the left or r i g h t fake h a n d , respectively, w a s h i d d e n f r o m v i e w u n d e r n e a t h a b o x cover w h i l e the r e m a i n i n g h a n d w a s visible. Procedure: Same as that i n E x p e r i m e n t 1.  Results RT CEs a n d E r r o r CEs w e r e again the d e p e n d e n t measures of interest. Significant effects w e r e f o l l o w e d u p u s i n g the same p r o c e d u r e s as i n d i c a t e d i n E x p e r i m e n t 1. A l l factors w e r e w i t h i n - o b s e r v e r factors a n d i n c l u d e d C o n d i t i o n ( N o Fake H a n d , Visible, H i d d e n , Left H a n d H i d d e n , a n d R i g h t H a n d H i d d e n ) , a n d Target-Distractor Side (Same, Opposite). There w e r e t w o m a i n f i n d i n g s . O n e , w h e n b o t h fake h a n d s w e r e h i d d e n f r o m v i e w u s i n g b o x covers, CEs w e r e of the same m a g n i t u d e as those observed w h e n b o t h fake h a n d s w e r e visible (see Figure 6). This suggests that observers w e r e still i n f e r r i n g the presence of the fake h a n d s , despite the fact t h a t t h e y w e r e covered a n d their shape disguised. I n other w o r d s , observers s t i l l m i s l o c a l i z e d tactile targets to the fake h a n d s w h e n b o t h w e r e h i d d e n . N o t e t h a t the CEs i n these t w o c o n d i t i o n s w e r e larger t h a n those f o u n d w h e n the fake h a n d s w e r e absent.  37  A.  B.  Same  200  200 ,—^  E  CO  E c  150  LU  y 100  I  Opposite  O tz  co cu  50  vt  TJ  c  CO  I  a> co  a>  'vt > o  m  c a>  a> •g CO  TJ  I  sz o  to  C  C  a> a) "a  TD  "5 "5 II  e O  o  sz u  o Fake Hand Condition  a>  150i  100 50  •a  TJ C  <=  CD  I  "a  vt  CO  I  >  c  o  O  £ u  ca cu ca  a; > o CD  CD TJ  CD TJ  TJ TJ  CO c a> T , I  CO C  C  a) a> O  CQ  TJ TJ  I  Q> C  O  O  TJ TJ  I  TJ  W  I  >  cu c c o  C O  O  £  (J 3 O  sz u o  Fake Hand Condition  Figure 6. Mean CEs (in ms) for Experiment 2 as a function of Fake Hand Condition (No Fake Hands, Both Visible, Both Hidden, One Hidden - Touch on Hidden Side, One Hidden - Touch on Visible Side) for (A) Same and (B) Opposite target-distractor presentations.  Two, when only one of the fake hands was hidden and the other was visible, CEs were significantly larger when targets and distractors were presented to the side of the visible fake hand than when they were presented to the side of the hidden fake hand (see Figure 6). This is a very different result from that obtained when either both fake hands are visible or both fake hands are hidden. This suggests that when there is both a visible and a hidden fake hand in competition observers are less likely to infer the presence of the hidden hand. Two main analyses were computed. First, a repeated measures A N O V A was computed using RT CEs as the dependent measure, and Condition (No Fake Hands, Both Visible, Both Hidden) and Target-Distractor Side (Same, Opposite) as within-observer factors, refer to Figure 6. Both the main effects of Condition 38  [F (2,50) = 4.07, p < .05, M S E = 7716] a n d Distractor Side w e r e significant [F ( 1 , 25) = 23.65, p < .01, M S E = 12367], as w a s the i n t e r a c t i o n [F (2,50) = 5.0, p < .05, M S E = 6038]. The i n t e r a c t i o n w a s e x a m i n e d m o r e closely b y l o o k i n g at the m a i n effect of C o n d i t i o n separately for each target-distractor t y p e (i.e., same a n d opposite side presentations; see Figure 6 A a n d 6B, respectively). The m a i n effect of C o n d i t i o n w a s significant o n l y f o r same-side target distractor presentations, F (2,50) = 5.44, p < .05, M S E = 10422. CEs w e r e smallest i n the N o Fake H a n d s c o n d i t i o n (77 ms) t h a n either the B o t h Visible (169 ms) or B o t h H i d d e n c o n d i t i o n s (137 m s ) , p's < .05. CEs i n the latter t w o c o n d i t i o n s d i d n o t d i f f e r s i g n i f i c a n t l y f r o m one another, p > .20. The same analysis as that above w a s c o m p u t e d u s i n g E r r o r CEs as the d e p e n d e n t measure. O n l y the m a i n effect of Target-Distractor Side w a s significant, w h e r e CEs w e r e larger w h e n targets a n d distractors w e r e presented to the same side (6.1%) versus the opposite side (1.8%) of f i x a t i o n , F ( 1 , 25) = 9.03, p < .01, M S E - 79. N o other effects w e r e significant, F's < 1. Speed accuracy tradeoffs w e r e n o t a concern as the error rates w e r e either the same across c o n d i t i o n s or their p a t t e r n resembled that f o u n d i n the RT CEs. The second analysis focused o n the t w o c o n d i t i o n s w h e r e either the left or r i g h t fake h a n d w a s h i d d e n , Left H a n d H i d d e n a n d R i g h t H a n d H i d d e n , respectively. The data w e r e o r g a n i z e d i n t o t w o factors t h a t each h a d t w o levels, Side of T o u c h ( H i d d e n , Visible) a n d Distractor Side (Same, O p p o s i t e ) . The factor Side of T o u c h referred to the side of space that the tactile s t i m u l i w a s d e l i v e r e d to, that is, w h e t h e r i t w a s the side of the h i d d e n fake h a n d or t h a t of the visible fake h a n d . Distractor Side referred to the side of space t h a t the distractor l i g h t  39  was d e l i v e r e d t o , that is, w h e t h e r i t w a s the same side as the tactile s t i m u l u s or the opposite side. A repeated measures A N O V A w a s c o m p u t e d o n the RT CEs u s i n g b o t h Side of T o u c h a n d Distractor Side as factors (refer to the t w o r i g h t m o s t bars i n Figure 6 A a n d 6B). The m a i n effect of Distractor Side w a s significant, F ( 1 , 25) = 18.72, p < .01, M S E = 9750. This was t e m p e r e d b y a significant Side of T o u c h x Distractor Side i n t e r a c t i o n , F ( 1 , 25) = 5.45, p < .03, M S E = 7968. T o better i n t e r p r e t the i n t e r a c t i o n , the s i m p l e m a i n effect of Side of T o u c h w a s e x a m i n e d separately for same a n d opposite side target-distractor presentations. The s i m p l e m a i n effect of Side of T o u c h w a s significant o n l y w h e n the distractor w a s presented to the same side of space as the target. W h e n this w a s the case, CEs w e r e s i g n i f i c a n t l y larger for targets presented o n the side of the visible fake h a n d (182 ms) c o m p a r e d to the side of the h i d d e n fake h a n d (106 m s ) , F ( 1 , 25) = 7.33, p < .05, M S E = 10278. W h e n the above analysis w a s repeated u s i n g m e a n e r r o r CEs as the d e p e n d e n t variable, a s i m i l a r p a t t e r n of results w a s observed, a n d speedaccuracy tradeoffs w e r e e l i m i n a t e d as a concern. The m a i n effect of Distractor Side w a s significant, F (1,25) = 14.33, p < .001, M S E = 54. This w a s t e m p e r e d b y a significant Side of T o u c h x Distractor Side i n t e r a c t i o n , F (1,25) = 6.09, p < .05, M S E = 86. The m a i n effect of Side of T o u c h w a s e x a m i n e d separately f o r Same a n d O p p o s i t e target-distractor presentations. W i t h same side target-distractor presentations, error CEs t e n d e d to be larger f o r targets presented t o the visible (12%) versus h i d d e n (7%) fake h a n d , F ( 1 , 25) = 3.07, p < .10, M S E = 111. W i t h opposite side target-distractor presentations, error CEs t e n d e d t o be larger w h e n targets w e r e presented to the h i d d e n h a n d (6%) t h a n the visible h a n d (2%), F ( 1 , 40  25) = 3.26, p < .10, M S E = 59. This last result, although not significant i n the RT CEs, is still consistent w i t h the pattern observed there. W i t h the exception of this last result, the error CEs were either the same across conditions or their pattern was the same as the RT CEs, and thus speed-accuracy tradeoffs were not a concern. Discussion In Experiment 2, there were two main findings. Each w i l l be discussed i n turn below. CEs are similar whether both fake hands are visible or hidden: Observers continue to infer the presence of both fake hands even when they are hidden under box covers. In other words, when observers are aware of the presence of the fake hands, but are unable to see them directly, they continue to mislocalize tactile targets to them. This implies that the fake hand effect can be elicited by partial visual information that reinforces that the fake hands are present. CEs are larger on the side of a visible versus hidden fake hand: This finding differs from that found when either both fake hands were visible, or both fake hands were hidden, neither of which differed from one another. It suggests that when there is potential for competition between a visible and hidden hand, observers w i l l weight the visual information more heavily so that full visual cues influence tactile localizations more than partial cues. This is an important finding as it indicates flexibility i n the observer's usage of the visual information that is available when there is a discrepancy between the seen and felt limb locations. In the absence of any other visual  41  information, observers w i l l respond to a hidden limb as though it were visible, but fail to do so when there is more salient visual information available. Experiments 1 and 2 provide further empirical support for the idea that tactile targets can be mislocalized towards a fake hand. The novel contribution of the present studies is the finding that tactile targets are not mislocalized to the extent that the tactile vibrations are felt at the fingers of the fake hand, or to the extent that observers behave as though their o w n hands are at the location specified by vision of the fake hands. These results make a quantitative contribution to the subjective reports collected by Pavani et al. (2000), which indicate most observers feel "as if the rubber hands were m y hands", or "as if I was feeling the tactile vibration i n the location where I saw the rubber hands", or even that it "seemed as if the lights were near to m y real hands" (p. 357). In particular, the present results imply that the experience is only about 70% of the full possible strength . 5  One possible explanation for this apparent discrepancy i n results from qualitative versus quantitative measures is that the observers tested i n Experiments 1 and 2 d i d not wear gloves on their hands to match the fake hands whereas observers tested i n Pavani et al. (2000) did. That is, it may be that feeling the soft plastic on their hands, as well as seeing the soft plastic fake hands, enhanced the experience of tactile mislocalizations. Experiment 3 was designed to test whether the fake hand effect was any stronger when observers could feel the rubber material on their o w n hands compared to w h e n their hands were bare.  This estimation came from dividing the largest C E i n the Fake H a n d condition (128 ms) from that i n the Real H a n d Baseline (189 ms), and multiplying by 100.  5  42  Experiment 3 The m a i n q u e s t i o n addressed i n the present e x p e r i m e n t w a s w h e t h e r a tactile sensory experience, i n a d d i t i o n to a v i s u a l one, c o u l d i n f l u e n c e the m a g n i t u d e of the fake h a n d effect. T h a t is, can the experience of feeling soft plastic o n one's h a n d s at the same t i m e as seeing the soft plastic of the fake hands enhance the fake h a n d effect. T o test this, observers w o r e a p a i r of soft plastic k i t c h e n gloves o n their h a n d s that m a t c h e d those used to construct the fake hands. The p r e d i c t i o n w a s that i f the feel of plastic o n one's h a n d s increases tactile mislocalizations, t h e n CEs w o u l d be larger w h e n observers w o r e a p a i r of soft plastic gloves c o m p a r e d to w h e n their h a n d s r e m a i n e d bare.  Methods Participants:  Fourteen u n d e r g r a d u a t e students at the U n i v e r s i t y of  B r i t i s h C o l u m b i a p a r t i c i p a t e d i n the 4 5 - m i n u t e e x p e r i m e n t a l session i n r e t u r n for p a r t i a l course credit. A l l observers r e p o r t e d n o r m a l or c o r r e c t e d - t o - n o r m a l v i s i o n . F u r t h e r descriptive i n f o r m a t i o n o n observers is p r o v i d e d i n Table 1. Observers w e r e n a i v e to the p u r p o s e of the e x p e r i m e n t a n d w e r e f u l l y debriefed u p o n completion. A p p a r a t u s : Similar to that used i n E x p e r i m e n t 2. Observers l a i d their arms o n the surface of a table ( w i t h i n a r m b o u n d a r i e s o n either side), a n d u n d e r n e a t h a smaller stand. T h e y h e l d a f o a m cube i n each h a n d . Each f o a m cube c o n t a i n e d a p a i r of tactile v i b r a t o r s as described i n E x p e r i m e n t 1. A black c l o t h w a s d r a p e d over the surface of the smaller stand o b s t r u c t i n g a v i e w of the observer's hands. A n a d d i t i o n a l f o a m cube w a s a l i g n e d above each of the observer's h a n d s b e l o w . This p a i r of cubes h e l d pairs of distractor L E D s . A g a i n ,  43  these w e r e as described i n E x p e r i m e n t 1. O n half of the trials, observers w e r e r e q u i r e d to w e a r a p a i r of p i n k soft plastic k i t c h e n gloves (a v a r i e t y of sizes w e r e available). A s w e l l , observers w o r e a p a i r of t h i n disposable g l o v e liners inside the k i t c h e n gloves f o r sanitary purposes. S t i m u l i : Same as that used i n E x p e r i m e n t 2. The target w a s one of f o u r possible tactile v i b r a t o r s , a n d the distractor one of f o u r possible L E D s . Design: Observers p a r t i c i p a t e d i n three t r a i n i n g sessions of 15 trials each, f o l l o w e d b y eight e x p e r i m e n t a l blocks of 96 trials. I n a d d i t i o n to the w i t h i n observer factors of C o n d i t i o n ( N o Fake H a n d , Fake H a n d ) , C o n g r u e n c y ( C o n g r u e n t , I n c o n g r u e n t ) , a n d Distractor Side (Same, O p p o s i t e ) , there w a s an a d d i t i o n a l w i t h i n - o b s e r v e r factor referred to as Observer Gloves ( N o n e , Plastic). Procedure: Same as that i n E x p e r i m e n t 2.  