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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 FAKE 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 this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of 1V(<^«,U>Q|^ The University of British Columbia Vancouver, Canada Date CXo~e. 3 o 1 2 o o 3 > DE-6 (2/88) Abstract Space-relevant tactile localizations may be influenced by non-tactile sources of information. This is evident in 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 in 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 would expect if the tactile targets were mislocalized to the locus of the fake hand (Experiment 1). Surprisingly, the fake hand effect d id 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 i i Table of Contents i i i List of Tables.. v List of Figures v i 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 i i i Results... 57 Discussion 62 General Discussion 64 Impl icat ions of the Present Results 68 Outs tand ing Questions and Future Direct ions 71 References 77 A p p e n d i x 87 List of Tables Table 1: Descriptive information for each Experiment Table 2: Breakdown of trial types in 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 t h ed.) (p. 231), by S. Coren, L . M . Ward, J.T. Enns, 1999, New 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 t h ed.) (p. 234), by S. Coren, L . M . Ward, J.T. Enns, 1999, New 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 and Driver (2000). (A) N o 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 11 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 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 -v i Touch on Hidden Side, One Hidden - 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 Hand Condition (No Fake Hand, Inconsistent, Consistent) and Observer Hand Orientation (Prone, Supine) for (A) Same and (B) Opposite target-distractor presentations 51 Figure 9. Mean C E (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 target-distractor presentations 60 v i i Introduction The human tactile system is often confronted with 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 with 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, in order to know where the event occurred in space, one must figure out where the hand is positioned in 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. When 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 & Wing, 2001). Within this region are groups of neurons that are devoted to particular parts of the body; the spatial layout of these neurons in the brain roughly corresponds to the spatial layout of their receptive fields under the skin's surface. In order to identify where in 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 wi th in the somatosensory cortex to different parts of the human body. From Sensation and Perception (5 t f ( ed.) (p. 231), by S. Coren, L.M. Ward, J.T. Enns, 1999, New York: Harcourt Bruce. Copyright 1950 by Macmillan Publishing Co., renewed 1978 by Theodore Rasmussen. Permission granted. 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 & Wing, 2001). The spatial sensitivity of the tactile sensory system is not uniform, rather, the areas of the body that are wel 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, in 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). E o V Z SO 45 -40 35 30 25 20 15 10 5 Upper arm * Shoulder . Forehead «- Forearm Palm Figure 2. Two-point tactile discrimination thresholds for different regions of the human body. From Sensation and Perception (5 t h ed.) (p. 234), by S. Coren, L.M. Ward, J.T. Enns, 1999, New York: Harcourt Bruce. Permission granted. 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 knowing 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 in space by relying on information from pressure sensors to determine the position of the joints, the amount of tension in a particular muscle or tendon, or changes in muscle length (Roberts & Wing, 2001). Information from these three sources (tactile, visual, proprioceptive) is thus combined in some way in 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 in 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; Howard & Templeton, 1966). A comparable illusion, referred to as the McGurk effect, is one in which observed l ip movements influence speech perception (e.g., McGurk & 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 uphil l when in neutral gear (Shimamura & Prinzmetal, 1999). Popular tourist attractions, these areas are clear examples of how 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 in the environment. Thus, rather than relying on signals from proprioception to indicate upright, for example, people tend to rely on vision, with 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 in other sensory modalities. Sekuler, Sekuler and Lau (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 in 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 in 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 known to be more precise, in 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-Will iams 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 in the direction 6 induced by the goggles. When the goggles are removed, the target-pointing responses are shifted in 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 with the non-target hand). This latter result suggests that the goggles d id not simply change the perceived location of the visual image, which would 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. As far as the observers knew, the view provided was always of their own 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 in 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 in order to determine the space-relevant location of tactile information. This makes them vulnerable to tactile and proprioceptive illusions when the visual information is inconsistent with 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 own 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, and/or they reported that they felt as though their own 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 with 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 in 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 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. 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. To avo id this, the tactile local izat ion and l imb pos i t ion measures ough t to be objective rather than subjective. Pavani , Spence and Dr iver (2000) recently repor ted results s imi lar to those of Botv in ick and Cohen (1998), bu t us ing methods that c i rcumvented some of the problems described above. That is, their key measure of tacti le and l i m b mislocal izat ions was indirect (i.e., observers were not asked d i rect ly about the perceived locat ion of ei ther), and objective (i.e., d i d not re ly o n subjective impressions repor ted b y observers). The t w o key condi t ions repor ted b y Pavani et al. (2000) w i l l be referred to here as the N o Fake H a n d cond i t i on (Figure 4A) and the Fake H a n d cond i t ion (Figure 4B). I n b o t h condi t ions, the authors had observers place their hands beneath a smal l stand, such that they were out-of-sight, and h o l d a foam cube i n each hand so that a tacti le v ib ra tor was posi t ioned under their f inger and one under their t h u m b (see Figure 4A) . The task of observers was to use a foot response to indicate, as qu ick ly as possible, the elevat ion of the target tactile v ib ra t ion (top or bo t tom) . A match ing pa i r of foam cubes, w h i c h he ld v isua l distractors i n the place of the tactile v ibra tors , was posi t ioned i n v i e w of observers o n the stand just above the cubes i n the observer's hands be low. One of the v isual distractors was presented s imul taneously w i t h the tactile target. The elevat ion of the v isua l distractor cou ld either be congruent (Figure 4C) or incongruent (Figure 4D) w i t h respect to that of the tactile v ib ra t ion . I n the Fake H a n d cond i t ion , a fake h a n d was posi t ioned above each of the observer's hands and was arranged so as to appear to ' h o l d ' the distractor l ights (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 No Fake Hand condition. In general, observers were faster and more accurate in 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 in the Fake Hand 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 in the Fake Hand 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 id 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 would be closer to the distractor lights, and the apparent target-distractor proximity would 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 id not include a baseline measure that could be used to objectively determine whether, in 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, in order to establish whether this is the case, the authors would 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 in the fake hand effect by introducing a condition wherein the observers held the tactile vibrations in 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 space-relevant 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 in the context of existing theories and predict the outcome of the manipulations introduced here. Thus, for each question addressed in 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? Do observers actually feel the tactile vibrations at the location occupied by the fake hand? This question was explored in 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 Hand condition), B) when the observer's own hand was at the level of the distractor lights (Real Hand Baseline condition), and C) when the observer's own hand was below the level of the distractor lights (No Fake Hand 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 CEs in the Fake Hand condition should be similar to those in the Real Hand Baseline condition and larger than those in the N o Fake Hand condition. Three additional questions of interest regarding the fake hand effect were explored in Experiment 1. First, in order to determine whether the pattern of 13 responses in the Fake Hand condition was relatively more similar to those in the Real Hand Baseline or the N o Fake Hand 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 Hand 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 with 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 would 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 in the location normally occupied by the fake hand was larger than that obtained when the fake hand was present. Two, 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 in 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 in 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 with the fake hand effect made the effect more compelling (i.e., increased CEs in the Fake Hand condition relative to the N o Fake Hand condition). Observers wore soft plastic kitchen gloves on their hands that were identical to those used for the fake hands. Even with the feel of plastic on their hands, however, the effect remained the same size as that observed in 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 with the observer's hands immediately below. When the posture of the fake hands was inconsistent with 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 in 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 in 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. When this particular mapping was combined with a supine observer hand orientation, the effect both weaker and in the reverse direction 2. This latter case indicates that the 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 wel l onto the toe and the thumb does not map wel l 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 in 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 2 The term 'reversed' is meant to distinguish the effect in this case where CEs are negative and in the direction of being larger for the Inconsistent Fake Hand condition from the standard effect where CEs are positive and larger in the Consistent Fake Hand condition. 