Results RT CEs a n d E r r o r CEs w e r e the m a i n measures of interest. T h e y w e r e calculated i n the same w a y as before ( I n c o n g r u e n t RT m i n u s C o n g r u e n t RT). The analyses consisted m a i n l y of repeated measures A N O V A s . Significant t w o w a y interactions of interest w e r e f o l l o w e d u p b y s i m p l e effects testing w h i l e significant m a i n effects of interest w e r e f o l l o w e d u p u s i n g Least Significant Difference testing. The m a i n f i n d i n g of interest i n the present s t u d y w a s t h a t CEs w e r e the same m a g n i t u d e i n the Fake H a n d c o n d i t i o n w h e t h e r observers experienced the feel of soft plastic o n their h a n d s or their h a n d s r e m a i n e d bare, see F i g u r e 7. A d d i t i o n a l l y , as expected, a target-distractor separation effect w a s o b t a i n e d w h e r e CEs w e r e consistently larger w h e n the target a n d distractor w e r e  44  presented o n the same side of f i x a t i o n (see Figure 7A) t h a n w h e n t h e y w e r e presented to opposite sides (see Figure 7B). A  Same Condition  200  •  B  Opposite  No Fake Hand 200  Fake Hand  I" %  160-1 120  HI  oco  Plastic None Observer Gloves  Plastic None Observer Gloves  Figure 7. Mean CE (in ms) for Experiment 3 as a function of Condition (No Fake Hands, Fake Hands), Observer Gloves (Plastic, None) for (A) Same and (B) Opposite side target-distractor presentations A 3-factor repeated measures A N O V A w a s c o m p u t e d u s i n g C o n d i t i o n ( N o Fake H a n d , Fake H a n d ) , Distractor Side (Same, O p p o s i t e ) a n d Observer Gloves ( N o n e , Plastic) as factors, a n d RT CEs as the d e p e n d e n t variable. See Figure 7. O n l y the m a i n effects of C o n d i t i o n [F (1,13) = 10.77, p < .01, M S E = .3143] a n d Distractor Side [F (1,13) = 16.93, p < .01, M S E = 3692] w e r e significant. A l l r e m a i n i n g p's > .10. These results indicate that CEs w e r e larger o v e r a l l i n the Fake H a n d c o n d i t i o n (94 ms) versus the N o Fake H a n d c o n d i t i o n (59 m s ) . A s expected, CEs w e r e larger o v e r a l l w h e n the target a n d distractor w e r e presented o n the same side of f i x a t i o n (100 ms) versus the opposite side (53 m s ) . N o n e of the m a i n effects or interactions i n v o l v i n g the Observer Gloves factor reached significance. This indicates t h a t CE m a g n i t u d e s w e r e the same regardless of w h e t h e r observers felt plastic o n their h a n d s or not.  45  The same analysis as that above w a s repeated u s i n g E r r o r CEs as the d e p e n d e n t variable. O n l y the m a i n effect of Distractor Side [F (1,13) = 11.73, p < .01, M S E = 33] a n d the t h r e e - w a y i n t e r a c t i o n of C o n d i t i o n x D i s t r a c t o r Side x Observer Gloves [F (1,13) = 8.82, p < .05, M S E = 8.5] w e r e significant. The threew a y i n t e r a c t i o n w a s e x a m i n e d b y l o o k i n g at the s i m p l e C o n d i t i o n x D i s t r a c t o r Side i n t e r a c t i o n separately for Gloves a n d N o Gloves. W h e n the data w a s restricted to the trials w h e r e observers w o r e gloves, the s i m p l e C o n d i t i o n x Distractor side i n t e r a c t i o n d i d n o t reach significance, F < 1. W h e n the data w a s restricted to trials w h e r e observers d i d n o t w e a r gloves, the s i m p l e i n t e r a c t i o n w a s significant, F (1,13) = 8.26, p < .05, M S E = 18. To u n d e r s t a n d it better, it w a s b r o k e n d o w n b y l o o k i n g at the s i m p l e effect of C o n d i t i o n f o r same a n d opposite side target-distractor presentations. W h e n targets a n d distractors w e r e presented o n the same side, E r r o r CEs w e r e larger o v e r a l l i n the Fake H a n d c o n d i t i o n (12%) t h a n i n the N o Fake H a n d c o n d i t i o n (4%), F (1,13) = 4.82, p < .05, M S E = 97. W h e n targets a n d distractors w e r e presented o n the opposite side, the s i m p l e m a i n effect of C o n d i t i o n d i d n o t reach significance, F < 1. Since the E r r o r CEs w e r e either n o t significant, or their p a t t e r n w a s consistent w i t h the p a t t e r n of the RT CEs, speed accuracy tradeoffs w e r e n o t a concern.  Discussion The results of E x p e r i m e n t 3 suggest that the feel of soft plastic does n o t enhance the fake h a n d effect. T h a t is, the m a g n i t u d e of the CEs r e m a i n e d the same w h e t h e r or n o t observers felt soft plastic o n their h a n d s . This suggests that the fake h a n d effect is n o t d r i v e n b y the tactile experience of w e a r i n g gloves t h a t m a t c h the v i s u a l i n f o r m a t i o n about the gloves. N o r is i t necessary t o have a perfect m a t c h i n either the texture or v i s u a l m a t e r i a l of the fake a n d the real 46  hands. D i d wearing gloves have the effect of weakening the tactile signal overall? This possibility is eliminated by the observation that the speed and accuracy of detection of the target tactile vibrations were not affected by whether observers wore gloves or not, which suggests that the tactile signal was unchanged by the gloves . 6  If not driven by the 'feel' of a glove, what factors then underlie the tendency to mislocalize tactile targets to the fake hand? It is clear from the previous experiments that there is a visual component to the effect, but it remains uncertain as to what specific visual aspects matter for generating it. Pavani et al. (2000) suggest that the mislocalization effect "is specific to the case in which the rubber hand is aligned so as to look plausibly like the participant's o w n hand" (p. 356). Recall that their alignment manipulation involved either keeping the fake hand aligned with the observer's hand below, or turning the fake hands outward so that they were misaligned w i t h the observer's hands but still continued to 'hold' the distractor lights. Note that this manipulation also involved a change i n posture. That is, when the fake hands were aligned w i t h the observer's hands, the postures of the real and fake hands matched, but when they were misaligned, the postures mismatched. The question that remains from this is whether observers w i l l continue to mislocalize tactile targets to the fake hand if the alignment between the real and fake hands is maintained, but the postures mismatch (e.g., fake hands i n a prone posture, real hands i n a supine posture).  Analyses using correct RTs and Errors as dependent variables revealed that the Observer Gloves main effect was not significant i n either analysis, nor d i d this factor interact w i t h any of the other factors, p's > .05. 6  47  Experiment 4 Pavani et al. (2000) reported that the fake hand effect disappears when the fake hands are misaligned w i t h the observer's hands. The authors asserted that the alignment of the hands was necessary for the fake hand effect. There are, however, at least two other possible explanations for the pattern of results that they obtained. One, by misaligning the fake hands w i t h the observer's hands, the authors also had the effect of changing the posture of the former w i t h respect to the latter. It could be that i n doing so, the fake hand's posture was seen as implausible (i.e., w o u l d elicit a different set of proprioceptive signals than the observer's hand was suggesting), and as a result, the fake hand could no longer be misperceived as belonging to the observer. Two, the authors' repositioning of the fake hands when they were misaligned meant that there was no longer a direct visual path to the distractor lights. If the role of the fake hand is to direct the observer's attention to the distractor lights by creating a straight line of sight, this role w o u l d be compromised by the misalignment of the fake hands. In order to test the contributions of postural matches i n the fake and real hands to the fake hand effect, Experiment 4 was designed such that the alignment of the hands was held constant (the hands were always pointing i n the direction of the distractor lights) and only the postures were changed. In this design, the postures of the fake hands and real hands either matched (both prone or both supine), or mismatched (one prone the other supine). If a match i n posture is an important factor i n generating the larger CEs i n the Fake H a n d versus N o Fake H a n d condition then one w o u l d expect this pattern only when the postures of the real and fake hands match but not when they mismatch,  48  despite that the hands are aligned i n both cases. If posture turns out to be a relevant factor to the fake hand effect, this w o u l d support the idea that it is the personal limb identification that the observer has w i t h the fake hands that is relevant rather than the subsidiary effect of aligning the observer's gaze w i t h the distractor lights. Note that if the latter is the important factor, the fake hand effect should persist whether the postures match or mismatch since the fake hands are pointed i n the direction of the distractor lights i n both cases. Methods Participants: A l l 28 participants were volunteer Psychology undergraduate students from the University of British Columbia. They received partial course credit for their participation i n the 45-minute experimental session. Descriptive information can be found i n Table 1. Apparatus: Similar to Experiment 3. Stimuli: Same as Experiment 3 Details: Observers participated i n three practice blocks of 15 trials (as described earlier). There were six experimental blocks of 96 trials. Observers were randomly assigned to one of two Observer H a n d Orientations: prone or supine. In the prone position, observers rested their finger on the top tactile vibrator and thumb on the bottom tactile vibrator, as before. In the supine position, observers adopted the reverse position (i.e., rested their finger on the bottom tactile vibrator and thumb on the top tactile vibrator). A l l remaining factors were within-observer and included: Fake H a n d Condition (No Fake H a n d , Prone, Supine), Distractor Side (Same, Opposite). Note that for a prone fake hand, the finger of the fake hand rested next to the upper distractor light, and the thumb next to the lower distractor light, as before. For a supine fake 49  hand, the finger was next to the lower distractor light, and the thumb next to the upper distractor light. Every observer participated i n 576 trials (6 blocks * 96 trials per block). O n 192 of those trials the orientation of the fake hands was consistent w i t h that of the observer's hands (e.g., observer's hands prone, fake hands prone). O n another 192 trials, the orientation of the fake hands was inconsistent w i t h that of the observer's hands (e.g., observer's hands prone, fake hands supine). O n the remaining 192 trials, the fake hands were absent. A t the end of each experimental block, the experimenter had to alter the setup slightly (e.g., remove the fake hands, change the orientation of the hands). Procedure: Similar to Experiment 3 w i t h one exception. Observers i n the Prone condition were instructed to respond using a toe lift when they localized the tactile vibration to their finger (top digit), and to respond w i t h a heel lift when they localized the tactile vibration to their thumb (bottom digit). This was as before. In contrast, observers i n the Supine condition were given the reverse instructions; to respond w i t h a toe lift when the tactile vibration was localized to their thumb (top digit), and respond w i t h a heel lift when the tactile vibration was localized to their finger (bottom digit). By giving these instructions to observers i n the supine condition, the stimulus-response mapping was held constant across limb orientations such that a tactile vibration on the top digit was always paired w i t h a toe lift response, and a tactile vibration on the bottom digit was always paired w i t h a heel lift response. Results  The measures of interest were again the RT CEs and Error CEs. M i x e d design A N O V A s were computed using Observer H a n d Orientation (Prone, 50  Supine) as a between-observer factor and Fake H a n d Condition (No Fake H a n d , Prone, Supine), and Distractor Side (Same, Opposite) as within-observer factors. Significant interactions and main effects were followed up using simple effects testing, and least significant difference testing. The main finding of the present study was that tactile mislocalizations were present only when the orientation of the fake hands was consistent with that of the observer's hands, see Figure 8. W h e n the two were inconsistent, CEs resembled those obtained when there were no fake hands, suggesting that tactile mislocalizations are less common when the fake hand's orientation is implausible with respect to the observer's hand. Additionally, when the observer's hands are supine, the fake hand effect is weaker overall than when the observer's hands are prone (i.e., CEs i n the Consistent condition for supine observer hands are i n the direction of being larger than those i n the N o Fake H a n d condition, but the two are statistically similar). A Same  B Opposite Fake Hand Condition •  No Fake Hand  51 Inconsistent •  Consistent  200 150 $100  Prone  Supine  O b s e r v e r H a n d Orientation  Prone  Supine  O b s e r v e r H a n d Orientation  Figure 8. Mean CE (in ms) for Experiment 4 as a function of Fake Hand Condition (No Fake Hand, Inconsistent, Consistent) and Observer Hand Orientation (Prone, Supine) for (A) Same and (B) Opposite target-distractor presentations.  