16 (e.g., slower RTs on Incongruent trials, faster RTs on Congruent trials, or a combination of both). There were four main questions addressed within Experiment 1: Are tactile vibrations mislocalized to the fake hands? To make such a judgment, one would first need to establish a baseline C E measure in 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 Hand Baseline condition was included in the present study wherein the observer's own 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 in the presence of the fake hands (Fake Hand condition), then their apparent location should be at the same location as the tactile vibrations in the Real Hand Baseline, and thus, the resulting CEs should be the same magnitude in the two conditions. That is, the magnitude of CEs in the Fake Hand condition should be comparable to CEs in the Real Hand Baseline condition where the tactile vibrations are actually at the location that is specified by vision of the fake hand in the Fake Hand condition. This is in contrast to the CEs in the N o Fake Hand condition where the tactile vibrators are vertically separated from the distractor lights. Are the dynamics similar for a fake versus real hand? Regardless of the absolute magnitude of CEs in the Fake Hand versus Real Hand 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 when 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 target-distractor horizontal separations and comparing the response patterns across conditions, it is possible to determine whether the pattern generated in the presence of the fake hand (Fake Hand condition) is more similar to the case where the observer's own hand is at the location specified by vision (i.e., Real Hand Baseline condition), or at the location specified by proprioception but below that of vision (i.e., N o Fake Hand condition). Evidence for the former would support the notion that observers behave, at least to some degree, as though the visible fake hand were their own. Are CEs influenced by viewing the limb in the N o Fake Hand condition? In the N o Fake Hand condition of the Pavani et al. (2000) study, the observer's own 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, Taylor-Clarke, & 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 own limb indirectly (Tipper et al., 1998; 2001). It is possible that CEs in the N o Fake Hand condition would 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 would 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 would 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 in both the N o Fake Hand and the Real Hand 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 in complete view of observers. The question remains then as to whether the fake hand effect would persist when the observers know 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 in 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 in a 45-minute experimental session in 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 exper iment and were f u l l y debriefed u p o n complet ion. Fur ther par t ic ipant i n fo rmat ion is p r o v i d e d i n Table 1. Procedure: The task of observers was to make a speeded forced-choice tactile local izat ion judgment . The target was a tactile v ib ra t i on app l ied to either the f inger or t h u m b of the observer's r igh t hand . Locat ion responses were made v ia t w o foot-pedals, one posi t ioned under the toes of the r igh t foot , and the other under the heel. A t rest, b o t h pedals were depressed 3 . To indicate that the tactile v ib ra t ion locat ion was at the f inger, observers released the peda l under their toes br ie f l y and then rested. To indicate that the tactile v ib ra t ion locat ion was at the t h u m b , they br ie f ly released the pedal under their heel, and then rested. Observers were instructed to ignore the s imultaneous onset of a distractor L E D w h i l e ma in ta in ing center f ixat ion and at tending to the target tacti le v ib ra t ion . Observers took short breaks between each exper imenta l b lock of tr ials. Appara tus : Figure 5 includes i l lustrat ions of the basic exper imenta l setup for each of the three condi t ions, i nc lud ing samples of the v is ib i l i t y man ipu la t ion : vis ible (Figure 5A) and h i d d e n (Figure 5B). A lso inc luded i n Figure 5 is an example of each of the three target-distractor separations (none, smal l , and large). A ch in rest was centered on a table. I t was used to stabalize the pos i t ion of the observer's head at a distance of 47 c m f r o m a red central f i xa t ion L E D . Cardboard boundar ies were constructed and secured on the lef t and r igh t sides 3 This took l i t t le- to-no effort on the observer's part . The mere w e i g h t of the observer's foot at rest was al l that was requi red to depress the pedals. S imi lar ly , l i t t le effort was needed to release the pedal . The toes or heel h a d to be raised just enough so that the pedal was no longer complete ly depressed. No te that this d i d not require the toes or heel to clear the pedal . 20 of the chin rest, and were used to maintain the position of the observer's forearms. Visible CO j ! 150 $ 1 0 0 Target-Distractor Separation • None S3 Small • Large No Fake Hand Fake Hand Condition Real Hand Baseline Hidden 200 150 S 100 c CO £ 50 Target-Distractor Separation • None 53 Small • Large No Fake Hand T - H Fake Hand Condition Real Hand Baseline 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 ibrators were Ot icon-A bone conduc t ion v ibrators of 100 Ohms. They were posi t ioned i n the upper and lower left h a n d corners of a foam cube. A T E N M A func t ion generator was used to del iver each tactile target. To mask the sound of the v ibrators, w h i t e noise was p layed t h r o u g h a pai r of headphones w o r n b y observers. T w o ye l low distractor LEDs were pos i t ioned i n the upper and lower r igh t h a n d corners of a second foam cube, w h i c h was placed on top of a ta l l str ip of foam r u n n i n g hor izonta l ly f r o m the back of one set of fo rearm boundar ies to the other. The fake h a n d , used at d i f ferent points d u r i n g the exper iment , was constructed us ing a r igh t -handed p i n k soft plastic d ishwash ing glove stuf fed w i t h cot ton bat t ing. W h e n the fake hand was present, i t rested on a sheet of black corkboard that was posi t ioned over top of the fo rearm boundar ies. No te that whenever the fake h a n d was present, this meant that the observer's o w n hands were under the corkboard and thus out of sight. Observers made their responses b y releasing one of t w o footpedals as qu ick ly and as accurately as possible. W h e n they local ized the tactile v ib ra t ion to their f inger observers were instructed to release the pedal under the toes, and w h e n they local ized the tactile v ib ra t ion to their t h u m b they were inst ructed to release the pedal under the heel. S t imul i : To produce a v isual distractor, one of the t w o LEDs was f lashed three t imes for a du ra t ion of 50 ms each t ime, w i t h each f lash separated b y a 50 ms ISI. To produce the tactile target, three 50 ms 200-Hz sine-wave signals separated b y 50 ms ISIs were sent f r o m one of the t w o tacti le v ibrators. The target and distractor were a lways presented s imul taneously. Target and 22 distractor locations were random with the constraint that each location was selected from equally often. Whenever observers made an error in 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 in total). Correct response times (RT) and mean percentage errors were recorded. Observers were randomly assigned to one of three conditions: N o Fake Hand (n = 14), Fake Hand (n = 14), or Real Hand 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 Hand and Fake Hand 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 Hand 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 wi th the observer's hand below, and it was aligned horizontally with the distractor cube. A foam cube was positioned between the index finger and thumb of the fake hand to enhance the similarities with the observer's hand (refer to the Fake Hand condition pictured in Figure 5). O n trials where the distractor cube was 23 posi t ioned beside the fake hand , the fake index f inger rested next to the upper distractor l igh t and the fake t h u m b next to the lower distractor l ight . The Real H a n d Baseline cond i t ion was the key comparat ive cond i t ion in t roduced i n the present s tudy. I t was s imi lar to the N o Fake H a n d cond i t ion w i t h the one except ion that the height of the observer's hand was raised to the level of the distractor cube b y hav ing observers rest their a r m on a th ick st r ip of foam. Three wi th in-observer factors were man ipu la ted : Congruency of target-distractor elevations, hor izonta l Separation between the distractor cube and target cube, and the V is ib i l i t y of the observer's hand or of the fake h a n d (when present). Each of these factors is described i n t u r n be low. Target-distractor elevations were either congruent or incongruent (Congruency factor). Congruent tr ials, i n this case, consisted of an upper L E D pai red w i t h a top- f inger v ib ra t ion , or a lower L E D pa i red w i t h a b o t t o m - t h u m b v ib ra t ion . Incongruent tr ials, i n contrast, consisted of an upper L E D pa i red w i t h a bo t tom- thumb v ib ra t ion or a lower L E D pa i red w i t h a top- f inger v ib ra t ion . Each target-distractor pair occurred equal ly of ten i n each block. The hor izonta l separation between the L E D distractor cube and the target tactile v ib ra t ion cube was man ipu la ted b y m o v i n g the distractor cube i n a l e f twa rd d i rect ion away f r o m the tactile v ib ra t ion cube. There were three possible separations: none (0 c m apart, distractor cube at r i gh t ) , smal l (15 c m apart, distractor cube at center), or large (30 c m apart, d istractor cube at far left). No te that w i t h i n any cond i t ion , the pos i t ion of the target tacti le v ib ra t ion cube remained constant i n that i t remained on the r igh t side. Thus, the distractor cube was always to the left of the target cube, and on ly the pos i t ion of the distractor cube was altered. This meant that w h e n the fake h a n d was present, i t remained 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 in the N o Fake Hand and Real Hand Baseline conditions, and the visibility of the fake hand in the Fake Hand 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 Hand condition, while a black cloth was used to hide the fake hand and the observer's hand in the remaining two conditions. The two within-observer factors, Separation (3 levels) and Visibil i ty (2 levels), were blocked resulting in 6 blocks in total. The order of blocks was randomized. Condition (3 levels) was a between-observer factor. A breakdown of the trial types is presented in 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 in any experimental condition 4. Only trials where observers made correct localization responses were included in the RT analyses. The RT Congruency Effect (CE) was calculated for each condition by finding the difference between the mean RT on Incongruent 4 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. 25 tr ials m inus the mean RT on Congruent tr ials. The analyses and discussion were m a i n l y focused o n the CE results for t w o reasons. First, the CE measure is a convenient s u m m a r y measure as its magn i tude is d i rect ly ind icat ive of the extent to w h i c h the onset of the LEDs interfered w i t h the speeded responses to the tactile v ibrat ions. That is, the larger the CE, the more effective the L E D as a distractor. Second, the relat ive magn i tude of the CE is ind i rec t ly ind icat ive of the perceived separat ion between the tactile v ibrat ions and the distractor l ights, where the larger the CE, the smaller the perceived target-distractor separation. The mean percentage errors were also analyzed as CEs ( Incongruent Errors - Congruent Errors) and this pat tern of data was compared to that of the CEs for RTs. I f either there were no signif icant effects observed i n the Error CE data, or i f the same pat tern of data was f o u n d across condi t ions i n b o t h RT CEs and Error CEs (e.g., the larger RT CEs pa i red w i t h the larger error CEs), then speed-accuracy tradeoffs were e l iminated as a concern. I n the event that oppos ing patterns were observed (e.g., the larger RT CEs pa i red w i t h the smaller error CEs), speed-accuracy tradeoffs had to be considered as a possibi l i ty . I n add i t i on to these m a i n analyses of interest us ing the CEs, ident ical analyses were computed us ing the mean Correct RT and mean Percentage Error data. This was done to ensure that the pat tern of results observed f r o m the CEs, w h i c h are difference scores, was s imi lar to that for the means. These latter analyses are presented for each Exper iment i n an A p p e n d i x . Since the results were always consistent w i t h the RT and Error CE analyses, they w i l l no t be discussed fur ther . Both the RT CEs and Error CEs were subjected to analyses of variance ( A N O V A ) i n v o l v i n g wi th in-observer factors of Separation (None, Smal l , Large) 26 and V is ib i l i t y (Visible, H i d d e n ) , and the between-observer factor of Cond i t i on (No Fake H a n d , Fake H a n d , Real H a n d Baseline). Signif icant three-way interactions were fo l l owed u p us ing Simple Interact ion Effects, w h i l e signi f icant t w o - w a y interact ions were fo l l owed u p us ing Simple Effects test ing. Signif icant m a i n effects were fo l l owed u p us ing Least Signif icant Dif ference (LSD) tests. As p ic tu red i n Figure 5, there were several notable f ind ings i n the present s tudy. First, w h e n the l imbs were visible and there was no separat ion between targets and distractors (see the sol id black bars i n Figure 5A) , there were signif icant differences i n the magn i tude of the CEs across condi t ions, w i t h the CE be ing the largest i n the Real H a n d Baseline cond i t ion , and smallest i n the N o Fake H a n d condi t ion. This suggests that a l though tactile targets were be ing mislocal ized to the fake h a n d w h e n i t was present, the fake h a n d effect was not of equal st rength to the presence of the observers' o w n l imbs i n the same locat ion. For the smal l and large target-distractor separations, the CEs were of s imi lar magn i tude across condi t ions. Second, the CEs tended to decrease w i t h increasing target-distractor separation for a l l condi t ions except the N o Fake H a n d cond i t ion . The fact that s imi lar patterns were observed i n the Fake H a n d and Real H a n d Baseline condi t ions suggests that there are some simi lar i t ies between the fake hand and a real han d despite differences i n the absolute magn i tude of CEs. T h i r d , h i d i n g the l imbs f r o m v i e w had very l i t t le impac t o n the magn i tude of the CEs relat ive to w h e n the l imb was visible (compare Figure 5B to 5A) . There was one except ion to this. W h e n there was no target-distractor separat ion (see sol id black bars i n Figure 5), CEs were larger i n the N o Fake H a n d cond i t ion w h e n the h a n d was h i d d e n (Figure 5B) than w h e n i t was v is ib le (Figure 5A) . 