51  W h e n RT CEs w e r e the d e p e n d e n t variable, the m a i n effect of Distractor Side w a s significant, a n d i n d i c a t e d that CEs w e r e larger o v e r a l l f o r same side target-distractor presentations t h a n f o r opposite side presentations (97 m s vs. 53 m s ) , F ( 1 , 26) = 25.16, p < .01, M S E = 812. The Fake H a n d C o n d i t i o n x Observer H a n d O r i e n t a t i o n i n t e r a c t i o n w a s also significant, F (2, 52) = 6.83, p < .01, M S E = 2077. T o better i n t e r p r e t this i n t e r a c t i o n , the s i m p l e effect of Fake H a n d C o n d i t i o n w a s e x a m i n e d separately for Prone a n d Supine observer h a n d orientations. W h e n the observer's h a n d s w e r e i n a p r o n e o r i e n t a t i o n , the m a i n effect of Fake H a n d C o n d i t i o n w a s significant, F (2, 26) = 5.14, p < .05, M S E = 1280. Least significant difference testing revealed that CEs w e r e largest w h e n the fake h a n d s w e r e p r o n e (94 m s ) , rather t h a n s u p i n e (72 ms) or absent (64 m s ) , p's < .05. The CEs d i d n o t d i f f e r i n the latter t w o cases, p > .20. W h e n the observer's h a n d s w e r e i n a s u p i n e o r i e n t a t i o n , the m a i n effect of Fake H a n d C o n d i t i o n w a s again significant, F (2, 26) = 3.80, p < .05, M S E = 2874. CEs w e r e largest o v e r a l l w h e n the fake h a n d s w e r e supine (95 ms) c o m p a r e d to p r o n e (55 m s ) , p < .05. N e i t h e r p a i r e d c o m p a r i s o n i n v o l v i n g the N o Fake H a n d c o n d i t i o n (72 ms) reached significance, p's > .10. The same analysis as that above w a s c o m p u t e d u s i n g E r r o r CEs as the d e p e n d e n t variable. The m a i n effect of Distractor Side w a s significant, F ( 1 , 26) = 22.68, p < .01, M S E = 3. O v e r a l l , CEs w e r e larger f o r same side (6.8%) versus opposite side (3.0%) target distractor presentations. Since the error CEs w e r e either n o t significant, or their p a t t e r n is s i m i l a r to t h a t f o u n d f o r RT CEs, this eliminates concern for speed accuracy tradeoffs.  52  Discussion The m o s t i m p o r t a n t o b s e r v a t i o n f r o m the above a n a l y s i s w a s that the fake h a n d effect w a s m o s t c o m p e l l i n g w h e n the o r i e n t a t i o n of the fake h a n d s w a s consistent w i t h that of the observer's h a n d s . I n fact, w h e n the orientations w e r e inconsistent, the p a t t e r n of C E s r e s e m b l e d those f o u n d i n the N o F a k e H a n d s c o n d i t i o n . These results suggest that the fake h a n d effect occurs w h e n observers can i n c o r p o r a t e the fake h a n d s i n t o their b o d y s c h e m a (i.e., treat t h e m as their o w n ) . It does not appear to be the case that the role of the fake h a n d s is s i m p l y to direct the observer's attention to the distractor l i g h t s , w h i c h w o u l d h a p p e n w h e t h e r the o r i e n t a t i o n of the fake h a n d s w e r e consistent or inconsistent w i t h the o r i e n t a t i o n of the observer's h a n d s . T h e present results are consistent w i t h the results of a s t u d y r e p o r t e d b y M a r a v i t a , Spence, K e n n e t t a n d D r i v e r (2002) w h e r e i n observers i n c o r p o r a t e d a t o o l h e l d i n their h a n d s i n t o their b o d y schema. I n a setup s i m i l a r to the one u s e d presently, observers h e l d a p a i r of plastic golf clubs i n their h a n d s , a n d the p a i r of tactile v i b r a t o r s w a s p o s i t i o n e d o n the h a n d l e of the t o o l so that there w a s a tactile v i b r a t o r o n the observer's finger a n d t h u m b . O b s e r v e r s w e r e to r e p o r t the l o c a t i o n of the target tactile v i b r a t i o n . A p a i r of distractor lights p o s i t i o n e d 75 c m a w a y f r o m observer's h a n d s , w e r e ' c o n n e c t e d ' to the tactile v i b r a t i o n s b y the tools. The authors r e p o r t e d the t y p i c a l p a t t e r n of C E s s u c h that C E s w e r e larger for same side versus opposite side target-distractor p a i r s . A s d e m o n s t r a t e d i n the present E x p e r i m e n t 1, the C E is i n f l u e n c e d b y targetdistractor separation. G i v e n the distance b e t w e e n targets a n d distractors u s e d b y M a r a v i t a et a l , the tools m u s t have successfully b r i d g e d that distance, just as the  53  fake h a n d s i n the present s t u d y have. These f i n d i n g s suggest t h a t the tools w e r e i n c o r p o r a t e d i n t o the observer's b o d y schema. Response m a p p i n g s w e r e assigned i n E x p e r i m e n t 4 such t h a t a tactile v i b r a t i o n d e l i v e r e d to the t o p d i g i t w a s a l w a y s p a i r e d w i t h a toe l i f t response ( t o p - t o e ) , a n d a tactile v i b r a t i o n d e l i v e r e d to the b o t t o m d i g i t w a s p a i r e d w i t h a heel l i f t response ( b o t t o m - h e e l ) . A r e these response m a p p i n g s relevant to the fake h a n d effect? W i l l the fake h a n d effect persist i f these response m a p p i n g s are reversed such t h a t a tactile v i b r a t i o n d e l i v e r e d to the t o p d i g i t is p a i r e d w i t h a heel l i f t response ( t o p - h e e l ) , a n d a tactile v i b r a t i o n d e l i v e r e d t o the b o t t o m d i g i t is p a i r e d w i t h a toe l i f t response (bottom-toe)? E x p e r i m e n t 5 w a s designed to address this question.  Experiment 5 There is a large literature d a t i n g f r o m the 1950's t h a t deals w i t h the influence of stimulus-response compatibilities o n response times (e.g., Fitts & Seeger, 1953, see A l l u i s i & W a r m , 1990). T y p i c a l l y , i n these e x p e r i m e n t s , observers are asked to m a k e a forced choice k e y press, a m a n u a l p o i n t i n g response, a j o y s t i c k m o v e m e n t , or a v e r b a l response t o a v i s u a l s t i m u l u s . Consistently w i t h i n that literature, response times are fastest w h e n there is a direct spatial translation b e t w e e n the s t i m u l u s a n d response (e.g., responses to a left v i s u a l s t i m u l u s are faster w h e n observers are asked to m a k e a left versus r i g h t keypress). A d d i t i o n a l l y , observers are faster to r e s p o n d w h e n the response t y p e is consistent w i t h the s t i m u l u s t y p e (e.g., m a n u a l response f o r a spatial s t i m u l u s or a v o c a l response for a v e r b a l s t i m u l u s ; Proctor a n d W a n g , 1997). I n  54  contrast, RTs are slowed when the spatial translation between stimulus and response is indirect or when the stimulus and response types differ. Chua and Weeks (1997) proposed a way i n w h i c h to conceptualize the factors involved i n stimulus-response compatibility effects. M a p p i n g refers to the assignment of a specific response to a given stimulus (e.g., left keypress for left stimulus, right keypress for right stimulus). Configuration relation refers to the layout of the stimulus display relative to the response array (e.g., parallel versus orthogonal stimulus display and response array). Global relation refers to the relative spatial location of the stimulus display and response array (e.g., stimulus display at the midline, response array aligned w i t h the shoulder on the right). Included within this latter division is the orientation of the observer w i t h respect to either the stimulus display or response array (Chua et al., 2001). A l l three factors contribute to a different extent to the stimulus-response compatibility effects. When the configuration relation is orthogonal, such that the stimulus display is vertical and the response array is horizontal, preferences for particular mappings seem to arise, such that a top-stimulus paired w i t h a right-response and a bottom-stimulus paired w i t h a left-response is faster than the opposite pairings (e.g., Michaels, 1989). These preferences tend to depend on the global relations (i.e., reverses when the response is made i n the left hemispace; Weeks, Proctor & Beyak, 1995). When Experiment 4 is considered within the framework of C h u a and Weeks (1997), the manipulations of interest were mainly at the mapping level and to a minimal extent at the global relation level (orientation of observer's arm), whereas the configuration relation remained constant (vertical stimulus 55  d i s p l a y p a i r e d w i t h a front-back response array). I t is possible t h a t i n Experiments 1-4, the response m a p p i n g s assigned to observers t o o k advantage of the m o s t direct translation b e t w e e n the vertical s t i m u l u s a r r a y a n d the front-back response array. T h a t is, it m a y be that the t r a n s l a t i o n b e t w e e n the t o p d i g i t to the toe l i f t ( f r o n t pedal) a n d b e t w e e n the b o t t o m d i g i t to the heel l i f t (back pedal) w e r e the m o s t direct stimulus-response translations for those p a r t i c u l a r arrays. If so, this raises the p o s s i b i l i t y that the reverse response m a p p i n g assignments, w h e r e i n a tactile v i b r a t i o n d e l i v e r e d to the top d i g i t is m a p p e d to a heel l i f t response, a n d a tactile v i b r a t i o n d e l i v e r e d to the b o t t o m d i g i t is m a p p e d to a toe l i f t response, m i g h t d i s r u p t the fake h a n d effect. E x p e r i m e n t 5 w a s designed to examine the possible role of response m a p p i n g i n the fake h a n d effect. T w o c o n d i t i o n s w e r e i n c l u d e d w h e r e i n observers w e r e asked to r e s p o n d to a tactile v i b r a t i o n o n the t o p d i g i t u s i n g a heel l i f t (back p e d a l ) , a n d to a tactile v i b r a t i o n o n the b o t t o m d i g i t u s i n g a toe l i f t (front pedal). These response m a p p i n g s w e r e the reverse of those assigned i n E x p e r i m e n t 4. The data f r o m the t w o c o n d i t i o n s i n E x p e r i m e n t 5 w e r e c o m b i n e d w i t h those f r o m E x p e r i m e n t 4 so that it w a s possible to l o o k at a l l c o m b i n a t i o n s of D i g i t - R e s p o n s e M a p p i n g ( t o p - t o e & b o t t o m - h e e l versus t o p - h e e l & b o t t o m - t o e ) a n d the t w o levels of Observer H a n d O r i e n t a t i o n (Prone, Supine) to see w h e t h e r the CEs w e r e the same u n d e r a l l c o m b i n a t i o n s . The same three fake h a n d c o n d i t i o n s as those tested i n E x p e r i m e n t 4 w e r e again i n t r o d u c e d : N o Fake H a n d s ( 1 / 3 of trials), Consistent Fake H a n d s ( 1 / 3 of trials), a n d Inconsistent Fake H a n d s ( 1 / 3 of trials).  56  Methods Participants: Twenty-eight volunteers from the undergraduate Psychology program at the University of British Columbia participated i n the 45minute experimental session for partial course credit. Descriptive information can be found i n Table 1. The remainder of the method was identical to Experiment 4 w i t h the exception that the response mappings were reversed. This meant that a tactile vibration delivered to the top digit was always paired w i t h a heel lift response, and a tactile vibration delivered to the bottom digit was always paired w i t h a toe lift response. Thus, for observers i n the Prone condition, the finger (top digit) was mapped to the heel, and the thumb (bottom digit) was mapped to the toe, while for the observers i n the Supine condition the thumb (top digit) was mapped to the heel, and the finger (bottom digit) was mapped to the toe. Results Because the focus of the present study was on the interaction between response mapping and observer hand orientation, the present data were combined w i t h that of the previous experiment so that it was a complete 2 x 2 design with two levels of Digit-Response M a p p i n g (top-toe & bottom-heel versus top-heel & bottom-toe) combined w i t h two levels of Observer H a n d Orientation (Prone, Supine). CEs were calculated i n the same w a y as i n previous experiments where Correct RTs or Errors on Congruent trials were subtracted from Correct RTs or Errors on Incongruent trials. Note that congruent and incongruent still refer to the elevation of the distractor light w i t h respect to the target tactile vibration.  57  The d e p e n d e n t measures of interest w e r e the RT CEs a n d E r r o r CEs. M i x e d design A N O V A s w e r e c o m p u t e d u s i n g Fake H a n d C o n d i t i o n ( N o Fake H a n d , Consistent, Inconsistent), a n d Distractor Side (Same, O p p o s i t e ) as w i t h i n observer factors, a n d Digit-Response M a p p i n g ( t o p - t o e & b o t t o m - h e e l versus t o p - h e e l & b o t t o m - t o e ) a n d Observer H a n d O r i e n t a t i o n (Prone, Supine) as between-observer factors. Significant interactions a n d effects w e r e f o l l o w e d u p u s i n g s i m p l e effects testing, a n d least significant difference testing. There are three m a i n f i n d i n g s . First, o v e r a l l CEs w e r e larger w h e n the fake h a n d s w e r e present a n d i n a p o s t u r e consistent w i t h t h a t of the observer's hands. This is as expected based o n the results of the p r e v i o u s e x p e r i m e n t . Second, the effect is w e a k e r o v e r a l l w h e n observers a d o p t a top-heel & b o t t o m - t o e m a p p i n g (see Figure 9) versus a top-toe & b o t t o m - h e e l m a p p i n g (see Figure 8). T h a t is, f o r the f o r m e r m a p p i n g assignment, CEs i n the Consistent Fake H a n d c o n d i t i o n are n o longer s i g n i f i c a n t l y larger t h a n those i n the remaining t w o conditions. Three, the fake h a n d effect p a t t e r n w a s altered i n the case w h e r e the observers' h a n d s w e r e i n a supine p o s i t i o n , a n d a t o p - h e e l & b o t t o m - t o e m a p p i n g w a s assigned ( i n this case, a tactile v i b r a t i o n localized to the t h u m b r e q u i r e d a heel l i f t response a n d a tactile v i b r a t i o n localized t o the f i n g e r r e q u i r e d a toe l i f t response). U n d e r these c o n d i t i o n s , the CEs w e r e t y p i c a l l y negative, a n d their absolute values smaller t h a n the CEs f o r the other c o m b i n a t i o n s (see the r i g h t m o s t bars of Figure 9 A ) . N o t e t h a t a n e g a t i v e CE i n this case reflects the fact that observers are faster o n i n c o n g r u e n t versus c o n g r u e n t trials. I n other w o r d s , u n d e r these c o n d i t i o n s , observers are faster to r e s p o n d , for e x a m p l e , to a tactile v i b r a t i o n o n the t h u m b (top d i g i t ) w h e n i t is 58  paired w i t h a bottom distractor light (incongruent), and slower when paired with a top distractor light (congruent). Interestingly, there was a trend, though it was not significant, for CEs to increase (in the negative direction) when the fake hand was present and its postural rotation was inconsistent w i t h that of the observer's hand (i.e., the fake hand was prone, the observer's hand was supine). A 2 within (Fake H a n d Condition and Distractor Side) and 2 between (Digit-Response M a p p i n g and Observer H a n d Orientation) mixed design A N O V A was computed. A l l of the main effects were significant, p's < .05: DigitResponse M a p p i n g [F (1, 52) = 18.22, p < .01, M S E = 13347], Observer H a n d Orientation [F (1, 52) = 9.13, p < .01, M S E = 13347], Condition [F (2.104) = 4.52, p < .05, M S E = 3257], and Distractor Side [F (1,52) = 14.09, M S E = 13347], see Figures 8 and 9. The main effect of Fake H a n d Condition indicated that CEs were larger overall i n the Consistent condition (61 ms) than either the N o Fake H a n d (45 ms) or Inconsistent condition (39 ms), p's < .05. The comparison between the latter two conditions d i d not reach significance, p > .20. The other main effects were tempered by significant interactions of Response M a p p i n g x Observer H a n d Orientation [F (1,52) = 7.89, p < .01, M S E = 13347], Distractor Side x Response M a p p i n g [F (1,52) = 4.68, p < .05, M S E = 4607], Distractor Side x Observer H a n d Orientation [F (1,52) = 10.91, p < .01, M S E = 4607], and Distractor Side x Response M a p p i n g x Observer H a n d Orientation [F (1, 52) = 7.34, p < .01, M S E = 4607].  59  A Same Fake Hand Condition • No Fake Hand S Inconsistent • Consistent  B Opposite  «  Prone Supine Observer Hand Orientation  150  Prone  Supine  Observer Hand Orientation  Figure 9. Mean CE (in ms) for a top-heel & bottom-toe digit-response mapping as a function of Fake Hand Condition (No Fake Hand, Inconsistent, Consistent) and Observer Hand Orientation (Prone, Supine) for (A) Same and (B) Opposite targetdistractor presentations.  The 3-way interaction above was followed up by looking at the simple interaction of Digit-Response M a p p i n g x Observer H a n d Orientation for same and opposite side target-distractor presentations. When targets and distractors were presented to the same side of fixation, the Response M a p p i n g x Observer H a n d Orientation interaction was significant, F (1, 52) = 10.38, p < .01, M S E = 12450. This was further broken d o w n by looking at the simple effect of Response M a p p i n g for prone and supine hand positions. When the observer's hands were prone, the main effect of Response M a p p i n g was not significant, F < 1. When the observer's hands were supine, CEs were larger overall when a top-toe and bottom-heel mapping was assigned (94 ms) versus a top-heel and bottom-toe mapping (-32 ms), F (1, 26) = 45.85, p < .01, M S E = 7192. W h e n targets and distractors were presented to the opposite side of fixation, the simple interaction of Digit-Response M a p p i n g x Observer H a n d Orientation was not significant, F (1,52) = 1.80, p > .10, M S E = 5504. 60  A f t e r e x a m i n i n g the data i n Figures 8 a n d 9, i t seemed a p p r o p r i a t e to d o f o u r a d d i t i o n a l , b u t m o r e specific comparisons to test w h e t h e r the fake h a n d effect v a r i e d across the f o u r c o m b i n a t i o n s of Digit-Response M a p p i n g a n d Observer H a n d O r i e n t a t i o n .  7  W h e n the observer's h a n d w a s p r o n e a n d a t o p -  toe & b o t t o m - h e e l m a p p i n g was assigned (see Figure 8), the m a i n effect of C o n d i t i o n w a s significant, F (2, 26) = 5.14, p < .05, M S E = 1280. CEs w e r e larger for the Consistent Fake H a n d c o n d i t i o n (94 ms) t h a n either the N o Fake H a n d (64 ms) or Inconsistent c o n d i t i o n s (72 m s ) , p's < .05. W h e n the observer's h a n d w a s p r o n e b u t a top-heel & b o t t o m - t o e m a p p i n g w a s assigned (see F i g u r e 9), the m a i n effect of C o n d i t i o n w a s n o longer significant, F (2, 26) < 1, M S E = 5390. CEs w e r e the same m a g n i t u d e f o r the N o Fake H a n d (59 m s ) , Consistent Fake H a n d (53 m s ) , a n d Inconsistent Fake H a n d c o n d i t i o n s (62 m s ) . W h e n the observer's h a n d w a s supine a n d a top-toe & b o t t o m - h e e l m a p p i n g w a s assigned (see Figure 8), the m a i n effect of C o n d i t i o n w a s again significant, F (2, 26) = 3.80, p < .05, M S E = 2875. The CEs i n the Consistent c o n d i t i o n (95 ms) w e r e larger t h a n those i n the Inconsistent c o n d i t i o n (55 m s ) , p < .05. N o comparisons w i t h the N o Fake H a n d c o n d i t i o n (70 ms) reached significance, p's > .10. F i n a l l y , w h e n the observer's h a n d w a s s u p i n e a n d a top-heel & b o t t o m - t o e m a p p i n g w a s assigned (see Figure 9), the m a i n effect of C o n d i t i o n o n l y a p p r o a c h e d significance, F (2, 26) = 2.60, p < .10, M S E = 3484. CEs w e r e larger i n the Inconsistent (-33 ms) versus the Consistent (3 ms) c o n d i t i o n s , p < .05. N o c o m p a r i s o n s w i t h the N o Fake H a n d c o n d i t i o n reached significance, p's > .20.  These comparisons w e r e c o m p u t e d even t h o u g h the C o n d i t i o n x D i g i t Response M a p p i n g x Observer H a n d O r i e n t a t i o n i n t e r a c t i o n w a s n o t significant. 7  61  The o v e r a l l analysis above w a s c o m p u t e d u s i n g E r r o r CEs as the d e p e n d e n t variable. O n l y the m a i n effects of Response M a p p i n g [F ( 1 , 52) = 15.89, p < .01, M S E = 62] a n d Distractor Side [F (1,52) = 19.30, p < .01, M S E = 31] w e r e significant. These m a i n effects w e r e t e m p e r e d b y significant interactions of Digit-Response M a p p i n g x Observer H a n d O r i e n t a t i o n [F ( 1 , 52) = 4.50, p < .05, M S E = 62], a n d Distractor Side x Digit-Response M a p p i n g [F ( 1 , 52) = 5.23, p < .05, M S E = 31]. These interactions w e r e f o l l o w e d u p b y first l o o k i n g at the s i m p l e m a i n effect of Digit-Response M a p p i n g f o r p r o n e a n d s u p i n e observer hands. W h e n the observer's h a n d s w e r e p r o n e , the s i m p l e m a i n effect w a s n o t significant, F ( 1 , 26) = 1.75, p > .20, M S E = 6 1 . W h e n the observer's h a n d s w e r e supine, CEs w e r e larger o v e r a l l f o r the t o p - t o e a n d b o t t o m - h e e l m a p p i n g (5%) versus the t o p - h e e l a n d b o t t o m - t o e m a p p i n g (0%), F ( 1 , 26) = 18.55, p < .01, M S E = 62. The Distractor Side x Digit-Response M a p p i n g i n t e r a c t i o n w a s f o l l o w e d u p b y l o o k i n g at the s i m p l e m a i n effect of Digit-Response M a p p i n g f o r same a n d opposite side target-distractor presentations. W h e n targets a n d distractors w e r e presented to the same side of f i x a t i o n , CEs w e r e larger w h e n the m a p p i n g assignment w a s t o p - t o e a n d b o t t o m - h e e l (7%) versus t o p - h e e l a n d b o t t o m - t o e (2%), F (1,52) = 16.27, p < .01, M S E = 65. W h e n targets a n d distractors w e r e presented to the opposite side of f i x a t i o n , CEs w e r e larger f o r a t o p - t o e a n d b o t t o m - h e e l m a p p i n g assignment (3%) versus t o p - h e e l a n d b o t t o m - t o e m a p p i n g assignment (1%), F (1,52) = 8.68, p < .01, M S E = 27.  Discussion There w e r e t w o m a i n f i n d i n g s i n this E x p e r i m e n t .  62  Digit-Response M a p p i n g and Observer H a n d Orientation influence the effect: The definitive pattern for the fake hand effect of larger CEs i n the Consistent Fake H a n d condition than the remaining two conditions is less pronounced when the observer adopts a top-heel & bottom-toe mapping. This is especially true i n the case where the observer's hand is supine. This suggests that the fake hand effect is sensitive to response mapping, and to posture when response mapping is manipulated. Note that when a top-toe & bottom-heel mapping was adopted, the effect persisted for either hand orientation. This suggests that the observer's hand orientation by itself does not influence the effect, but rather what matters are the digit-response mappings and the visible fake hand orientations that it is paired with. Negative CEs for Supine Hands and T o p - H e e l & Bottom-Toe Mapping: The negative CEs indicate that observers are faster to localize the tactile target when the elevation of the distractor light is incongruent rather than congruent w i t h the elevation of the tactile target. This reverse C E pattern is only observed for this particular combination of observer hand orientation and digitresponse mapping. Additionally, when the orientation of the fake hand is prone, and thus the visual information is inconsistent w i t h the supine orientation of the observer's hand, the reversed C E pattern tends to be more pronounced, although not significantly so. This latter result suggests that i n this particular condition observers may have mentally rotated their hand to the prone orientation, as consistent w i t h the available visual information.  63  General D i s c u s s i o n Touch is a proximal sense in that tactile sensations typically arise through direct skin contact (Cholewiak & Collins, 1991). The tactile receptors under the skin are plentiful enough that relatively accurate body-relevant localizations are possible. This makes it fairly easy to tell that something is touching your finger instead of your thumb. But, it is a rather different matter to be able to judge the precise location of this touch i n the three dimensional space that surrounds you. The touch to your finger w i l l feel the same, for example, whether your finger is in front of your body, or behind your back. To know where the touch occurred in the surrounding space, the body-relevant tactile information must be combined w i t h other sensory information such as is given through proprioception and vision (e.g., Botvinick & Cohen, 1998; Pavani et al., 2000). The first question of the present study was " C a n tactile targets be mislocalized to a new spatial location that is initially specified solely by vision?" The subjective impressions collected from observers i n the Pavani et al. (2000) study suggested that the tactile vibrations were mislocalized to the digits of a fake hand. The authors, however, d i d not include an appropriate baseline measure w i t h which to objectively confirm the perceived locations of the tactile vibrations. D i d observers behave as though the tactile vibrations were presented at the location specified by vision of the fake hands? The initial step i n addressing this question was to introduce a new baseline condition wherein the observer's hands were positioned i n the location normally occupied by the fake hands (Experiment 1). By doing so, it was possible to measure tactile localization responses when the true location of the  64  tactile v i b r a t i o n s w a s the same as the subjective location of the tactile v i b r a t i o n s n o r m a l l y i n d u c e d b y the presence of the fake h a n d s . This k e y baseline c o n d i t i o n revealed that, i n contrast to the q u a l i t a t i v e a n d subjective reports of observers, the q u a n t i t a t i v e m e a s u r e m e n t of the fake h a n d effect i n d i c a t e d t h a t the experience w a s n o t i d e n t i c a l to that of h a v i n g