27 This finding suggests that the visual cues to the vertical separation between targets and distractors when the observer's hands were visible in 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 down 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 Hand Baseline condition (189 ms), smaller for the Fake Hand condition (128 ms), and smallest for the N o Fake Hand 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 Hand 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 Hand 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 did not reach significance (p > .05). Finally, the main effect of Separation approached significance for the N o Fake Hand 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) did 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 id not reach significance, p > .10. The overall three-way interaction was also broken down by computing the simple interaction of Condition x Visibili ty at each level of Separation in 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 did 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 Visibili ty was significant only for the N o Fake Hand 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 CEs that were significant were in the same direction as the RT CEs suggesting that speed-accuracy 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 in the present study. The implications of each are discussed in turn below. Larger CEs in the Real Hand Baseline versus Fake Hand 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 Hand condition) compared to when vision and proprioception indicate the spatial alignment between targets and distractors (Real Hand 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 would lead one to believe. It is, however, consistent with 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 with increases in target-distractor separation: This pattern was observed in both the Fake Hand and Real Hand Baseline conditions but not in the N o Fake Hand condition. This is consistent with the 30 idea that the fake h a n d is s imi lar to a real hand for observers (note that this can be the case even t h o u g h the absolute CE data shows that responses i n the presence of the fake h a n d are weaker than those obtained w i t h the real hand) . I t is interest ing to note that i n these t w o condi t ions CEs were largest overal l w h e n there was no target-distractor separation despite the fact that at the smal l separat ion, the distractor l ights were located at the center of gaze. I f seeing the distractors were the key factor, one w o u l d expect that i t w o u l d be at this locat ion that the l ights w o u l d have the greatest impact on target local izat ion responses. That they had their greatest impact w h e n they were next to the target v ibrat ions and away f r o m the center of gaze indicates that i t is the spat ial p r o x i m i t y between the targets and distractors that is of most impor tance. This target-distractor separation effect is consistent w i t h earlier reports suggest ing that the greater the discrepancy i n i n fo rmat ion w i t h i n the v isua l and propr iocept ive sources, the less evidence for v isual bias (Over, 1966; see Welch & War ren , 1980). The fact that the above separation pat tern was not observed i n the N o Fake H a n d cond i t ion indicates that the vert ical separat ion between targets and distractors is enough to weaken the magn i tude of the CE. I n fact, i t was on ly i n this cond i t ion that CEs tended to be h igher at the center of gaze (smal l separation) than w h e n there was no separat ion between targets and distractors. Further, since the separat ion effect was present i n the Fake H a n d cond i t ion , i t suggests that the fake h a n d must have successfully b r i dged the ver t ica l target-distractor separation that was apparent i n the N o Fake H a n d cond i t ion . 31 Little-to-no effect of limb visibility on CEs: The only condition in which there was an effect of visibility was the N o Fake Hand 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 Hand 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 CE . Partial vision of the fake hand is sufficient for the effect: As noted above, hiding the fake hand in the Fake Hand condition did not significantly change the magnitude of the CEs. That is, the CEs tended to be larger in the Fake Hand condition than in the N o Fake Hand 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 in Experiment 1 was a modified version of that used by Pavani et al. (2000). The modifications were necessary in 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 in 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 did 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 in Experiment 1, it was expected that CEs would 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 would 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 in some way by this competition. Experiment 2 There were two main goals of Experiment 2 . The first was to determine whether observers would 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 Hand condition w i l l be similar 33 to those in the N o Fake Hand condition). This result would indicate that seeing the features of the hand (e.g., shape) is an important part in generating mislocalizations, and that the conclusions drawn in Experiment 1 that partial vision of the fake hand is sufficient for mislocalizations would 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 would 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 would 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 with 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 would 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 in both the left and right hands, and in the Fake Hand condition, a fake hand was positioned above each of the observer's hands. In the N o Fake Hand condition, the fake hands were absent and only the distractor lights and central feedback cube were visible. With 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 in direct competition with 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. M e t h o d s Participants: Thirteen undergraduate students at the University of British Columbia participated in the one-hour experimental session for partial course credit. Personal descriptive information is provided in 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 in 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 in each hand. Each foam cube contained a pair of tactile vibrators as described in 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 wi th 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. On the remaining block, the fake hands were absent. The five conditions were completed in random order. Stimuli: Same as that used in Experiment 1, wi th the exception that there were now four possible target locations and four possible distractor light locations. Locations were randomly selected with 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 within-observer factors. Observers participated in five randomly ordered Fake Hand Conditions: N o Fake Hand, Both Visible, Both Hidden, Left Hand Hidden, and Right Hand Hidden. 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 Hand condition, there were no fake hands on the stand, only the distractor light cubes and the feedback cube in 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 in view, while in the Both Hidden condition, both fake hands were 36 h i d d e n beneath box covers. I n the Left H a n d H i d d e n and Right H a n d H i d d e n condi t ions, the left or r igh t fake hand , respectively, was h i d d e n f r o m v i e w underneath a box cover w h i l e the remain ing hand was vis ible. Procedure: Same as that i n Exper iment 1. Results RT CEs and Error CEs were again the dependent measures of interest. Signif icant effects were fo l l owed u p us ing the same procedures as ind icated i n Exper iment 1. A l l factors were wi th in-observer factors and inc luded C o n d i t i o n (No Fake H a n d , Visible, H i d d e n , Lef t H a n d H i d d e n , and R igh t H a n d H i d d e n ) , and Target-Distractor Side (Same, Opposi te). There were t w o m a i n f ind ings. One, w h e n b o t h fake hands were h i d d e n f r o m v i e w us ing box covers, CEs were of the same magn i tude as those observed w h e n bo th fake hands were visible (see Figure 6). This suggests that observers were st i l l i n fe r r ing the presence of the fake hands, despite the fact that they were covered and their shape disguised. I n other w o r d s , observers st i l l mis local ized tactile targets to the fake hands w h e n b o t h were h idden . N o t e that the CEs i n these t w o condi t ions were larger than those f o u n d w h e n the fake hands were absent. 3 7 A . S a m e B. O p p o s i t e 200 ,—^ CO E 150 c y 100 I 50 vt T J c CO I a> co a> 'vt > o m c a> T J a> •g CO I C C a> a) sz TD "a o "5 "5 to I I e o O sz u o a> •a CO <= CD "a vt I > c o O £ u O E 200 150i LU O 100 tz co cu 50 T J C ca I cu ca a; > o CD T J T J O CQ CD T J CO C C a) a> T J T J T J T J I I Q> C C O O £ (J 3 O CD T J CO c a> T , I T J W I > cu c c o O sz u o Fake Hand Condition 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 3 8 [F (2,50) = 4.07, p < .05, MSE = 7716] and Distractor Side were signi f icant [F (1 , 25) = 23.65, p < .01, MSE = 12367], as was the interact ion [F (2,50) = 5.0, p < .05, MSE = 6038]. The interact ion was examined more closely b y l ook ing at the m a i n effect of Cond i t i on separately for each target-distractor type (i.e., same and opposite side presentations; see Figure 6A and 6B, respectively). The m a i n effect of Cond i t i on was signif icant on ly for same-side target distractor presentat ions, F (2,50) = 5.44, p < .05, MSE = 10422. CEs were smallest i n the N o Fake Hands cond i t ion (77 ms) than either the Both Visible (169 ms) or Both H i d d e n condi t ions (137 ms) , p's < .05. CEs i n the latter t w o condi t ions d i d not d i f fer s igni f icant ly f r o m one another, p > .20. The same analysis as that above was computed us ing Error CEs as the dependent measure. O n l y the m a i n effect of Target-Distractor Side was signif icant, where CEs were larger w h e n targets and distractors were presented to the same side (6.1%) versus the opposite side (1.8%) of f i xa t ion , F ( 1 , 25) = 9.03, p < .01, MSE - 79. N o other effects were signif icant, F's < 1. Speed accuracy tradeoffs were not a concern as the error rates were either the same across condi t ions or their pat tern resembled that f o u n d i n the RT CEs. The second analysis focused on the t w o condi t ions where either the left or r i gh t fake h a n d was h i d d e n , Lef t H a n d H i d d e n and Right H a n d H i d d e n , respectively. The data were organized in to t w o factors that each had t w o levels, Side of Touch ( H i d d e n , Visible) and Distractor Side (Same, Opposi te) . The factor Side of Touch referred to the side of space that the tactile s t i m u l i was del ivered to, that is, whether i t was the side of the h i d d e n fake h a n d or that of the vis ible fake hand . Distractor Side referred to the side of space that the distractor l igh t 39 was del ivered to, that is, whether i t was the same side as the tactile s t imulus or the opposite side. A repeated measures A N O V A was computed o n the RT CEs us ing bo th Side of Touch and Distractor Side as factors (refer to the t w o r igh tmos t bars i n Figure 6A and 6B). The m a i n effect of Distractor Side was signi f icant, F (1 , 25) = 18.72, p < .01, MSE = 9750. This was tempered b y a signi f icant Side of Touch x Distractor Side interact ion, F (1 , 25) = 5.45, p < .03, MSE = 7968. To better in terpret the interact ion, the s imple m a i n effect of Side of Touch was examined separately for same and opposite side target-distractor presentat ions. The s imple m a i n effect of Side of Touch was signi f icant on ly w h e n the distractor was presented to the same side of space as the target. W h e n this was the case, CEs were s igni f icant ly larger for targets presented on the side of the vis ible fake hand (182 ms) compared to the side of the h i d d e n fake hand (106 ms) , F (1 , 25) = 7.33, p < .05, MSE = 10278. W h e n the above analysis was repeated us ing mean error CEs as the dependent var iable, a s imi lar pat tern of results was observed, and speed-accuracy tradeoffs were e l iminated as a concern. The m a i n effect of Distractor Side was signif icant, F (1,25) = 14.33, p < .001, MSE = 54. This was tempered b y a signi f icant Side of Touch x Distractor Side interact ion, F (1,25) = 6.09, p < .05, MSE = 86. The m a i n effect of Side of Touch was examined separately for Same and Opposi te target-distractor presentations. W i t h same side target-distractor presentations, error CEs tended to be larger for targets presented to the visible (12%) versus h i d d e n (7%) fake hand , F (1 , 25) = 3.07, p < .10, MSE = 111. W i t h opposite side target-distractor presentations, error CEs tended to be larger w h e n targets were presented to the h i d d e n hand (6%) than the vis ible h a n d (2%), F (1 , 40 25) = 3.26, p < .10, M S E = 59. This last result, although not significant in the RT CEs, is still consistent with the pattern observed there. Wi th 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 in 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 in 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 own 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 my hands", or "as if I was feeling the tactile vibration in the location where I saw the rubber hands", or even that it "seemed as if the lights were near to my real hands" (p. 357). In particular, the present results imply that the experience is only about 70% of the full possible strength5. One possible explanation for this apparent discrepancy in results from qualitative versus quantitative measures is that the observers tested in Experiments 1 and 2 d id not wear gloves on their hands to match the fake hands whereas observers tested in Pavani et al. (2000) did. That is, it may be that feeling the soft plastic on their hands, as wel l 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 own hands compared to when their hands were bare. 5 This estimation came from dividing the largest C E in the Fake Hand condition (128 ms) from that in the Real Hand Baseline (189 ms), and multiplying by 100. 42 E x p e r i m e n t 3 The m a i n quest ion addressed i n the present exper iment was whether a tactile sensory experience, i n add i t i on to a v isual one, cou ld inf luence the magn i tude of the fake h a n d effect. That is, can the experience of feel ing soft plastic on one's hands at the same t ime as seeing the soft plastic of the fake hands enhance the fake h a n d effect. To test this, observers w o r e a pa i r of soft plastic k i tchen gloves on their hands that matched those used to construct the fake hands. The pred ic t ion was that i f the feel of plastic o n one's hands increases tactile mislocal izat ions, then CEs w o u l d be larger w h e n observers w o r e a pai r of soft plastic gloves compared to w h e n their hands remained bare. Methods Part icipants: Fourteen undergraduate students at the Un ive rs i t y of Br i t ish Co lumb ia par t ic ipated i n the 45-minute exper imenta l session i n re tu rn for par t ia l course credit . A l l observers repor ted n o r m a l or corrected- to-normal v is ion. Further descr ipt ive in fo rmat ion on observers is p r o v i d e d i n Table 1. Observers were naive to the purpose of the exper iment and were f u l l y debr iefed u p o n comple t ion. Appara tus : Simi lar to that used i n Exper iment 2. Observers la id their arms o n the surface of a table ( w i t h i n a r m boundar ies o n either side), and underneath a smaller stand. They he ld a foam cube i n each hand . Each foam cube contained a pai r of tactile v ibrators as described i n Exper iment 1. A black c loth was draped over the surface of the smaller stand obst ruct ing a v i e w of the observer's hands. A n add i t iona l foam cube was a l igned above each of the observer's hands be low. This pai r of cubes he ld pairs of distractor LEDs. A g a i n , 43 these were as described i n Exper iment 1. O n hal f of the tr ials, observers were requi red to wear a pai r of p i n k soft plastic k i tchen gloves (a var ie ty of sizes were available). As w e l l , observers wo re a pai r of t h i n disposable g love l iners inside the k i tchen gloves for sanitary purposes. S t imul i : Same as that used i n Exper iment 2. The target was one of four possible tactile v ibrators, and the distractor one of four possible LEDs. Design: Observers par t ic ipated i n three t ra in ing sessions of 15 tr ials each, f o l l owed b y eight exper imenta l blocks of 96 tr ials. I n add i t i on to the w i t h i n -observer factors of Cond i t i on (No Fake H a n d , Fake H a n d ) , Congruency (Congruent , Incongruent ) , and Distractor Side (Same, Opposi te) , there was an add i t iona l wi th in-observer factor referred to as Observer Gloves (None, Plastic). Procedure: Same as that i n Exper iment 2. Results RT CEs and Error CEs were the m a i n measures of interest. They were calculated i n the same w a y as before ( Incongruent RT m inus Congruent RT). The analyses consisted m a i n l y of repeated measures A N O V A s . Signif icant t w o -w a y interactions of interest were fo l l owed u p b y s imple effects test ing w h i l e signi f icant m a i n effects of interest were fo l l owed u p us ing Least Signif icant Difference test ing. The m a i n f i n d i n g of interest i n the present s tudy was that CEs were the same magn i tude i n the Fake H a n d cond i t ion whether observers experienced the feel of soft plastic on their hands or their hands remained bare, see Figure 7. A d d i t i o n a l l y , as expected, a target-distractor separat ion effect was obta ined where CEs were consistently larger w h e n the target and distractor were 44 presented o n the same side of f ixat ion (see Figure 7A) than w h e n they were presented to opposite sides (see Figure 7B). A Same B Opposite 200 Condition • No Fake Hand Fake Hand Plastic None Observer Gloves 200 I" 160-1 % 120 HI o co 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 was compu ted us ing C o n d i t i o n (No Fake H a n d , Fake H a n d ) , Distractor Side (Same, Opposi te) and Observer Gloves (None, Plastic) as factors, and RT CEs as the dependent var iable. See Figure 7. O n l y the m a i n effects of Cond i t i on [F (1,13) = 10.77, p < .01, MSE = .3143] and Distractor Side [F (1,13) = 16.93, p < .01, MSE = 3692] were signif icant. A l l rema in ing p's > .10. These results indicate that CEs were larger overa l l i n the Fake H a n d cond i t ion (94 ms) versus the N o Fake H a n d cond i t i on (59 ms). As expected, CEs were larger overal l w h e n the target and distractor were presented on the same side of f i xa t ion (100 ms) versus the opposite side (53 ms). 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 that CE magni tudes were the same regardless of whether observers felt plastic on their hands or not . 45 The same analysis as that above was repeated us ing Error CEs as the dependent variable. O n l y the m a i n effect of Distractor Side [F (1,13) = 11.73, p < .01, MSE = 33] and the three-way interact ion of C o n d i t i o n x Distractor Side x Observer Gloves [F (1,13) = 8.82, p < .05, MSE = 8.5] were signif icant. The three-w a y interact ion was examined b y look ing at the s imple C o n d i t i o n x Distractor Side interact ion separately for Gloves and N o Gloves. W h e n the data was restr icted to the tr ials where observers wo re gloves, the s imple C o n d i t i o n x Distractor side interact ion d i d not reach signif icance, F < 1. W h e n the data was restr icted to tr ials where observers d i d not wear gloves, the s imple interact ion was signif icant, F (1,13) = 8.26, p < .05, MSE = 18. To unders tand i t better, i t was b roken d o w n b y l ook ing at the s imple effect of Cond i t i on for same and opposite side target-distractor presentations. W h e n targets and distractors were presented o n the same side, Error CEs were larger overal l i n the Fake H a n d cond i t ion (12%) than i n the N o Fake H a n d cond i t ion (4%), F (1,13) = 4.82, p < .05, MSE = 97. W h e n targets and distractors were presented o n the opposi te side, the s imple m a i n effect of Cond i t i on d i d not reach signif icance, F < 1. Since the Error CEs were either not signif icant, or their pat tern was consistent w i t h the pat tern of the RT CEs, speed accuracy tradeoffs were not a concern. Discussion The results of Exper iment 3 suggest that the feel of soft plastic does not enhance the fake h a n d effect. That is, the magn i tude of the CEs remained the same whether or not observers felt soft plastic on their hands. This suggests that the fake h a n d effect is not d r i ven b y the tactile experience of w e a r i n g gloves that match the v isua l i n fo rmat ion about the gloves. N o r is i t necessary to have a perfect match i n either the texture or v isua l mater ia l of the fake and 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 gloves6. 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 own 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 with the observer's hands but still continued to 'hold ' the distractor lights. Note that this manipulation also involved a change in posture. That is, when the fake hands were aligned with 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 in a prone posture, real hands in a supine posture). 6 Analyses using correct RTs and Errors as dependent variables revealed that the Observer Gloves main effect was not significant in either analysis, nor d id this factor interact with any of the other factors, p's > .05. 47 Experiment 4 Pavani et al. (2000) reported that the fake hand effect disappears when the fake hands are misaligned with 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 wi th the observer's hands, the authors also had the effect of changing the posture of the former wi th respect to the latter. It could be that in doing so, the fake hand's posture was seen as implausible (i.e., would 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 would be compromised by the misalignment of the fake hands. In order to test the contributions of postural matches in 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 in 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 in posture is an important factor in generating the larger CEs in the Fake Hand versus N o Fake Hand condition then one would 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 in both cases. If posture turns out to be a relevant factor to the fake hand effect, this would support the idea that it is the personal limb identification that the observer has with the fake hands that is relevant rather than the subsidiary effect of aligning the observer's gaze with 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 in the direction of the distractor lights in 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 in the 45-minute experimental session. Descriptive information can be found in Table 1. Apparatus: Similar to Experiment 3. Stimuli: Same as Experiment 3 Details: Observers participated in 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 Hand 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 Hand Condition (No Fake Hand, 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 in 576 trials (6 blocks * 96 trials per block). O n 192 of those trials the orientation of the fake hands was consistent wi th 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 with 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 with one exception. Observers in 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 wi th a heel lift when they localized the tactile vibration to their thumb (bottom digit). This was as before. In contrast, observers in the Supine condition were given the reverse instructions; to respond with a toe lift when the tactile vibration was localized to their thumb (top digit), and respond with a heel lift when the tactile vibration was localized to their finger (bottom digit). By giving these instructions to observers in 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 with a toe lift response, and a tactile vibration on the bottom digit was always paired with a heel lift response. Results The measures of interest were again the RT CEs and Error CEs. Mixed design A N O V A s were computed using Observer Hand Orientation (Prone, 50 Supine) as a between-observer factor and Fake Hand Condition (No Fake Hand, 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. When 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 in the Consistent condition for supine observer hands are in the direction of being larger than those in the N o Fake Hand condition, but the two are statistically similar). A Same B Opposite Fake Hand Condi t ion • No Fake Hand 51 Inconsistent • Consistent 200 150 $ 1 0 0 Prone Supine Observer Hand Orientat ion Prone Supine Observer Hand Orientat ion 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 were the dependent var iable, the m a i n effect of Distractor Side was signif icant, and indicated that CEs were larger overa l l for same side target-distractor presentations than for opposite side presentations (97 ms vs. 53 ms) , F ( 1 , 26) = 25.16, p < .01, MSE = 812. The Fake H a n d C o n d i t i o n x Observer H a n d Or ienta t ion interact ion was also signif icant, F (2, 52) = 6.83, p < .01, MSE = 2077. To better in terpret this interact ion, the s imple effect of Fake H a n d Cond i t i on was examined separately for Prone and Supine observer h a n d orientat ions. W h e n the observer's hands were i n a prone or ientat ion, the m a i n effect of Fake H a n d Cond i t i on was signif icant, F (2, 26) = 5.14, p < .05, MSE = 1280. Least s igni f icant difference test ing revealed that CEs were largest w h e n the fake hands were prone (94 ms) , rather than supine (72 ms) or absent (64 ms) , p's < .05. The CEs d i d not d i f fer i n the latter t w o cases, p > .20. W h e n the observer's hands were i n a supine or ientat ion, the m a i n effect of Fake H a n d C o n d i t i o n was again signif icant, F (2, 26) = 3.80, p < .05, MSE = 2874. CEs were largest overal l w h e n the fake hands were supine (95 ms) compared to p rone (55 ms) , p < .05. Nei ther pa i red compar ison i n v o l v i n g the N o Fake H a n d cond i t ion (72 ms) reached signif icance, p's > .10. The same analysis as that above was computed us ing Error CEs as the dependent variable. The m a i n effect of Distractor Side was signi f icant, F (1 , 26) = 22.68, p < .01, MSE = 3. Overa l l , CEs were larger for same side (6.8%) versus opposite side (3.0%) target distractor presentations. Since the error CEs were either not signif icant, or their pat tern is s imi lar to that f o u n d for RT CEs, this el iminates concern for speed accuracy tradeoffs. 