one's o w n h a n d i n the same location as the fake h a n d . Rather, the fake h a n d effect r e s u l t e d i n a w e a k e r measure of p e r c e p t u a l c o n g r u e n c y t h a n w h e n the observer's h a n d s o c c u p i e d that same location. Despite these q u a n t i t a t i v e differences, h o w e v e r , there w e r e q u a l i t a t i v e similarities i n response patterns across these t w o c o n d i t i o n s i n the w a y that observers r e s p o n d e d to m a n i p u l a t i o n s of target-distractor spatial separation. This latter result is consistent w i t h observer's subjective impressions i n d i c a t i n g that they i d e n t i f i e d w i t h the l i m b to some extent. I n o r d e r to d e t e r m i n e w h e t h e r the fake h a n d effect d e p e n d e d o n observers seeing the fake h a n d , or w h e t h e r it was sufficient to have p a r t i a l v i s u a l i n f o r m a t i o n consistent w i t h its presence, the fake h a n d w a s h i d d e n u n d e r a c l o t h cover ( E x p e r i m e n t 1) or u n d e r a b o x cover that e l i m i n a t e d v i s u a l i n f o r m a t i o n a b o u t its shape ( E x p e r i m e n t 2). The fake h a n d effect persisted i n b o t h cases. This is the first piece of evidence t h a t the effect is n o t d r i v e n p u r e l y b y direct v i s i o n , b u t rather, m a y arise f r o m existing k n o w l e d g e t h a t is r e i n f o r c e d t h r o u g h p a r t i a l v i s i o n . The results of E x p e r i m e n t 2, h o w e v e r , also i n d i c a t e d t h a t w h e n there w a s b o t h a h i d d e n fake h a n d a n d a visible fake h a n d , the effect w a s smaller o n the side of the h i d d e n h a n d . This suggests that observers can reassign w e i g h t s to d i f f e r e n t sources of i n f o r m a t i o n d e p e n d i n g o n w h a t is available so t h a t they can m a k e the best estimate about the l o c a t i o n of a tactile s t i m u l u s . That 65  is, in the case where there is direct visual information i n one location, and indirect visual information i n another, the former is weighted more heavily than the latter. Experiment 3 was designed to address the question of whether the fake hand effect could be enhanced by the availability of additional tactile information that was congruent w i t h the fake hands - the feel of soft plastic on one's hands. The results revealed that the magnitude of the effect was the same whether observers wore gloves or not. This suggests both that the fake hand effect does not depend on a perfect visual or tactual match between the fake hand and the observer's hand, and that continuous tactile reinforcement, as that gained from wearing the soft plastic gloves, does not contribute to the effect. In Experiment 4, the rotational posture of the fake hands was manipulated such that their posture was either consistent or inconsistent w i t h that of the observer's hands. The fake hand effect was present only i n the former case. That is, the effect disappeared when the rotational posture of the fake hands was inconsistent w i t h that of the observer's hands. This was true despite the fact that the fake hands were always aligned w i t h the observer's hands below and were positioned to 'hold' the distractor lights. These results suggest both that the fake hand must be a plausible extension of the observer's body, and that the effect is not simply a product of the fake hands creating a direct line of sight to the distractor lights, because they do so for either rotational posture. The digit-response mapping assigned throughout Experiments 1-4 was the same - respond to a tactile vibration on the top digit using the toe lift response and a tactile vibration on the bottom digit using the heel lift response (top-toe & bottom-heel). Using the basic experimental design of Experiment 4, 66  the digit-response mapping assignment was reversed i n Experiment 5 - respond to a tactile vibration on the top digit using the heel lift and a tactile vibration on the bottom digit using the toe lift (top-heel & bottom-toe). By combining the data from Experiments 4 and 5, it was possible to look at the magnitude of the fake hand effect for different combinations of observer hand orientation (prone, supine) and digit-response mapping (top-toe & bottom-heel versus top-heel & bottom-toe). Interestingly, the fake hand effect was weaker for the top-heel & bottomtoe mapping than for the reverse mapping. Furthermore, when the observer's hands were i n a supine orientation and a top-heel & bottom-toe mapping was assigned, the effect was not only weaker overall, but the C E s were i n the negative direction. This result suggests that the fake hand effect is not only sensitive to the rotational posture of the fake hand (Experiment 4), but it is also sensitive to posture when particular response mappings are adopted such that when the hand is supine, the finger or bottom digit does not map w e l l onto the toe and the thumb or top digit does not map well onto the heel. It is worthwhile to highlight that a negative C E indicates that observers were faster to respond to the location of the tactile vibration when the location of the distractor light was incongruent rather than congruent. These negative C E are unique to this particular combination, and tend to increase when the fake hands are i n a prone orientation (i.e., inconsistent). One way to reconcile this pattern of results w i t h the others is by imagining that observers responded by first mentally rotating their hand into a prone or default position before responding. This w o u l d have the effect of making the negative CEs positive, and a prone fake hand consistent w i t h the  67  imagined posture of the observer's hand. More research is needed to explore this possibility.  • Implications of the Present Results  The present results have a number of implications for understanding how body schemas are formed. In turn, these principles can be used to improve such things as the construction and fitting of limb prostheses, and remote surgery. Formation of Body Schemas: A s noted throughout the present study, tactile localization i n threedimensional space is influenced by vision, even when the available visual and proprioceptive information conflict. K n o w i n g that the tactile stimuli are delivered to the digits of their o w n hand, but mislocalizing them towards the fake hand implies that perceived limb position is also vulnerable to vision, and that observers must somehow incorporate the fake hand within their body schema (e.g., adopt the fake hand as an extension of the body). This is consistent w i t h the subjective reports collected from previous studies (e.g., Botvinick & Cohen, 1998; Pavani et a l , 2000). Since the fake hand effect was sensitive to a variety of manipulations, this suggests that the human body schema is flexible under some conditions but is inflexible under others. The finding of a fake hand effect under both full and partial vision conditions, for instance, indicates that body schemas may be modified by indirect visual cues. In contrast, the absence of the fake hand effect when the rotational posture of the fake hand was inconsistent w i t h that of the observer's hand suggests that changes to the observer's body schema likely require that the fake hand be seen as a plausible extension of the observer's body.  68  This latter notion is consistent w i t h the earlier findings of Pavani et al. (2000) that to obtain the fake hand effect the fake hands had to be aligned w i t h the observer's body i n a plausible way. More research is needed to determine what is flexible about body schema and what is not. Acceptance of Prosthetic Limbs: Further research on what factors make the fake hands plausible extensions of an observer's body can be applied to the use of prosthetic limbs such that they are constructed so as to be a more accepted and natural part of the user's body. The present results suggest, for example, that the texture or feel of a prosthetic limb may not be as important a factor as the consistency between the seen posture of the limb and the expected posture. Ramachandran and Hirstein (1998) tested patients w h o were missing a limb and w h o reported experiencing a 'phantom' i n its place (i.e., a nonvisible limb). The authors used a mirror to give the phantom limb patients the impression that they could see their phantom (actually just a reflection of the normal hand) i n which case, patients reported experiencing a touch on the phantom limb when tactile stimuli were delivered to the normal hand. One way to think about these results is that the authors provided phantom limb patients with visual feedback of the limb that was sufficient for patients to accept that the limb was present. This mirror technique w o u l d not likely have been effective if the patients believed that the posture of the phantom was different from the seen posture. The results of the present study suggest that another w a y to provide feedback about the limb is to alter one's metacognitions about where their limb is in space, what posture it is in, and that it is functioning. This could perhaps be done by first having the patients participate i n an imagery session, where they 69  try to picture their limb i n an assigned orientation. Once patients report having done this successfully, the imagery tasks could then be expanded to include imagining different movements like the opening and closing of the p a l m prior to employing the mirror technique. Human-Machine Interactions: Another possible application of the present results to the real w o r l d is in the domain of design issues w i t h respect to remote surgery. Remote surgery is the process by which a surgeon, for example, manipulates tools held by a robotic arm through joystick manipulation from one location, while the patient being operated on and the robotic arm manipulated are in a separate location. To do this requires that that the surgeon receives visual feedback of the remote site i n a location that is removed from the actual site (e.g., doctor i n Vancouver performing an operation on a patient i n Burnaby, w i t h surgical feedback provided on a video monitor). Visual feedback i n this case is thus separated from the proprioceptive feedback, i n much the same way that the visible fake hands i n the present study were spatially separated from the location of the observer's hands. Research indicates that the location of this visual feedback is likely an important factor i n performance (Hanna, Shimi, & Cuschieri, 1998; MacKenzie, Graham, Cao, & Lomax, 1999; M a n d r y k & MacKenzie, 2000). Results reported by M a n d r y k and MacKenzie (2000), for instance, suggest that there may be an advantage to superimposing the image from the video monitor above where the controls are manipulated rather than at a 90° angle, as is typical when looking up at a monitor. The present results suggest that the tactual congruence between the robotic arms and the surgeon's arms may not be an issue, but that matching the  posture o f the robotic arms w i t h respect t o the surgeon's a r m s m a y be a critical factor f o r successful use o f the tools. I t m a y be p r o b l e m a t i c , f o r e x a m p l e , t o have the robotic a r m s o p e r a t i n g i n a supine p o s t u r e w h i l e the surgeon's arms are p r o n e . A d d i t i o n a l l y , the present results suggest that the c o m b i n a t i o n o f the p o s t u r e o f the arms a n d the m a p p i n g b e t w e e n the available v i s u a l i n f o r m a t i o n (stimulus) a n d the r e q u i r e d action (response) m a y also i n f l u e n c e the efficiency a n d accuracy o f the surgeon's responses. F u r t h e r research is n e e d e d , h o w e v e r , t o d e t e r m i n e w h e t h e r r o t a t i o n a l inconsistencies b e t w e e n the fake h a n d s a n d the observer's h a n d s are as i m p o r t a n t w h e n they are explicable b y s o m e t h i n g such as camera angle. I f there is a reasonable e x p l a n a t i o n f o r these differences (e.g., m i r r o r reversal, o r projected o n a m o n i t o r so d e p e n d e n t o n camera angle), t h e n i t m a y be the case that the fake h a n d effect w i l l persist even w h e n the r o t a t i o n a l postures are inconsistent.  Outstanding Questions and Future Directions Is the Fake H a n d Effect Generalizable t o N o n - H a n d Objects? Measures w e r e taken i n the present s t u d y t o construct t h e fake h a n d so that i t w a s u n d e n i a b l y h a n d - l i k e . The soft plastic glove, f o r instance, w a s stuffed w i t h c o t t o n b a t t i n g t o give i t a f u l l appearance, i n c l u d i n g the d i g i t s o f the fake h a n d . B u t , is i t critical that the v i s u a l object next to the distractor l i g h t s w a s a hand? Perhaps there are critical properties o f the fake h a n d t h a t l e d t o the tactile mislocalizations. I f so, there is a chance that these p r o p e r t i e s can also be f o u n d i n n o n - h a n d objects. T h e question t h u s remains as t o w h a t i t is a b o u t the presence of the fake h a n d t h a t leads t o tactile mislocalizations.  