52 Discussion The most i m p o r t a n t observation f r o m the above analysis was that the fake h a n d effect was most c o m p e l l i n g w h e n the orientat ion of the fake hands was consistent w i t h that of the observer's hands. In fact, w h e n the orientations were inconsistent, the pattern of C E s resembled those f o u n d i n the N o Fake H a n d s condi t ion . These results suggest that the fake h a n d effect occurs w h e n observers can incorporate the fake hands into their b o d y schema (i.e., treat t h e m as their own) . 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 ights, w h i c h w o u l d h a p p e n whether the orientat ion of the fake hands were consistent or inconsistent w i t h the orientat ion of the observer's hands. The present results are consistent w i t h the results of a s t u d y reported b y M a r a v i t a , Spence, Kennett a n d D r i v e r (2002) w h e r e i n observers incorporated a tool h e l d i n their hands into their b o d y schema. In a setup s i m i l a r to the one used presently, observers h e l d a pa i r of plastic golf clubs i n their hands , a n d the pa ir of tactile v ibrators was pos i t ioned o n the handle of the tool so that there was a tactile v ibrator o n the observer's finger a n d t h u m b . Observers were to report the locat ion of the target tactile v i b r a t i o n . A pa ir of distractor l ights p o s i t i o n e d 75 c m a w a y f r o m observer's hands , were 'connected' to the tactile v ibrat ions b y the tools. The authors reported the typ ica l pattern of C E s such that C E s were larger for same side versus opposite side target-distractor pairs . A s demonstrated i n the present E x p e r i m e n t 1, the C E is in f luenced b y target-distractor separation. G i v e n the distance between targets a n d distractors used b y M a r a v i t a et a l , the tools must have successfully b r i d g e d that distance, just as the 53 fake hands i n the present s tudy have. These f ind ings suggest that the tools were incorporated in to the observer's b o d y schema. Response mapp ings were assigned i n Exper iment 4 such that a tacti le v ib ra t ion del ivered to the top d ig i t was always pa i red w i t h a toe l i f t response ( top- toe) , and a tactile v ib ra t ion del ivered to the b o t t o m d ig i t was pa i red w i t h a heel l i f t response (bot tom-heel) . A re these response mapp ings relevant to the fake ha n d effect? W i l l the fake hand effect persist i f these response mapp ings are reversed such that a tactile v ib ra t ion del ivered to the top d ig i t is pa i red w i t h a heel l i f t response ( top-heel) , and a tactile v ib ra t ion de l ivered to the b o t t o m d ig i t is pa i red w i t h a toe l i f t response (bottom-toe)? Exper iment 5 was designed to address this quest ion. Experiment 5 There is a large l i terature da t ing f r o m the 1950's that deals w i t h the inf luence of st imulus-response compat ibi l i t ies on response t imes (e.g., Fitts & Seeger, 1953, see A l l u i s i & W a r m , 1990). Typ ica l ly , i n these exper iments, observers are asked to make a forced choice key press, a m a n u a l p o i n t i n g response, a joyst ick movement , or a verbal response to a v isua l s t imulus. Consistent ly w i t h i n that l i terature, response t imes are fastest w h e n there is a direct spatial t ranslat ion between the s t imulus and response (e.g., responses to a left v isual s t imulus are faster w h e n observers are asked to make a left versus r igh t keypress). A d d i t i o n a l l y , observers are faster to respond w h e n the response type is consistent w i t h the s t imulus type (e.g., manua l response for a spatial s t imulus or a vocal response for a verbal s t imulus; Proctor and 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 in which to conceptualize the factors involved in stimulus-response compatibility effects. Mapping 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 with the shoulder on the right). Included within this latter division is the orientation of the observer with 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 with a right-response and a bottom-stimulus paired with 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 in the left hemispace; Weeks, Proctor & Beyak, 1995). When Experiment 4 is considered within the framework of Chua 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 display pa i red w i t h a f ront-back response array). I t is possible that i n Exper iments 1-4, the response mapp ings assigned to observers took advantage of the most direct t ranslat ion between the vert ical s t imulus array and the f ront-back response array. That is, i t m a y be that the t ranslat ion between the top d ig i t to the toe l i f t ( f ront pedal) and between the b o t t o m d ig i t to the heel l i f t (back pedal) were the most direct st imulus-response translat ions for those par t icu lar arrays. I f so, this raises the possib i l i ty that the reverse response m a p p i n g assignments, where in a tactile v ib ra t ion del ivered to the top d ig i t is m a p p e d to a heel l i f t response, and a tactile v ib ra t ion del ivered to the b o t t o m d ig i t is m a p p e d to a toe l i f t response, m i g h t d is rup t the fake hand effect. Exper iment 5 was 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 condi t ions were inc luded w h e r e i n observers were asked to respond to a tactile v ib ra t ion o n the top d ig i t us ing a heel l i f t (back pedal) , and to a tactile v ib ra t ion o n the b o t t o m d ig i t us ing a toe l i f t ( f ront pedal) . These response mapp ings were the reverse of those assigned i n Exper iment 4. The data f r o m the t w o condi t ions i n Exper iment 5 were combined w i t h those f r o m Exper iment 4 so that i t was possible to look at a l l combinat ions of Dig i t -Response M a p p i n g ( top- toe & bo t tom-hee l versus top -hee l & bo t tom- toe) and the t w o levels of Observer H a n d Or ienta t ion (Prone, Supine) to see whether the CEs were the same under al l combinat ions. The same three fake hand condi t ions as those tested i n Exper iment 4 were again in t roduced: N o Fake Hands (1 /3 of tr ials), Consistent Fake Hands (1 /3 of t r ia ls), and Inconsistent Fake Hands (1 /3 of tr ials). 56 Methods Participants: Twenty-eight volunteers from the undergraduate Psychology program at the University of British Columbia participated in the 45-minute experimental session for partial course credit. Descriptive information can be found in Table 1. The remainder of the method was identical to Experiment 4 wi th the exception that the response mappings were reversed. This meant that a tactile vibration delivered to the top digit was always paired with a heel lift response, and a tactile vibration delivered to the bottom digit was always paired with a toe lift response. Thus, for observers in 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 in 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 with that of the previous experiment so that it was a complete 2 x 2 design with two levels of Digit-Response Mapping (top-toe & bottom-heel versus top-heel & bottom-toe) combined with two levels of Observer Hand Orientation (Prone, Supine). CEs were calculated in the same way as in 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 wi th respect to the target tactile vibration. 57 The dependent measures of interest were the RT CEs and Error CEs. M i x e d design A N O V A s were computed us ing Fake H a n d C o n d i t i o n (No Fake H a n d , Consistent, Inconsistent), and Distractor Side (Same, Opposi te) as w i t h i n -observer factors, and Digit-Response M a p p i n g ( top- toe & bo t tom-hee l versus top-hee l & bo t tom- toe) and Observer H a n d Or ienta t ion (Prone, Supine) as between-observer factors. Signif icant interactions and effects were f o l l o w e d u p us ing s imple effects test ing, and least signif icant dif ference test ing. There are three m a i n f ind ings. First, overal l CEs were larger w h e n the fake hands were present and i n a posture consistent w i t h that of the observer's hands. This is as expected based on the results of the prev ious exper iment . Second, the effect is weaker overal l w h e n observers adopt a top-heel & bot tom-toe m a p p i n g (see Figure 9) versus a top-toe & bot tom-heel m a p p i n g (see Figure 8). That is, for the former m a p p i n g assignment, CEs i n the Consistent Fake H a n d cond i t ion are no longer s igni f icant ly larger than those i n the remain ing t w o condi t ions. Three, the fake hand effect pat tern was altered i n the case where the observers' hands were i n a supine pos i t ion, and a top-hee l & b o t t o m - t o e m a p p i n g was assigned ( in this case, a tactile v ib ra t ion local ized to the t h u m b requi red a heel l i f t response and a tactile v ib ra t ion local ized to the f inger requi red a toe l i f t response). Under these condi t ions, the CEs were typ ica l ly negat ive, and their absolute values smaller than the CEs for the other combinat ions (see the r igh tmost bars of Figure 9A) . No te that a negat ive CE i n this case reflects the fact that observers are faster on incongruent versus congruent tr ials. I n other w o r d s , under these condi t ions, observers are faster to respond, for example, to a tactile v ib ra t ion on the t h u m b (top d ig i t ) w h e n i t is 58 paired with 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 with that of the observer's hand (i.e., the fake hand was prone, the observer's hand was supine). A 2 within (Fake Hand Condition and Distractor Side) and 2 between (Digit-Response Mapping and Observer Hand Orientation) mixed design A N O V A was computed. A l l of the main effects were significant, p's < .05: Digit-Response Mapping [F (1, 52) = 18.22, p < .01, M S E = 13347], Observer Hand 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 Hand Condition indicated that CEs were larger overall in the Consistent condition (61 ms) than either the N o Fake Hand (45 ms) or Inconsistent condition (39 ms), p's < .05. The comparison between the latter two conditions did not reach significance, p > .20. The other main effects were tempered by significant interactions of Response Mapping x Observer Hand Orientation [F (1,52) = 7.89, p < .01, M S E = 13347], Distractor Side x Response Mapping [F (1,52) = 4.68, p < .05, M S E = 4607], Distractor Side x Observer Hand Orientation [F (1,52) = 10.91, p < .01, M S E = 4607], and Distractor Side x Response Mapping x Observer Hand 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 « 150 Prone Supine Observer Hand Orientation 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 target-distractor presentations. The 3-way interaction above was followed up by looking at the simple interaction of Digit-Response Mapping x Observer Hand Orientation for same and opposite side target-distractor presentations. When targets and distractors were presented to the same side of fixation, the Response Mapping x Observer Hand Orientation interaction was significant, F (1, 52) = 10.38, p < .01, M S E = 12450. This was further broken down by looking at the simple effect of Response Mapping for prone and supine hand positions. When the observer's hands were prone, the main effect of Response Mapping 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. When targets and distractors were presented to the opposite side of fixation, the simple interaction of Digit-Response Mapping x Observer Hand Orientation was not significant, F (1,52) = 1.80, p > .10, M S E = 5504. 60 Af te r examin ing the data i n Figures 8 and 9, i t seemed appropr ia te to do four add i t iona l , bu t more specific comparisons to test whether the fake hand effect var ied across the four combinat ions of Digit-Response M a p p i n g and Observer H a n d Or ienta t ion. 7 W h e n the observer's h a n d was prone and a top-toe & bot tom-heel m a p p i n g was assigned (see Figure 8), the m a i n effect of Cond i t i on was signif icant, F (2, 26) = 5.14, p < .05, MSE = 1280. CEs were larger for the Consistent Fake H a n d cond i t ion (94 ms) than either the N o Fake H a n d (64 ms) or Inconsistent condi t ions (72 ms) , p's < .05. W h e n the observer's hand was prone b u t a top-heel & bot tom-toe m a p p i n g was assigned (see Figure 9), the m a i n effect of Cond i t i on was no longer signif icant, F (2, 26) < 1, MSE = 5390. CEs were the same magn i tude for the N o Fake H a n d (59 ms) , Consistent Fake H a n d (53 ms) , and Inconsistent Fake H a n d condi t ions (62 ms). W h e n the observer's hand was supine and a top-toe & bot tom-heel m a p p i n g was assigned (see Figure 8), the m a i n effect of Cond i t i on was again signif icant, F (2, 26) = 3.80, p < .05, MSE = 2875. The CEs i n the Consistent cond i t ion (95 ms) were larger than those i n the Inconsistent cond i t ion (55 ms) , p < .05. N o comparisons w i t h the N o Fake H a n d cond i t ion (70 ms) reached signif icance, p's > .10. F ina l ly , w h e n the observer's h a n d was supine and a top-heel & bot tom-toe m a p p i n g was assigned (see Figure 9), the m a i n effect of Cond i t i on on ly approached signif icance, F (2, 26) = 2.60, p < .10, MSE = 3484. CEs were larger i n the Inconsistent (-33 ms) versus the Consistent (3 ms) condi t ions, p < .05. N o compar isons w i t h the N o Fake H a n d cond i t ion reached signif icance, p's > .20. 