71  If it is the case that the fake hand effect depends on the realistic construction of the hand, then one might expect that the effect w o u l d be enhanced if a more realistic hand were used (e.g., mannequin hand). This possibility, however, is minimized by the present finding that the fake hand effect persisted even when the fake hands were no longer directly visible (Experiments 1-2), although it could be the case that the visual cues available were sufficient to allow observers to maintain a mental image of the hand. Another possibility to consider is that the positioning of the finger and thumb of the fake hand helped to highlight the two distinct elevations of the distractor lights, and this, i n turn, influenced tactile mislocalizations. If so, it is likely that any object that highlighted the elevations w o u l d produce the same results. Imagine, for instance, that a pair of dentures were positioned to 'bite' the distractor lights, such that the upper set of teeth were positioned next to the top light, and the lower set of teeth positioned next to the bottom light. Or, alternatively, imagine that w o r d labels such as 'top light' and 'bottom light' were assigned to the distractor light elevations. W o u l d these manipulations lead to tactile mislocalizations? Again, although both a possibility, the present findings of a fake hand effect when the digits of the fake hand were no longer visible suggest that drawing attention to the elevations may not be a critical factor . 8  W o u l d an imagined hand be sufficient for the fake hand effect? In the Fake H a n d conditions of the present study, observers always saw the fake hand as it was put i n position, and thus always saw it before it was  Consistent w i t h the elevation proposal, one could argue that the box-cover led to the fake hand effect because it too has a defined top and bottom. But, since the same results were found using the cloth cover, which has a less defined shape, this possibility seems unlikely.  8  72  h i d d e n f r o m v i e w . The c u r r e n t t h i n k i n g o n the fake h a n d effect is t h a t the available v i s u a l i n f o r m a t i o n is critical f o r p r o d u c i n g the effect. C o u l d it be the case, h o w e v e r , that s i m p l y i m a g i n i n g the presence of a fake h a n d n e x t to the distractor l i g h t s w o u l d be sufficient f o r the effect i n the absence of a n y v i s u a l cues to its presence? To test this, one c o u l d c o m p a r e the effect across t w o g r o u p s , w h e r e one o n l y i m a g i n e d the fake h a n d w a s present, a n d the other actually saw the fake h a n d . N o t e that the results of the v i s i b i l i t y m a n i p u l a t i o n 9  i n the present s t u d y cannot d i s t i n g u i s h b e t w e e n w h e t h e r the p a r t i a l v i s u a l cues to the fake h a n d ' s presence w e r e critical, or w h e t h e r w h e n the fake h a n d w a s h i d d e n , observers w e r e r e l y i n g o n a m e n t a l image of the h a n d . Is the fake h a n d effect m u l t i s e n s o r y or m u l t i i n f o r m a t i o n a l ? Consistent w i t h the q u e s t i o n above is the idea that the fake h a n d effect is the result of i n t e g r a t i n g the available v i s u a l a n d tactile i n f o r m a t i o n . A c c o r d i n g to Pavani et al. (2000), f o r example, the distractor lights c a p t u r e d the l o c a t i o n of the tactile v i b r a t i o n s a n d this was enhanced b y the presence of the fake h a n d . O n e m i g h t ask, h o w e v e r , w h e t h e r it w a s necessary that the distractors w e r e v i s u a l (i.e., d i f f e r e n t m o d a l i t y t h a n the target m o d a l i t y ) . A n o t h e r w a y to t h i n k a b o u t this is to ask w h e t h e r the fake h a n d effect is a p r o d u c t of p r e s e n t i n g i n f o r m a t i o n i n m o r e t h a n one m o d a l i t y (e.g., v i s u a l a n d tactile), or s i m p l y the p r o d u c t of p r e s e n t i n g m u l t i p l e sources of i n f o r m a t i o n . T o test this, one c o u l d test w h e t h e r an a d d i t i o n a l source of tactile i n f o r m a t i o n leads to the same result.  I n such a n e x p e r i m e n t , the g r o u p p a r t i c i p a t i n g i n the I m a g i n e d Fake H a n d c o n d i t i o n w o u l d a l w a y s have to participate i n the N o Fake H a n d c o n d i t i o n first, as the instructions to i m a g i n e a fake h a n d m i g h t interfere w i t h the results of the N o Fake H a n d c o n d i t i o n w e r e it to come first.  9  73  W h a t is the Role of A t t e n t i o n i n the Fake H a n d Effect? E v e n t h o u g h observers are t o l d that the elevation of the distractor lights is irrelevant to their task of l o c a l i z i n g the target tactile v i b r a t i o n s , the distractor lights nevertheless influence responses. This suggests t h a t the onset of the lights m i g h t capture a t t e n t i o n , a n d d o so even w h e n they are separated i n space f r o m the tactile v i b r a t i o n s , as evidenced b y the measurable CEs at the large v i b r a t i o n l i g h t separation i n the present E x p e r i m e n t 1. This raises the q u e s t i o n of w h e t h e r the fake h a n d effect w o u l d be r e d u c e d i f a t t e n t i o n w e r e w i t h d r a w n f r o m the lights (e.g., b y h a v i n g observers p e r f o r m a secondary task such as c o u n t i n g b a c k w a r d s i n threes f r o m a g i v e n n u m b e r ) . A n o t h e r w a y of t h i n k i n g a b o u t this is to ask w h a t w o u l d h a p p e n to the fake h a n d effect if observers w e r e i n s t r u c t e d to attend to the distractor lights rather t h a n i g n o r e t h e m . A l t h o u g h such m a n i p u l a t i o n s w o u l d l i k e l y change the o v e r a l l m a g n i t u d e of the CEs, the relative difference b e t w e e n CEs i n the N o Fake H a n d versus Fake H a n d c o n d i t i o n s w o u l d l i k e l y persist since the fake h a n d effect seems to be s o m e t h i n g over a n d above the a t t e n t i o n c a p t u r i n g effects of the distractor l i g h t s (i.e., CEs increase s i m p l y b y h a v i n g the fake h a n d ' h o l d ' the distractor l i g h t s ) . I n the present E x p e r i m e n t 3, the m a g n i t u d e of the fake h a n d effect d i d n o t change w h e n observers w o r e a p a i r of soft plastic gloves o n t h e i r h a n d s . I t w a s c o n c l u d e d t h a t p r o v i d i n g observers w i t h tactile i n f o r m a t i o n t h a t w a s c o n g r u e n t w i t h the fake h a n d d i d n o t alter the effect. A n o t h e r i n t e r p r e t a t i o n of these results, h o w e v e r , is that w e a r i n g the gloves h a d the a d d e d effect of d r a w i n g a t t e n t i o n to the observer's h a n d s , thereby canceling o u t a n y benefits of p r o v i d i n g observers w i t h c o n g r u e n t tactile i n f o r m a t i o n . There are at least a c o u p l e of w a y s to d e t e r m i n e w h e t h e r this m i g h t be the case. O n e w a y is t o a l l o w observers to  74  handle samples of soft plastic or to wear the plastic gloves for a short period of time prior to, but not during, the task. Another is to increase attention to the observer's hand by, for example, blowing hot air over it during the task. Either manipulation w o u l d allow one to separate the effects of having access to congruent tactile information from those of drawing attention to the observer's hand. W h y is Space an Important Factor i n the Fake H a n d Effect? In the present Experiment 1, the fake hand effect was minimized when the distractor lights were separated in space from the tactile vibrations. This finding is consistent w i t h the notion that there are bimodal neurons for the hand that have both a visual and tactile receptive field that are spatially linked such that when the hand moves, the visual receptive field moves w i t h the tactile receptive field (see evidence for bimodal neurons i n macaque monkeys i n Graziano & Gross, 1993; 1995; Iriki, Tanaka & Iwamura, 1996). In the case of the present study, when there is no horizontal separation between the visual and tactile stimuli, the receptive fields of the bimodal neurons for the hand are likely aligned, resulting in an overadditive neuronal response relative to the case where the visual and tactile information sources are separated i n space, and the visual information no longer falls within the visual receptive field of the bimodal neuron. A r e there individual differences i n the fake hand effect? Although the majority of observers i n the present study showed the fake hand effect, a small number d i d not. Additionally, for some observers the effect was larger than for others. What factors are responsible for these individual differences? One possibility is that the observers who showed a smaller fake 75  hand effect are less susceptible to suggestion than others. To test this, suggestibility tests could be administered at some point during the experiment. If suggestibility turns out to be a contributing factor to the fake hand effect, it w o u l d be a natural next step to test the magnitude of the effect i n children who are likely more susceptible to suggestion than adults. Time-line of the fake hand effect? Since Condition (No Fake H a n d , Fake Hand) was a within-observer manipulation i n Experiment 3, and it was a blocked factor, it was possible to ask whether the fake hand effect was present within the first block of trials, or whether it took some experience with and without the fake hand for the effect to emerge. It was found that the fake hand effect was present within the first two blocks of trials whether the Fake H a n d condition came first or followed the N o Fake H a n d condition . This is consistent w i t h the findings of Botvinick and 10  Cohen (1998) w h o reported a fake hand effect within the first three minutes of their manipulation. It is likely that observers need only enough time to recognize the consistent temporal congruency between the visual and tactile information sources before the fake hand effect emerges. It w o u l d be interesting to see whether the fake hand effect could be disrupted simply by including trials of both temporally matched and mismatched stimuli together.  A mixed design A N O V A was computed on the RT CEs i n Experiment 3 using Condition (No Fake H a n d , Fake Hand) as the within-observer factor and Order (Fake H a n d first, Fake H a n d second) as the between-observer factor. O n l y the main effect of Condition reached significance, F (1,12) = 9.0, p < .05, M S E = 252. Remaining p's > .15. 10  76  References A l l u i s i , E.A., & W a r m , J.S. (1990). T h i n g s that go together: A r e v i e w of s t i m u l u s response c o m p a t i b i l i t y a n d related effects. I n R.W. Proctor & T.G. Reeve (Eds.). Stimulus-response c o m p a t i b i l i t y : A n i n t e g r a t e d perspective. N o r t h H o l l a n d : Elsevier Science Publishers B.V. Bertelson, P., & Aschersleben, G. (1998). 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(1995). Stimulus-response compatibility for vertically oriented stimuli and horizontally oriented responses: Evidence for spatial coding. The Quarterly Journal of Experimental Psychology, 48A, 367-383. Welch, R. B., and D . H . Warren. (1986). "Intersensory interactions." In K . R. Boff, L. Kaufman, & J. P. Thomas (Eds.), Handbook of perception and human performance (c. 25). N e w York: Wiley Welch, R.B., Widawski, M . H . , Harrington, J., & Warren, D . H . (1979). A n examination of the relationship between visual capture and prism adaptation. Perception and Psychophysics, 25,126-132.  83  Table 1 D e s c r i p t i v e i n f o r m a t i o n f o r each E x p e r i m e n t . No Experiment  N  Males  Right  Left  Handed  Handed  Females  Corrective  Contacts  Glasses  Eyewear El  42  11  31  40  2  14  14  14  E2  13  7  6  12  1  5  6  2  E3  14  2  12  14  0  7  4  3  E4  28  11  17  28  0  8  13  7  E5  28  9  19  28  0  11  10  7  E4+E5  56  20  36  56  0  19  23  14  84  Table 2 Breakdown of trial types i n Experiment 1-5.  Experiment/  |  Between Observer  |  Manipulations  |  Exp 1  |  N = 42  Within-Observer Manipulations  Condition  |  Visibility  Separation  Congruency  N o Fake H a n d  |  Visible  None  Congruent  Fake H a n d  |  Hidden  Small  Incongruent  Real H a n d Baseline |  Large 6 blocks (1 block = 96 trials each) = 576 trials 2 * 3 * 2 = 12 trial types -> 48 trials/type  Exp 2  N = 13 Condition  T-D Side  Congruency  N o Fake H a n d  Same  Congruent  Both Visible  Opposite  Incongruent  Both H i d d e n One H i d d e n H i d d e n Side One H i d d e n Visible Side 5 blocks (1 block = 96 trials each) = 480 trials 5 * 2 * 2 = 20 trial types -> 24 trials/type 85  Exp 3  N = 14  |  |  Exp 4  Condition  Gloves  T-D Side  |  N o Fake H a n d  None  Same  |  Fake H a n d  Plastic  Opposite  Congruency Congruent Incongruent  |  8 blocks (1 b l o c k = 96 trials each) = 768 trials  |  2 * 2 * 2 * 2 = 16 t r i a l types -> 48 t r i a l s / t y p e  N = 28  Observer Hand Orientation  Condition  T-D Side  Congruency  Prone  |  N o Fake H a n d  Same  Congruent  Supine  |  Consistent  Opposite  Incongruent  Inconsistent 6 blocks (1 b l o c k = 96 trials each) = 576 trials 3* 2 * 2 = 12 t r i a l types - > 48 t r i a l s / t y p e * Response M a p p i n g assignment is T o p Tactile V i b r a t i o n t o Toe Pedal Response a n d B o t t o m Tactile V i b r a t i o n t o H e e l Pedal Response ( T o p - T o e a n d Bottom-Heel).  Exp 5  N = 28  Same as E x p e r i m e n t 4, except Response M a p p i n g assignment is T o p Tactile V i b r a t i o n t o H e e l Pedal Response a n d B o t t o m Tactile V i b r a t i o n t o Toe Pedal Response ( T o p - H e e l a n d B o t t o m - T o e ) 86  Appendix Experiment 1: In order to ensure that using difference scores referred to as C E s d i d not significantly alter the interpretation of the results, additional analyses were computed using the correct RTs as the dependent variable and Congruency (Congruent, Incongruent) as a factor. Interestingly, most of the effects were observed for the Incongruent trials, but not for Congruent trials. The pattern of results within the Incongruent trials was similar to that observed within the RT CEs, w i t h one exception. W h e n the data was restricted to visible trials where there was no target-distractor separation, the differences i n correct RTs for the Real H a n d Baseline and Fake H a n d condition d i d not reach significance, whereas CEs were significantly larger in the former condition than in the latter. A mixed design A N O V A was computed using correct RTs as the dependent variable, Separation (None, Small, Large), Visibility (Visible, Hidden) and Congruency (Congruent, Incongruent) as within-observer factors, and Condition (No Fake H a n d , Fake H a n d , Real H a n d Baseline) as a betweenobserver factor. Significant interactions were followed up using Simple Interactions or Simple Effects testing, while significant main effects were followed up using Least Significant Difference testing (LSD). The main effects of Separation [F (2, 78) = 12.79, p < .001] and Congruency [F (1, 39) = 239, p < .001] were significant. These main effects were tempered by significant interactions of Separation x Condition [F (4, 78) = 2.90, p < .05], Congruency x Condition, [F (2,39) = 4.30, p < .05], and Separation x Congruency [F (2, 78) = 19.13, p < .001]. The three-way interaction of Separation x 87  Congruency x Condition was also significant [F (4, 78) = 5.95, p < .01], as was the four-way interaction, F (4, 78) = 2.59, p < .05. First, the four-way interaction was first broken d o w n into its component Condition x Separation x Congruency interaction at each level of Visibility. The three-way interaction was significant only when the limbs were visible, F (4, 78) = 7.99, p < .001. This interaction was broken d o w n into its component Condition x Separation interaction for Congruent and Incongruent trials. The Condition x Separation interaction was significant only on Incongruent trials, F (4, 78) = 6.26, p < .001. W h e n the simple effect of Condition was tested at each level of Separation, it was significant only when there was no horizontal targetdistractor separation, F (2, 39) = 6.50, p < .01. Least significant difference testing revealed that RTs were significantly faster i n the N o Fake H a n d condition (577 ms) than i n either the Fake H a n d (717 ms) or Real H a n d Baseline (766 ms) conditions. There was no difference in RTs between the latter two conditions." The simple effect of Separation was also tested at each level of Condition, and was significant only for the Fake H a n d [F (2, 26) = 6.66, p < .01] and Real H a n d Baseline conditions [F (2, 26) = 12.93, p < .001]. Least significant difference testing revealed that for the Fake H a n d condition, RTs were significantly slower when there was no target-distractor separation (717 ms) than when the separation was large (628 ms), p < .05. N o comparisons w i t h the small separation (671 ms) reached significance, p's > .05. For the Real H a n d Baseline condition, RTs were significantly slower when there was no target-distractor  This finding deviates slightly from the C E data where CEs were significantly larger overall i n the Real H a n d Baseline condition than i n the Fake H a n d condition. n  88  separation (766 ms) t h a n w h e n either there w a s a s m a l l (640 ms) or large separation (601 m s ) , p's < .01. The r e m a i n i n g c o m p a r i s o n d i d n o t reach significance, p > .10. The same o v e r a l l analysis as that r e p o r t e d above w a s c o m p u t e d u s i n g percentage errors as the d e p e n d e n t variable. Since the error rates w e r e either n o t significant, or w e r e i n the same d i r e c t i o n as the correct RTs, speed accuracy tradeoffs w e r e n o t a concern. The m a i n effects of Separation [F (2, 78) = 4.42, p < .05] a n d C o n g r u e n c y [F ( 1 , 39) = 74.39, p < .01] w e r e significant. This w a s t e m p e r e d b y a significant Separation x C o n g r u e n c y i n t e r a c t i o n , F (2, 78) = 6.98, p < .01. This i n t e r a c t i o n w a s e x a m i n e d b y l o o k i n g at the s i m p l e effect of Separation f o r C o n g r u e n t a n d I n c o n g r u e n t trials. Separation w a s a significant factor o n l y f o r I n c o n g r u e n t trials, F (2, 78) = 6.40, p < .01. Least significant difference testing revealed that errors w e r e larger o v e r a l l w h e n there w a s n o target-distractor separation (9%) t h a n f o r either the s m a l l (6%) or large (6%) separations, p's < .01. T h e e r r o r rates for the latter t w o separations d i d n o t d i f f e r f r o m one another, p > .20.  Experiment 2: Correct RT Analysis # 1: M o s t of the significant effects w e r e observed i n the m e a n I n c o n g r u e n t RTs b u t n o t i n the m e a n C o n g r u e n t RTs. O v e r a l l , the p a t t e r n of I n c o n g r u e n t RTs w a s s i m i l a r to that for the RT CEs w i t h t w o exceptions. O n e , the m a i n effect of Distractor Side w a s n o t significant i n the correct RT data b u t w a s significant i n the RT CE data. T w o , w h e n the data analysis w a s restricted to opposite-side  89  target distractor presentations, the main effect of Condition was significant i n the Correct RTs but not the RT CEs. A repeated measures A N O V A was computed on correct RTs using Condition (No Fake Hands, Both Visible, Both Hidden), Target-Distractor Side (Same, Opposite), and Congruency (Congruent, Incongruent) as within-observer factors. The main effects of Congruency [F (1, 25) = 75.31, p < .01] and Condition [F (2,50) = 5.62, p < .01] were significant . These main effects were tempered by 12  a significant Congruency x Condition interaction [F (2, 50) = 5.46, p < .01], a Congruency x Distractor Side interaction [F (1, 25) = 20.54, p < .001], and a Congruency x Condition x Distractor Side interaction, F (2,50) = 6.52, p < .01. To better understand the significant three-way interaction, the simple interaction of Condition x Distractor Side was examined separately for Congruent and Incongruent trials. When the data was limited to Incongruent trials, the Condition x Distractor Side interaction was significant, F (2,50) = 5.91, p < .01. This was examined further by looking at the simple effect of Condition separately for same and opposite side target-distractor presentations. The simple effect of Condition was significant for same side target-distractor presentations, p < .01. Simple effects testing revealed that RTs were significantly faster i n the N o Fake Hands condition (644 ms) than either the Both Visible (764 ms) or Both H i d d e n (711 ms) conditions, F (2,50) = 8.64, p < .01. The comparison between the latter two conditions d i d not reach significance, p > .05. W h e n the targetdistractor presentations were on opposite sides of fixation, the main effect of  The main effect of Distractor Side d i d not reach significance i n the correct RT data but was significant i n the RT C E data. 12  90  C o n d i t i o n w a s significant, F (2,50) = 3.48, p < .05.  13  A g a i n , RTs w e r e  s i g n i f i c a n t l y faster i n the N o Fake H a n d s c o n d i t i o n (629 ms) t h a n either the B o t h Visible (668 ms) or B o t h H i d d e n (669 ms) c o n d i t i o n s , F (2, 50) = 8.64, p < .01. The c o m p a r i s o n b e t w e e n the latter t w o c o n d i t i o n s d i d n o t reach significance, p > .20. W h e n the data w a s l i m i t e d to C o n g r u e n t trials, o n l y the m a i n effect of Distractor Side w a s significant w h e r e RTs w e r e faster o v e r a l l f o r same side target-distractor presentations (581 ms) t h a n for opposite side presentations (615 m s ) , F ( 1 , 25) = 13.62, p < .01.  Error data analysis #1: The o v e r a l l analysis r e p o r t e d above w a s repeated u s i n g m e a n percentage errors as the d e p e n d e n t variable. Significant effects w e r e a g a i n seen o n l y o n I n c o n g r u e n t trials w h e r e the patterns w e r e s i m i l a r to the E r r o r CEs. I n general, the effects w e r e either n o t significant or w e r e i n the d i r e c t i o n of the Correct RTs, so speed-accuracy tradeoffs w e r e n o t a concern. The m a i n effect of C o n g r u e n c y [F (1,25) = 23.78, p < .01] a n d TargetDistractor Side [F ( 1 , 25) = 9.62, p < .01] w e r e significant. These w e r e t e m p e r e d b y a significant C o n g r u e n c y x Target-Distractor Side i n t e r a c t i o n , F ( 1 , 25) = 9.03, p < .01. This i n t e r a c t i o n w a s f o l l o w e d u p b y l o o k i n g at the s i m p l e effect of Target-Distractor Side f o r i n c o n g r u e n t a n d c o n g r u e n t trials. T h e s i m p l e effect w a s significant o n l y o n I n c o n g r u e n t trials, w h e r e observers m a d e m o r e errors o v e r a l l for same side (9%) versus opposite side target-distractor presentations (4%), F ( 1 , 25) = 19.24, p < .01. R e m a i n i n g p > .20.  This is d i f f e r e n t f r o m the RT CEs w h e r e the m a i n effect of C o n d i t i o n w a s significant o n l y f o r same side target-distractor presentations. 13  91  Correct RT Analysis # 2: Significant effects w e r e observed o n l y f o r the m e a n I n c o n g r u e n t RTs b u t n o t the m e a n C o n g r u e n t RTs. O v e r a l l , the p a t t e r n of I n c o n g r u e n t RTs w a s s i m i l a r to that for the RT CEs. A repeated measures A N O V A w a s c o m p u t e d o n correct RTs u s i n g Side of T o u c h ( H i d d e n , V i s i b l e ) , Distractor Side (Same, O p p o s i t e ) , a n d C o n g r u e n c y ( C o n g r u e n t , I n c o n g r u e n t ) as factors. The m a i n effect of C o n g r u e n c y [F ( 1 , 25) = 51.69, p < .01] a n d that of Distractor Side [F ( 1 , 25) = 5.34, p < .05] w e r e significant. These m a i n effects w e r e t e m p e r e d b y a significant C o n g r u e n c y x Distractor Side i n t e r a c t i o n [F ( 1 , 25) = 18.72, p < .001], Side of T o u c h x Distractor Side i n t e r a c t i o n [F ( 1 , 25) = 12.51, p < .01], a n d a C o n g r u e n c y x Side of T o u c h x Distractor Side i n t e r a c t i o n , F ( 1 , 25) = 5.45, p < .05. T o better i n t e r p r e t the t h r e e - w a y i n t e r a c t i o n , the s i m p l e i n t e r a c t i o n of Side of T o u c h x Distractor Side w a s e x a m i n e d separately f o r C o n g r u e n t a n d I n c o n g r u e n t trials. W h e n the data w a s l i m i t e d to I n c o n g r u e n t trials, the Side of T o u c h x Distractor Side i n t e r a c t i o n w a s significant, F ( 1 , 25) = 9.89, p < .05. This i n t e r a c t i o n w a s f u r t h e r e x a m i n e d b y l o o k i n g at the s i m p l e effect of D i s t r a c t o r Side w h e n the tactile s t i m u l u s w a s presented o n the side of the visible fake h a n d or o n the side of the h i d d e n fake h a n d . The s i m p l e effect of D i s t r a c t o r Side w a s significant o n l y w h e n targets a n d distractors w e r e presented o n the side of the visible fake h a n d , F ( 1 , 25) = 20.85, p < .01. R e m a i n i n g p > .20. O v e r a l l , RTs w e r e slower f o r same side target-distractor presentations (771 ms) versus opposite side presentations (658 m s ) . W h e n the data w a s l i m i t e d to C o n g r u e n t trials, the s i m p l e i n t e r a c t i o n of Side of T o u c h x Distractor Side d i d n o t reach significance, F ( 1 , 25) = 1.55, p > ,20. 92  Error data analysis #2: The o v e r a l l analysis r e p o r t e d above w a s repeated u s i n g m e a n percentage errors as the d e p e n d e n t variable. Significant effects w e r e again seen o n l y o n I n c o n g r u e n t trials w h e r e the patterns w e r e the same as the E r r o r CEs. Since the effects w e r e either n o t significant or w e r e i n the d i r e c t i o n of the Correct RTs, speed-accuracy tradeoffs w e r e n o t a concern. The m a i n effects of C o n g r u e n c y [F (1,25) = 22.50, p < .01] a n d TargetDistractor Side [F ( 1 , 25) = 14.18, p < .01] w e r e significant. These w e r e t e m p e r e d b y significant interactions of C o n g r u e n c y x Distractor Side [F ( 1 , 25) = 15.11, p < .01] a n d C o n g r u e n c y x Distractor Side x Side of T o u c h [F ( 1 , 25) = 6.51, p < .05]. T o better u n d e r s t a n d the t h r e e - w a y i n t e r a c t i o n , the s i m p l e i n t e r a c t i o n of Side of T o u c h x Distractor Side w a s e x a m i n e d f o r I n c o n g r u e n t a n d C o n g r u e n t trials. The s i m p l e i n t e r a c t i o n w a s significant o n l y o n I n c o n g r u e n t trials, F ( 1 , 25) = 4.23, p < .05. R e m a i n i n g p > .05. The s i m p l e effect of Distractor Side w a s e x a m i n e d separately f o r t o u c h o n the o c c l u d e d or visible side. The s i m p l e effect of Distractor Side w a s significant o n l y w h e n the tactile s t i m u l u s w a s presented o n the side of the visible fake h a n d , w h e r e errors w e r e h i g h e r f o r same side (13%) versus opposite side (4%) presentations, F (1,25) = 18.10, p < .01.  