7 These comparisons were computed even t h o u g h the C o n d i t i o n x D ig i t -Response M a p p i n g x Observer H a n d Or ientat ion interact ion was no t signif icant. 61 The overal l analysis above was computed us ing Error CEs as the dependent 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, MSE = 62] and Distractor Side [F (1,52) = 19.30, p < .01, MSE = 31] were signif icant. These m a i n effects were tempered b y signi f icant interact ions of Digit-Response M a p p i n g x Observer H a n d Or ienta t ion [F ( 1 , 52) = 4.50, p < .05, MSE = 62], and Distractor Side x Digit-Response M a p p i n g [F ( 1 , 52) = 5.23, p < .05, MSE = 31]. These interactions were fo l l owed u p b y f i rst l o o k i n g at the s imple m a i n effect of Digit-Response M a p p i n g for prone and supine observer hands. W h e n the observer's hands were prone, the s imple m a i n effect was not signif icant, F (1 , 26) = 1.75, p > .20, MSE = 61. W h e n the observer's hands were supine, CEs were larger overal l for the top- toe and bo t tom-hee l m a p p i n g (5%) versus the top-hee l and bo t tom- toe m a p p i n g (0%), F ( 1 , 26) = 18.55, p < .01, MSE = 62. The Distractor Side x Digit-Response M a p p i n g in teract ion was f o l l o w e d u p b y look ing at the s imple m a i n effect of Digit-Response M a p p i n g for same and opposite side target-distractor presentations. W h e n targets and distractors were presented to the same side of f ixat ion, CEs were larger w h e n the m a p p i n g assignment was top- toe and bo t tom-hee l (7%) versus top-hee l and bo t tom- toe (2%), F (1,52) = 16.27, p < .01, MSE = 65. W h e n targets and distractors were presented to the opposi te side of f ixat ion, CEs were larger for a top - toe and bo t tom-hee l m a p p i n g assignment (3%) versus top-hee l and 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, MSE = 27. Discussion There were t w o m a i n f ind ings i n this Exper iment . 62 Digit-Response Mapping and Observer Hand Orientation influence the effect: The definitive pattern for the fake hand effect of larger CEs in the Consistent Fake Hand condition than the remaining two conditions is less pronounced when the observer adopts a top-heel & bottom-toe mapping. This is especially true in 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 Top-Heel & 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 with the elevation of the tactile target. This reverse C E pattern is only observed for this particular combination of observer hand orientation and digit-response mapping. Additionally, when the orientation of the fake hand is prone, and thus the visual information is inconsistent with 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 in this particular condition observers may have mentally rotated their hand to the prone orientation, as consistent with the available visual information. 63 General Discussion 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 in 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 with 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 "Can tactile targets be mislocalized to a new spatial location that is initially specified solely by vision?" The subjective impressions collected from observers in the Pavani et al. (2000) study suggested that the tactile vibrations were mislocalized to the digits of a fake hand. The authors, however, d id not include an appropriate baseline measure with 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 in addressing this question was to introduce a new baseline condition wherein the observer's hands were positioned in 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 ibrat ions was the same as the subjective locat ion of the tacti le v ibrat ions no rma l l y induced b y the presence of the fake hands. This key baseline cond i t ion revealed that, i n contrast to the qual i tat ive and subjective reports of observers, the quant i tat ive measurement of the fake hand effect ind icated that the experience was not ident ical to that of hav ing one's o w n h a n d i n the same locat ion as the fake hand . Rather, the fake h a n d effect resul ted i n a weaker measure of perceptual congruency than w h e n the observer's hands occupied that same locat ion. Despite these quant i tat ive differences, however , there were qual i tat ive simi lar i t ies i n response patterns across these t w o condi t ions i n the w a y that observers responded to manipu la t ions of target-distractor spatial separat ion. This latter result is consistent w i t h observer's subjective impressions ind ica t ing that they ident i f ied w i t h the l imb to some extent. I n order to determine whether the fake h a n d effect depended o n observers seeing the fake hand , or whether i t was suff ic ient to have par t ia l v isua l i n fo rmat ion consistent w i t h its presence, the fake hand was h i d d e n under a c loth cover (Exper iment 1) or under a box cover that e l iminated v isua l i n fo rma t ion about its shape (Exper iment 2). The fake h a n d effect persisted i n b o t h cases. This is the f i rst piece of evidence that the effect is not d r i v e n pu re l y b y direct v is ion , b u t rather, m a y arise f r o m exist ing knowledge that is re in forced t h r o u g h par t ia l v is ion. The results of Exper iment 2, however , also ind icated that w h e n there was b o t h a h i d d e n fake hand and a visible fake h a n d , the effect was smaller on the side of the h i d d e n hand. This suggests that observers can reassign weights to d i f ferent sources of i n fo rmat ion depend ing o n w h a t is available so that they can make the best estimate about the locat ion of a tacti le s t imulus. That 65 is, in the case where there is direct visual information in one location, and indirect visual information in 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 with 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 with that of the observer's hands. The fake hand effect was present only in the former case. That is, the effect disappeared when the rotational posture of the fake hands was inconsistent with that of the observer's hands. This was true despite the fact that the fake hands were always aligned with 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 in 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 & bottom-toe mapping than for the reverse mapping. Furthermore, when the observer's hands were in a supine orientation and a top-heel & bottom-toe mapping was assigned, the effect was not only weaker overall, but the CEs were in 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 wel 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 in a prone orientation (i.e., inconsistent). One way to reconcile this pattern of results wi th the others is by imagining that observers responded by first mentally rotating their hand into a prone or default position before responding. This would have the effect of making the negative CEs positive, and a prone fake hand consistent wi th 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: As noted throughout the present study, tactile localization in three-dimensional space is influenced by vision, even when the available visual and proprioceptive information conflict. Knowing that the tactile stimuli are delivered to the digits of their own 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 with 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 wi th 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 with the earlier findings of Pavani et al. (2000) that to obtain the fake hand effect the fake hands had to be aligned with the observer's body in 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 who were missing a limb and who reported experiencing a 'phantom' in 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) in 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 would 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 way 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 in an imagery session, where they 69 try to picture their limb in 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 palm prior to employing the mirror technique. Human-Machine Interactions: Another possible application of the present results to the real wor ld is in the domain of design issues with 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 in a location that is removed from the actual site (e.g., doctor in Vancouver performing an operation on a patient in Burnaby, with surgical feedback provided on a video monitor). Visual feedback in this case is thus separated from the proprioceptive feedback, in much the same way that the visible fake hands in 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 in performance (Hanna, Shimi, & Cuschieri, 1998; MacKenzie, Graham, Cao, & Lomax, 1999; Mandryk & MacKenzie, 2000). Results reported by Mandryk 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 of the robot ic arms w i t h respect to the surgeon's arms m a y be a cri t ical factor for successful use of the tools. I t m a y be problemat ic , for example, to have the robot ic arms operat ing i n a supine posture w h i l e the surgeon's arms are prone. A d d i t i o n a l l y , the present results suggest that the combina t ion of the posture of the arms and the m a p p i n g between the available v isua l i n fo rma t ion (st imulus) and the requi red action (response) m a y also inf luence the eff iciency and accuracy of the surgeon's responses. Further research is needed, however , to determine whether rotat ional inconsistencies between the fake hands and the observer's hands are as impor tan t w h e n they are explicable b y someth ing such as camera angle. I f there is a reasonable explanat ion for these differences (e.g., m i r r o r reversal, or projected on a mon i to r so dependent on camera angle), then i t m a y be the case that the fake h and effect w i l l persist even w h e n the rotat ional postures are inconsistent. Outstanding Questions and Future Directions Is the Fake H a n d Effect Generalizable to N o n - H a n d Objects? Measures were taken i n the present s tudy to construct the fake h a n d so that i t was unden iab ly hand- l ike. The soft plastic glove, for instance, was stuf fed w i t h cot ton bat t ing to give i t a f u l l appearance, i nc lud ing the d ig i ts of the fake hand . But , is i t cr i t ical that the v isua l object next to the distractor l ights was a hand? Perhaps there are cri t ical propert ies of the fake h a n d that led to the tactile mislocal izat ions. I f so, there is a chance that these propert ies can also be f o u n d i n non-hand objects. The quest ion thus remains as to w h a t i t is about the presence of the fake h a n d that leads to tactile mislocal izat ions. 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 would 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, in turn, influenced tactile mislocalizations. If so, it is likely that any object that highlighted the elevations would 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 word labels such as 'top light' and 'bottom light' were assigned to the distractor light elevations. Would 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 factor8. Would an imagined hand be sufficient for the fake hand effect? In the Fake Hand conditions of the present study, observers always saw the fake hand as it was put in position, and thus always saw it before it was 8 Consistent with 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. 72 h i d d e n f r o m v iew. The current t h i n k i n g on the fake hand effect is that the available v isua l i n fo rmat ion is cr i t ical for p r o d u c i n g the effect. C o u l d i t be the case, however , that s imp ly imag in ing the presence of a fake h a n d next to the distractor l ights w o u l d be suff icient for the effect i n the absence of any v isua l cues to its presence? To test this, one cou ld compare the effect across t w o groups, where one on ly imag ined the fake hand was present, and the other actual ly saw the fake hand 9 . No te that the results of the v is ib i l i t y m a n i p u l a t i o n i n the present s tudy cannot d is t ingu ish between whether the par t ia l v isua l cues to the fake hand's presence were cr i t ical, or whether w h e n the fake h a n d was h i d d e n , observers were re ly ing o n a menta l image of the hand . Is the fake h a n d effect mul t isensory or mul t i in fo rmat iona l? Consistent w i t h the quest ion above is the idea that the fake h a n d effect is the result of in tegrat ing the available v isual and tactile i n fo rmat ion . Acco rd ing to Pavani et al. (2000), for example, the distractor l ights captured the locat ion of the tactile v ibrat ions and this was enhanced b y the presence of the fake hand . One m i g h t ask, however , whether i t was necessary that the distractors were v isual (i.e., d i f ferent moda l i t y than the target moda l i t y ) . Ano the r w a y to t h i n k about this is to ask whether the fake hand effect is a p roduc t of present ing i n fo rmat ion i n more than one moda l i t y (e.g., v isual and tacti le), or s i m p l y the p roduc t of present ing mu l t i p le sources of in fo rmat ion . To test this, one cou ld test whether an add i t iona l source of tactile i n fo rmat ion leads to the same result. 