Experiment 3: M o s t of the significant effects w e r e observed i n the I n c o n g r u e n t R T data, b u t n o t i n the C o n g r u e n t RT data. I n the f o r m e r case, the p a t t e r n o f results w a s s i m i l a r to that observed f o r the RT CEs.  93  A f o u r factor repeated-measures A N O V A w a s c o m p u t e d u s i n g correct RTs as the d e p e n d e n t variable, a n d C o n d i t i o n ( N o Fake H a n d , Fake H a n d ) , Distractor Side (Same, O p p o s i t e ) , Observer Gloves ( N o n e , Plastic), a n d C o n g r u e n c y ( C o n g r u e n t , I n c o n g r u e n t ) as factors. The m a i n effects of C o n d i t i o n [F (1,13) = 8.62, p < .05] a n d C o n g r u e n c y [F (1,13) = 27.73, p < .05] w e r e significant . These w e r e t e m p e r e d b y significant interactions of C o n d i t i o n x 14  C o n g r u e n c y [F (1,13) = 10.77, p < .01], a n d Distractor Side x C o n g r u e n c y [F ( 1 , 13) = 16.93, p < .01. B o t h interactions w e r e f o l l o w e d u p u s i n g s i m p l e effects testing. W h e n the analysis w a s restricted to C o n g r u e n t trials, o n l y the s i m p l e m a i n effect of Distractor Side w a s significant w h e r e RTs w e r e faster o v e r a l l w h e n the target a n d distractor w e r e presented to the same side of f i x a t i o n (569 ms) versus the opposite side (590 m s ) , F (1,13) = 34.06, p < .001. W h e n the analysis w a s restricted to I n c o n g r u e n t trials, b o t h the s i m p l e m a i n effects of C o n d i t i o n [F (1,13) = 10.44, p < .01] a n d Distractor Side [F (1,13) = 7.13, p < .05] w e r e significant. RTs w e r e faster i n the N o Fake H a n d c o n d i t i o n (636 ms) versus the Fake H a n d c o n d i t i o n (677 m s ) , a n d faster w h e n the target a n d distractor w e r e presented to opposite sides of f i x a t i o n (643 ms) versus the same side (670 m s ) . The o v e r a l l analysis r e p o r t e d above w a s repeated u s i n g m e a n percentage errors as the d e p e n d e n t variable. Significant effects w e r e observed m a i n l y o n I n c o n g r u e n t trials w h e r e the patterns w e r e the same as the E r r o r CEs. Since the effects w e r e either n o t significant or w e r e i n the d i r e c t i o n of the Correct RTs, speed-accuracy tradeoffs w e r e n o t a concern.  The m a i n effect of Distractor Side was n o t significant i n the correct RT analysis b u t w a s i n the RT CE analysis. 14  94  The same analysis as that above w a s c o m p u t e d u s i n g errors as the d e p e n d e n t variable. The m a i n effect of C o n g r u e n c y w a s significant, F (1,13) = 10.53, p < .01. This w a s t e m p e r e d b y significant interactions of C o n d i t i o n x Distractor Side [F (1,13) = 5.54, p < .05], a n d C o n g r u e n c y x D i s t r a c t o r Side [F ( 1 , 13) = 11.73, p < .01]. The f o u r - w a y interaction also reached significance, F (1,13) = 8.82, p < .05. T o better c o m p r e h e n d the f o u r - w a y i n t e r a c t i o n , i t w a s b r o k e n d o w n i n t o the s i m p l e t h r e e - w a y interaction of C o n d i t i o n x C o n g r u e n c y x Distractor Side w h e n observers either w o r e gloves or d i d n o t . W h e n observers w o r e gloves o n their h a n d s , the s i m p l e t h r e e - w a y i n t e r a c t i o n d i d n o t reach significance, F (1,13) < 1, ns. W h e n observers d i d n o t w e a r gloves, the s i m p l e t h r e e - w a y i n t e r a c t i o n w a s significant, F (1,13) = 8.26, p < .05. T h i s w a s f u r t h e r f o l l o w e d u p b y l o o k i n g at the s i m p l e Distractor Side x C o n d i t i o n i n t e r a c t i o n for I n c o n g r u e n t a n d C o n g r u e n t trials. The s i m p l e i n t e r a c t i o n w a s significant o n l y for I n c o n g r u e n t trials, F (1,13) = 6.95, p < .05. R e m a i n i n g p > .20. The s i m p l e effect of C o n d i t i o n w a s e x a m i n e d f o r same a n d opposite side target-distractor presentations, a n d reached significance o n l y for the f o r m e r w h e r e observers t e n d e d to m a k e m o r e errors i n the Fake H a n d c o n d i t i o n (14%) t h a n the N o Fake H a n d c o n d i t i o n (6%), F (1,13) = 4.25, p < .07.  Experiment 4: A m i x e d design A N O V A was c o m p u t e d u s i n g correct RT as the d e p e n d e n t variable, Observer H a n d O r i e n t a t i o n (Prone, Supine) as a b e t w e e n observer factor, a n d Fake H a n d C o n d i t i o n ( N o Fake H a n d s , Prone, Supine), Distractor Side (Same, O p p o s i t e ) , a n d C o n g r u e n c y ( C o n g r u e n t , I n c o n g r u e n t ) as w i t h i n - o b s e r v e r factors. The m a i n effects of Observer H a n d O r i e n t a t i o n [F ( 1 , 26) 95  = 23.46, p < .01], Congruency [F (1, 26) = 63.97, p < .01], and Fake H a n d Condition were significant, F (2, 52) = 5.48, p < .01 . These main effects were 15  tempered by significant interactions of Congruency x Distractor Side [F (1,26) = 25.16, p < .01], Fake H a n d Condition x Distractor Side [F (2,52) = 4.11, p < .05], and Congruency x Fake H a n d Condition x Observer H a n d Orientation [F (2, 52) = 6.83, p < .01]. The Fake H a n d Condition x Distractor Side interaction was followed up by looking at the simple main effect of Fake H a n d Condition for same and opposite side target-distractor presentations. The simple main effect was significant only for same side target-distractor presentations, F (2, 52) = 8.0, p < .01. M e a n RTs were slower overall when the fake hand was prone (748 ms) than either supine (701 ms) or absent (697 ms), p's < .05. The comparison between the latter two was not significant, p > .20. The three-way interaction was followed up by examining the simple interaction of Fake H a n d Condition x Congruency for prone and supine observer hand postures. When the observer's hands were prone, the simple interaction was significant, F (2, 26) = 5.14. This was further examined by looking at the simple main effect of Fake H a n d Condition for congruent and incongruent trials. The simple main effect was significant only i n the latter case, F (2,26) = 8.19, p < .01. RTs were slower when the fake hands were prone (662 ms), than when either supine (622 ms) or absent (616 ms), p's < .01. W h e n the observer's hands were supine, the simple interaction of Fake H a n d Condition x Congruency was significant, F (2, 26) = 3.80, p < .05. The simple main effect of Fake H a n d  The main effect of Distractor Side was not significant i n the correct RT data but was i n the RT C E data. 15  C o n d i t i o n w a s significant o n l y o n C o n g r u e n t trials, F (2, 26) = 3.40, p < .05. RTs w e r e s i g n i f i c a n t l y slower w h e n the fake h a n d s w e r e p r o n e (838 m s ) t h a n supine (778 ms) or absent (776 m s ) , p's < .05. The o v e r a l l analysis r e p o r t e d above w a s repeated u s i n g m e a n percentage errors as the d e p e n d e n t variable. Significant effects w e r e observed o n l y o n I n c o n g r u e n t trials w h e r e the patterns w e r e the same as the E r r o r CEs. Since the effects w e r e either n o t significant or w e r e i n the d i r e c t i o n of the Correct RTs, speed-accuracy tradeoffs w e r e n o t a concern. The m a i n effects of C o n g r u e n c y [F ( 1 , 26) = 51.77, p < .01] a n d D i s t r a c t o r Side [F ( 1 , 26) = 6.59, p < .05] w e r e significant. These w e r e t e m p e r e d b y a significant C o n g r u e n c y x Distractor Side i n t e r a c t i o n , F ( 1 , 26) = 22.68, p < .01. This w a s f o l l o w e d u p b y e x a m i n i n g the s i m p l e effect of D i s t r a c t o r Side f o r I n c o n g r u e n t a n d C o n g r u e n t trials. The s i m p l e effect w a s s i g n i f i c a n t o n l y o n i n c o n g r u e n t trials w h e r e observers t e n d e d to m a k e m o r e errors o n same side (11%) versus opposite side target-distractor presentations (8%), F ( 1 , 26) = 19.20, p < .01.  Experiment 5: A m i x e d design A N O V A was c o m p u t e d u s i n g correct RTs as the d e p e n d e n t variable, Observer H a n d O r i e n t a t i o n (Prone, Supine), a n d D i g i t Response M a p p i n g (top-toe & b o t t o m - h e e l , top-heel & b o t t o m - t o e ) as b e t w e e n observer factors a n d C o n g r u e n c y ( C o n g r u e n t , I n c o n g r u e n t ) , Fake H a n d C o n d i t i o n ( N o Fake H a n d , Consistent, Inconsistent), a n d D i s t r a c t o r Side (Same, Opposite) as w i t h i n - o b s e r v e r factors. The m a i n effect of C o n g r u e n c y w a s significant, F ( 1 , 52) = 58.99, p < .01. This w a s t e m p e r e d b y s i g n i f i c a n t 97  interactions of C o n g r u e n c y x Observer H a n d O r i e n t a t i o n [F ( 1 , 52) = 9.13, p < .01], C o n g r u e n c y x Digit-Response M a p p i n g [F (1,52) = 18.22, p < .01], C o n g r u e n c y x C o n d i t i o n [F (2,104) = 4.52, p < .05], C o n g r u e n c y x D i s t r a c t o r Side [F ( 1 , 52) = 14.09, p < .01], C o n g r u e n c y x Observer H a n d O r i e n t a t i o n x D i g i t Response M a p p i n g [F ( 1 , 52) = 7.89, p < .01], C o n g r u e n c y x D i s t r a c t o r Side x Observer H a n d O r i e n t a t i o n [F ( 1 , 52) = 10.91, p < .01], C o n g r u e n c y x Distractor Side x Digit-Response M a p p i n g [F (1,52) = 4.68, p < .05], C o n g r u e n c y x Distractor Side x Observer H a n d O r i e n t a t i o n x Digit-Response M a p p i n g [F (1,52) = 7.34, p < .01]. The C o n d i t i o n x Distractor Side x Observer H a n d O r i e n t a t i o n interaction w a s also significant, F (2,104) = 3.23, p < .05. The f o u r - w a y interaction w a s e x a m i n e d b y l o o k i n g at the s i m p l e i n t e r a c t i o n of Distractor Side x Observer H a n d O r i e n t a t i o n x Digit-Response M a p p i n g for I n c o n g r u e n t a n d C o n g r u e n t trials. The i n t e r a c t i o n w a s significant o n l y for I n c o n g r u e n t trials, F ( 1 , 52) = 7.15, p <.05. The s i m p l e i n t e r a c t i o n of Distractor Side x Digit-Response M a p p i n g w a s e x a m i n e d f o r p r o n e a n d s u p i n e observer h a n d orientations. I t w a s significant o n l y w h e n the observer's arms w e r e s u p i n e , F (1,26) = 8.19, p < .01. The s i m p l e effect of D i s t r a c t o r Side a p p r o a c h e d significance o n l y for the top-heel & b o t t o m - t o e m a p p i n g , w h e r e observers w e r e faster to localize the target w h e n the distractor w a s presented to the same side (622 ms) versus the opposite side of f i x a t i o n (646 m s ) , F (1,13) = 4.51, p < .06. The C o n d i t i o n x Distractor Side x Observer H a n d O r i e n t a t i o n i n t e r a c t i o n w a s e x a m i n e d b y l o o k i n g at the s i m p l e i n t e r a c t i o n of C o n d i t i o n x Observer H a n d O r i e n t a t i o n f o r same a n d opposite side target-distractor presentations. The i n t e r a c t i o n w a s significant o n l y for same side target-distractor presentations, 98  F (2,104) = 4.30, p < .05. The s i m p l e effect of C o n d i t i o n w a s e x a m i n e d f o r p r o n e a n d supine observer a r m postures. W h e n the observer's a r m w a s p r o n e , the s i m p l e m a i n effect of C o n d i t i o n a p p r o a c h e d significance, F (2, 54) = 2.51, p < .10. M e a n RTs i n the Consistent Fake H a n d c o n d i t i o n (757 ms) w e r e s i g n i f i c a n t l y slower t h a n the RTs i n the Inconsistent Fake H a n d c o n d i t i o n (716 ms) b u t n o t the N o Fake H a n d c o n d i t i o n (730 ms). W h e n the observer's a r m w a s s u p i n e , the s i m p l e m a i n effect of C o n d i t i o n a p p r o a c h e d significance, F (2, 52) = 2.68, p < .10. M e a n RTs w e r e s i g n i f i c a n t l y slower i n the Inconsistent Fake H a n d c o n d i t i o n (759 ms) t h a n the N o Fake H a n d c o n d i t i o n (724 m s ) , b u t d i d n o t d i f f e r f r o m the Consistent Fake H a n d c o n d i t i o n (729 ms). The same o v e r a l l analysis as that r e p o r t e d above w a s c o m p u t e d u s i n g m e a n percentage error as the d e p e n d e n t variable. The m a i n effects of C o n g r u e n c y [F (1,52) = 51.61, p < .01] a n d Distractor Side [F ( 1 , 52) = 6.80, p < .05] w e r e significant. These w e r e t e m p e r e d b y significant interactions of C o n g r u e n c y x Digit-Response M a p p i n g [F ( 1 , 52) = 13.36, p < .01], C o n g r u e n c y x Distractor Side [F (1,52) = 15.47, p < .01], C o n g r u e n c y x D i s t r a c t o r Side x D i g i t Response M a p p i n g [F (1,52) = 6.52, p < .05]. The Observer H a n d O r i e n t a t i o n x Digit-Response M a p p i n g interaction w a s also significant, F ( 1 , 52) = 6.99, p < .05. The t h r e e - w a y interaction w a s e x a m i n e d b y l o o k i n g at the s i m p l e interaction of Distractor Side x Digit-Response M a p p i n g f o r C o n g r u e n t a n d I n c o n g r u e n t trials. The s i m p l e interaction a p p r o a c h e d significance o n l y o n the i n c o n g r u e n t trials, F ( 1 , 52) = 2.97, p < .10. The s i m p l e m a i n effect of Distractor Side w a s significant o n l y for a top-toe & b o t t o m - h e e l m a p p i n g assignment, F ( 1 , 26) = 19.20, p <.01. E r r o r CEs w e r e larger o v e r a l l o n the same side (11%) versus the opposite side (8%). 99  In compliance with the Canadian Privacy Legislation some supporting forms may have been removed from this dissertation. While these forms may be included in the document page count, their removal does not represent any loss of content from the dissertation.  

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