9 I n such an exper iment , the g roup par t ic ipat ing i n the Imag ined Fake H a n d cond i t ion w o u l d a lways have to part ic ipate i n the N o Fake H a n d cond i t ion f i rst , as the instruct ions to imagine a fake hand m i g h t interfere w i t h the results of the N o Fake H a n d cond i t ion were i t to come first. 73 What is the Role of A t ten t ion i n the Fake H a n d Effect? Even t h o u g h observers are to ld that the elevat ion of the distractor l ights is i r relevant to their task of local iz ing the target tactile v ibra t ions, the distractor l ights nevertheless inf luence responses. This suggests that the onset of the l ights m i g h t capture at tent ion, and do so even w h e n they are separated i n space f r o m the tactile v ibrat ions, as evidenced b y the measurable CEs at the large v ib ra t ion-l ight separat ion i n the present Exper iment 1. This raises the quest ion of whether the fake h a n d effect w o u l d be reduced i f at tent ion were w i t h d r a w n f r o m the l ights (e.g., b y hav ing observers p e r f o r m a secondary task such as coun t ing backwards i n threes f r o m a g iven number ) . Ano ther w a y of t h i n k i n g about this is to ask w h a t w o u l d happen to the fake h a n d effect i f observers were instructed to at tend to the distractor l ights rather than ignore them. A l t h o u g h such manipu la t ions w o u l d l ike ly change the overal l magn i tude of the CEs, the relat ive difference between CEs i n the N o Fake H a n d versus Fake H a n d condi t ions w o u l d l i ke ly persist since the fake hand effect seems to be someth ing over and above the at tent ion captur ing effects of the distractor l ights (i.e., CEs increase s imp ly b y hav ing the fake h a n d ' h o l d ' the distractor l ights) . I n the present Exper iment 3, the magn i tude of the fake h a n d effect d i d not change w h e n observers w o re a pai r of soft plastic gloves on their hands. I t was concluded that p r o v i d i n g observers w i t h tactile i n fo rma t ion that was congruent w i t h the fake h a n d d i d not alter the effect. Ano ther in terpre ta t ion of these results, however , is that wear ing the gloves had the added effect of d r a w i n g at tent ion to the observer's hands, thereby canceling out any benefits of p r o v i d i n g observers w i t h congruent tactile in fo rmat ion . There are at least a couple of ways to determine whether this m i g h t be the case. One w a y is to a l low 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 would allow one to separate the effects of having access to congruent tactile information from those of drawing attention to the observer's hand. Why is Space an Important Factor in the Fake Hand 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 with 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 wi th the tactile receptive field (see evidence for bimodal neurons in macaque monkeys in 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 in space, and the visual information no longer falls within the visual receptive field of the bimodal neuron. Are there individual differences in the fake hand effect? Although the majority of observers in the present study showed the fake hand effect, a small number did 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 would be a natural next step to test the magnitude of the effect in children who are likely more susceptible to suggestion than adults. Time-line of the fake hand effect? Since Condition (No Fake Hand, Fake Hand) was a within-observer manipulation in 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 Hand condition came first or followed the N o Fake Hand condition 1 0. This is consistent wi th the findings of Botvinick and Cohen (1998) who 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 would be interesting to see whether the fake hand effect could be disrupted simply by including trials of both temporally matched and mismatched stimuli together. 1 0 A mixed design A N O V A was computed on the RT CEs in Experiment 3 using Condition (No Fake Hand, Fake Hand) as the within-observer factor and Order (Fake Hand first, Fake Hand second) as the between-observer factor. Only the main effect of Condition reached significance, F (1,12) = 9.0, p < .05, M S E = 252. 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T ipper , S.P., Phi l l ips, N. , Dancer, C , L l o y d , D., H o w a r d , L.A., & McGlone , F. (2001). V is ion inf luences tactile percept ion at b o d y sites that cannot be v i e w e d direct ly. Exper imenta l Bra in Research, 139,160-167. 82 Warren, D . H . , & Rossano, M.J. (1991). Intermodality relations: Vis ion and touch. In M . A . Heller & W. Schiff (Eds.). The psychology of touch. Hillsdale, NJ: Lawrence Erlbaum Associates. Warren, D . H . , & Schmitt, T.L. (1978). On the plasticity of visual-proprioceptive bias effects. Journal of Experimental Psychology: Human Perception and Performance, 4, 302-310. Weeks, D.J., Proctor, R.W., & Beyak, B. (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). New 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 Descr ipt ive i n fo rmat ion for each Exper iment . Exper iment N Ma les Females R i g h t H a n d e d Le f t H a n d e d N o Correct ive Eyewear Contacts Glasses E l 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 in Experiment 1-5. Experiment/ | Within-Observer Manipulations Between Observer | Manipulations | Exp 1 N = 42 | Condition | Visibility Separation Congruency N o Fake Hand | Visible None Congruent Fake Hand | Hidden Small Incongruent Real Hand Baseline | Large 6 blocks (1 block = 96 trials each) = 576 trials 2 * 3 * 2 = 12 trial types -> 48 trials/type Exp 2 N = 13 T-D Side Congruency Same Congruent Opposite Incongruent Condition N o Fake Hand Both Visible Both Hidden One Hidden Hidden Side One Hidden Visible Side 5 blocks (1 block = 96 trials each) = 480 trials 5 * 2 * 2 = 20 trial types -> 24 trials/type 85 E x p 3 N = 14 | | Condition Gloves T-D Side Congruency | N o Fake H a n d None Same Congruent | Fake H a n d Plastic Opposi te Incongruent | 8 blocks (1 b lock = 96 tr ials each) = 768 tr ials | 2 * 2 * 2 * 2 = 16 t r ia l types -> 48 t r i a l s / t y p e E x p 4 N = 28 Observer Hand Orientation Condition T-D Side Congruency Same Congruent Opposi te Incongruent Prone | N o Fake H a n d Supine | Consistent Inconsistent 6 blocks (1 b lock = 96 tr ials each) = 576 tr ials 3* 2 * 2 = 12 t r ia l types -> 48 t r i a l s / t y p e * Response M a p p i n g assignment is Top Tactile V ib ra t ion to Toe Pedal Response and Bot tom Tacti le V ib ra t ion to Heel Pedal Response (Top-Toe and Bot tom-Hee l ) . Exp 5 N = 28 Same as Exper iment 4, except Response M a p p i n g assignment is Top Tacti le V ib ra t ion to Heel Pedal Response and Bo t tom Tacti le V ib ra t i on to Toe Pedal Response (Top-Hee l and Bot tom-Toe) 86 Appendix Experiment 1: In order to ensure that using difference scores referred to as CEs did 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, with one exception. When the data was restricted to visible trials where there was no target-distractor separation, the differences in correct RTs for the Real Hand Baseline and Fake Hand condition d id 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), Visibili ty (Visible, Hidden) and Congruency (Congruent, Incongruent) as within-observer factors, and Condition (No Fake Hand, Fake Hand, Real Hand Baseline) as a between-observer 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 down 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 down 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. When the simple effect of Condition was tested at each level of Separation, it was significant only when there was no horizontal target-distractor separation, F (2, 39) = 6.50, p < .01. Least significant difference testing revealed that RTs were significantly faster in the N o Fake Hand condition (577 ms) than in either the Fake Hand (717 ms) or Real Hand 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 Hand [F (2, 26) = 6.66, p < .01] and Real Hand Baseline conditions [F (2, 26) = 12.93, p < .001]. Least significant difference testing revealed that for the Fake Hand 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 with the small separation (671 ms) reached significance, p's > .05. For the Real Hand Baseline condition, RTs were significantly slower when there was no target-distractor n This finding deviates slightly from the C E data where CEs were significantly larger overall in the Real Hand Baseline condition than in the Fake Hand condition. 88 separation (766 ms) than w h e n either there was a smal l (640 ms) or large separation (601 ms) , p's < .01. The remain ing compar ison d i d not reach signif icance, p > .10. The same overal l analysis as that repor ted above was compu ted us ing percentage errors as the dependent variable. Since the error rates were either not signif icant, or were i n the same di rect ion as the correct RTs, speed accuracy tradeoffs were not a concern. The m a i n effects of Separation [F (2, 78) = 4.42, p < .05] and Congruency [F ( 1 , 39) = 74.39, p < .01] were signif icant. This was tempered b y a signi f icant Separation x Congruency interact ion, F (2, 78) = 6.98, p < .01. This interact ion was examined b y look ing at the s imple effect of Separation for Congruent and Incongruent tr ials. Separation was a signi f icant factor on ly for Incongruent tr ials, F (2, 78) = 6.40, p < .01. Least signif icant dif ference test ing revealed that errors were larger overa l l w h e n there was no target-distractor separat ion (9%) than for either the smal l (6%) or large (6%) separations, p's < .01. The error rates for the latter t w o separations d i d not d i f fer f r o m one another, p > .20. Experiment 2: Correct RT Analysis # 1: Most of the signi f icant effects were observed i n the mean Incongruent RTs bu t not i n the mean Congruent RTs. Overa l l , the pat tern of Incongruent RTs was s imi lar to that for the RT CEs w i t h t w o exceptions. One, the m a i n effect of Distractor Side was no t s igni f icant i n the correct RT data b u t was signi f icant i n the RT CE data. T w o , w h e n the data analysis was restr icted to opposite-side 89 target distractor presentations, the main effect of Condition was significant in 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 significant1 2. These main effects were tempered by 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 in the N o Fake Hands condition (644 ms) than either the Both Visible (764 ms) or Both Hidden (711 ms) conditions, F (2,50) = 8.64, p < .01. The comparison between the latter two conditions did not reach significance, p > .05. When the target-distractor presentations were on opposite sides of fixation, the main effect of 1 2 The main effect of Distractor Side did not reach significance in the correct RT data but was significant in the RT C E data. 90 Cond i t i on was signif icant, F (2,50) = 3.48, p < .05. 1 3 A g a i n , RTs were s igni f icant ly faster i n the N o Fake Hands cond i t ion (629 ms) than either the Both Visible (668 ms) or Both H i d d e n (669 ms) condi t ions, F (2, 50) = 8.64, p < .01. The compar ison between the latter t w o condi t ions d i d not reach signif icance, p > .20. W h e n the data was l im i ted to Congruent tr ials, on ly the m a i n effect of Distractor Side was signif icant where RTs were faster overa l l for same side target-distractor presentations (581 ms) than for opposi te side presentations (615 ms) , F (1 , 25) = 13.62, p < .01. Error data analysis #1: The overa l l analysis repor ted above was repeated us ing mean percentage errors as the dependent variable. Signif icant effects were again seen on ly on Incongruent tr ials where the patterns were s imi lar to the Error CEs. I n general, the effects were either not signif icant or were i n the d i rec t ion of the Correct RTs, so speed-accuracy tradeoffs were not a concern. The m a i n effect of Congruency [F (1,25) = 23.78, p < .01] and Target-Distractor Side [F (1 , 25) = 9.62, p < .01] were signif icant. These were tempered b y a signif icant Congruency x Target-Distractor Side in teract ion, F ( 1 , 25) = 9.03, p < .01. This interact ion was fo l l owed u p b y l ook ing at the s imple effect of Target-Distractor Side for incongruent and congruent tr ials. The s imple effect was signi f icant on l y on Incongruent tr ials, where observers made more errors overal l for same side (9%) versus opposite side target-distractor presentations (4%), F (1 , 25) = 19.24, p < .01. Remain ing p > .20. 1 3 This is d i f ferent f r o m the RT CEs where the m a i n effect of C o n d i t i o n was signif icant on ly for same side target-distractor presentations. 91 Correct RT Analysis # 2: Signif icant effects were observed on ly for the mean Incongruent RTs bu t not the mean Congruent RTs. Overa l l , the pat tern of Incongruent RTs was s imi lar to that for the RT CEs. A repeated measures A N O V A was computed o n correct RTs us ing Side of Touch ( H i d d e n , Vis ib le) , Distractor Side (Same, Opposi te) , and Congruency (Congruent , Incongruent) as factors. The m a i n effect of Congruency [F ( 1 , 25) = 51.69, p < .01] and that of Distractor Side [F (1 , 25) = 5.34, p < .05] were signif icant. These m a i n effects were tempered b y a signi f icant Congruency x Distractor Side interact ion [F (1 , 25) = 18.72, p < .001], Side of Touch x Distractor Side interact ion [F (1 , 25) = 12.51, p < .01], and a Congruency x Side of Touch x Distractor Side interact ion, F (1 , 25) = 5.45, p < .05. To better in terpret the three-way interact ion, the s imple in teract ion of Side of Touch x Distractor Side was examined separately for Congruent and Incongruent tr ials. W h e n the data was l im i ted to Incongruent tr ials, the Side of Touch x Distractor Side interact ion was signif icant, F (1 , 25) = 9.89, p < .05. This interact ion was fur ther examined b y look ing at the s imple effect of Distractor Side w h e n the tactile s t imulus was presented o n the side of the vis ible fake hand or o n the side of the h i d d e n fake hand . The s imple effect of Distractor Side was signif icant on ly w h e n targets and distractors were presented o n the side of the visible fake hand , F ( 1 , 25) = 20.85, p < .01. Remain ing p > .20. Overa l l , RTs were slower for same side target-distractor presentations (771 ms) versus opposi te side presentations (658 ms). W h e n the data was l im i ted to Congruent tr ials, the s imple in teract ion of Side of Touch x Distractor Side d i d not reach signif icance, F ( 1 , 25) = 1.55, p > ,20. 92 Error data analysis #2: The overal l analysis repor ted above was repeated us ing mean percentage errors as the dependent variable. Signif icant effects were again seen on ly on Incongruent tr ials where the patterns were the same as the Error CEs. Since the effects were either no t signif icant or were i n the d i rect ion of the Correct RTs, speed-accuracy tradeoffs were not a concern. The m a i n effects of Congruency [F (1,25) = 22.50, p < .01] and Target-Distractor Side [F (1 , 25) = 14.18, p < .01] were signif icant. These were tempered b y signi f icant interact ions of Congruency x Distractor Side [F ( 1 , 25) = 15.11, p < .01] and Congruency x Distractor Side x Side of Touch [F ( 1 , 25) = 6.51, p < .05]. To better unders tand the three-way interact ion, the s imple in teract ion of Side of Touch x Distractor Side was examined for Incongruent and Congruen t tr ials. The s imple interact ion was signi f icant on ly o n Incongruent tr ials, F ( 1 , 25) = 4.23, p < .05. Remain ing p > .05. The s imple effect of Distractor Side was examined separately for touch on the occluded or visible side. The s imple effect of Distractor Side was signi f icant on ly w h e n the tactile s t imu lus was presented on the side of the visible fake hand , where errors were h igher for same side (13%) versus opposite side (4%) presentations, F (1,25) = 18.10, p < .01. Experiment 3: Most of the signi f icant effects were observed i n the Incongruent RT data, b u t not i n the Congruent RT data. I n the fo rmer case, the pa t te rn of results was simi lar to that observed for the RT CEs. 93 A four factor repeated-measures A N O V A was compu ted us ing correct RTs as the dependent var iable, and Cond i t i on (No Fake H a n d , Fake H a n d ) , Distractor Side (Same, Opposi te) , Observer Gloves (None, Plastic), and Congruency (Congruent , Incongruent) as factors. The m a i n effects of Cond i t i on [F (1,13) = 8.62, p < .05] and Congruency [F (1,13) = 27.73, p < .05] were signi f icant 1 4 . These were tempered b y signi f icant interact ions of C o n d i t i o n x Congruency [F (1,13) = 10.77, p < .01], and Distractor Side x Congruency [F (1 , 13) = 16.93, p < .01. Both interactions were fo l l owed u p us ing s imple effects test ing. W h e n the analysis was restr icted to Congruent tr ials, on ly the s imple m a i n effect of Distractor Side was signif icant where RTs were faster overa l l w h e n the target and distractor were presented to the same side of f i xa t ion (569 ms) versus the opposi te side (590 ms) , F (1,13) = 34.06, p < .001. W h e n the analysis was restr icted to Incongruent tr ials, bo th the s imple m a i n effects of C o n d i t i o n [F (1,13) = 10.44, p < .01] and Distractor Side [F (1,13) = 7.13, p < .05] were signif icant. RTs were faster i n the N o Fake H a n d cond i t ion (636 ms) versus the Fake H a n d cond i t ion (677 ms) , and faster w h e n the target and distractor were presented to opposite sides of f i xa t ion (643 ms) versus the same side (670 ms). The overal l analysis repor ted above was repeated us ing mean percentage errors as the dependent variable. Signif icant effects were observed m a i n l y o n Incongruent tr ials where the patterns were the same as the Error CEs. Since the effects were either not signif icant or were i n the d i rect ion of the Correct RTs, speed-accuracy tradeoffs were not a concern. 1 4 The m a i n effect of Distractor Side was not s igni f icant i n the correct RT analysis b u t was i n the RT CE analysis. 94 The same analysis as that above was computed us ing errors as the dependent variable. The m a i n effect of Congruency was signi f icant, F (1,13) = 10.53, p < .01. This was tempered b y signif icant interact ions of C o n d i t i o n x Distractor Side [F (1,13) = 5.54, p < .05], and Congruency x Dist ractor Side [F (1 , 13) = 11.73, p < .01]. The fou r -way interact ion also reached signif icance, F (1,13) = 8.82, p < .05. To better comprehend the fou r -way interact ion, i t was b roken d o w n in to the s imple three-way interact ion of C o n d i t i o n x Congruency x Distractor Side w h e n observers either wo re gloves or d i d not . W h e n observers wo re gloves o n their hands, the s imple three-way interact ion d i d no t reach signif icance, F (1,13) < 1, ns. W h e n observers d i d not wear gloves, the s imple three-way interact ion was signif icant, F (1,13) = 8.26, p < .05. This was fur ther f o l l owed u p b y l ook ing at the s imple Distractor Side x C o n d i t i o n in teract ion for Incongruent and Congruent tr ials. The s imple interact ion was signi f icant on ly for Incongruent tr ials, F (1,13) = 6.95, p < .05. Remain ing p > .20. The s imple effect of Cond i t i on was examined for same and opposi te side target-distractor presentations, and reached significance on ly for the fo rmer where observers tended to make more errors i n the Fake H a n d cond i t ion (14%) than the N o Fake H a n d cond i t ion (6%), F (1,13) = 4.25, p < .07. Experiment 4: A m ixed design A N O V A was computed us ing correct RT as the dependent var iable, Observer H a n d Or ienta t ion (Prone, Supine) as a between-observer factor, and Fake H a n d Cond i t i on (No Fake Hands , Prone, Supine), Distractor Side (Same, Opposi te) , and Congruency (Congruent , Incongruent) as wi th in-observer factors. The m a i n effects of Observer H a n d Or ien ta t ion [F (1 , 26) 95 = 23.46, p < .01], Congruency [F (1, 26) = 63.97, p < .01], and Fake Hand Condition were significant, F (2, 52) = 5.48, p < .011 5. These main effects were tempered by significant interactions of Congruency x Distractor Side [F (1,26) = 25.16, p < .01], Fake Hand Condition x Distractor Side [F (2,52) = 4.11, p < .05], and Congruency x Fake Hand Condition x Observer Hand Orientation [F (2, 52) = 6.83, p < .01]. The Fake Hand Condition x Distractor Side interaction was followed up by looking at the simple main effect of Fake Hand 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. Mean 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 Hand 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 Hand Condition for congruent and incongruent trials. The simple main effect was significant only in 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. When the observer's hands were supine, the simple interaction of Fake Hand Condition x Congruency was significant, F (2, 26) = 3.80, p < .05. The simple main effect of Fake Hand 1 5 The main effect of Distractor Side was not significant in the correct RT data but was in the RT C E data. Cond i t i on was signi f icant on ly o n Congruent tr ials, F (2, 26) = 3.40, p < .05. RTs were s igni f icant ly s lower w h e n the fake hands were prone (838 ms) than supine (778 ms) or absent (776 ms) , p's < .05. The overal l analysis repor ted above was repeated us ing mean percentage errors as the dependent variable. Signif icant effects were observed on ly o n Incongruent tr ials where the patterns were the same as the Er ror CEs. Since the effects were either no t signif icant or were i n the d i rect ion of the Correct RTs, speed-accuracy tradeoffs were not a concern. The m a i n effects of Congruency [F (1 , 26) = 51.77, p < .01] and Distractor Side [F (1 , 26) = 6.59, p < .05] were signif icant. These were tempered b y a signi f icant Congruency x Distractor Side interact ion, F ( 1 , 26) = 22.68, p < .01. This was fo l l owed u p b y examin ing the s imple effect of Distractor Side for Incongruent and Congruent tr ials. The s imple effect was s igni f icant on l y on incongruent tr ials where observers tended to make more errors o n same side (11%) versus opposi te side target-distractor presentations (8%), F ( 1 , 26) = 19.20, p < .01. Experiment 5: A m ixed design A N O V A was computed us ing correct RTs as the dependent var iable, Observer H a n d Or ienta t ion (Prone, Supine), and D ig i t -Response M a p p i n g (top-toe & bot tom-heel , top-heel & bot tom-toe) as between-observer factors and Congruency (Congruent, Incongruent ) , Fake H a n d Cond i t i on (No Fake H a n d , Consistent, Inconsistent), and Distractor Side (Same, Opposite) as wi th in-observer factors. The m a i n effect of Congruency was signif icant, F ( 1 , 52) = 58.99, p < .01. This was tempered b y s igni f icant 97 interactions of Congruency x Observer H a n d Or ienta t ion [F ( 1 , 52) = 9.13, p < .01], Congruency x Digit-Response M a p p i n g [F (1,52) = 18.22, p < .01], Congruency x Cond i t i on [F (2,104) = 4.52, p < .05], Congruency x Distractor Side [F (1 , 52) = 14.09, p < .01], Congruency x Observer H a n d Or ien ta t ion x D ig i t -Response M a p p i n g [F (1 , 52) = 7.89, p < .01], Congruency x Dist ractor Side x Observer H a n d Or ienta t ion [F ( 1 , 52) = 10.91, p < .01], Congruency x Distractor Side x Digit-Response M a p p i n g [F (1,52) = 4.68, p < .05], Congruency x Distractor Side x Observer H a n d Or ientat ion x Digi t-Response M a p p i n g [F (1,52) = 7.34, p < .01]. The Cond i t i on x Distractor Side x Observer H a n d Or ien ta t ion interact ion was also signif icant, F (2,104) = 3.23, p < .05. The fou r -way interact ion was examined b y l ook ing at the s imple interact ion of Distractor Side x Observer H a n d Or ien ta t ion x Digi t-Response M a p p i n g for Incongruent and Congruent tr ials. The in teract ion was signi f icant on ly for Incongruent tr ials, F (1 , 52) = 7.15, p <.05. The s imple in teract ion of Distractor Side x Digit-Response M a p p i n g was examined for p rone and supine observer h a n d orientat ions. I t was signif icant on ly w h e n the observer's arms were supine, F (1,26) = 8.19, p < .01. The s imple effect of Dist ractor Side approached signif icance on ly for the top-heel & bot tom-toe m a p p i n g , where observers were faster to localize the target w h e n the distractor was presented to the same side (622 ms) versus the opposite side of f i xa t ion (646 ms) , F (1,13) = 4.51, p < .06. The Cond i t i on x Distractor Side x Observer H a n d Or ien ta t ion interact ion was examined b y look ing at the s imple interact ion of C o n d i t i o n x Observer H a n d Or ienta t ion for same and opposite side target-distractor presentations. The interact ion was signif icant on ly for same side target-distractor presentat ions, 98 F (2,104) = 4.30, p < .05. The s imple effect of C o n d i t i o n was examined for prone and supine observer a r m postures. W h e n the observer's a r m was prone, the s imple m a i n effect of Cond i t i on approached signif icance, F (2, 54) = 2.51, p < .10. Mean RTs i n the Consistent Fake H a n d cond i t ion (757 ms) were s igni f icant ly s lower than the RTs i n the Inconsistent Fake H a n d cond i t ion (716 ms) b u t not the N o Fake H a n d cond i t ion (730 ms). W h e n the observer's a r m was supine, the s imple m a i n effect of Cond i t i on approached signif icance, F (2, 52) = 2.68, p < .10. M e a n RTs were s igni f icant ly s lower i n the Inconsistent Fake H a n d cond i t ion (759 ms) than the N o Fake H a n d cond i t ion (724 ms) , bu t d i d no t d i f fer f r o m the Consistent Fake H a n d cond i t ion (729 ms). The same overal l analysis as that repor ted above was c o m p u t e d us ing mean percentage error as the dependent variable. The m a i n effects of Congruency [F (1,52) = 51.61, p < .01] and Distractor Side [F ( 1 , 52) = 6.80, p < .05] were signif icant. These were tempered b y signi f icant interact ions of Congruency x Digit-Response M a p p i n g [F (1 , 52) = 13.36, p < .01], Congruency x Distractor Side [F (1,52) = 15.47, p < .01], Congruency x Distractor Side x D ig i t -Response M a p p i n g [F (1,52) = 6.52, p < .05]. The Observer H a n d Or ienta t ion x Digit-Response M a p p i n g interact ion was also signif icant, F ( 1 , 52) = 6.99, p < .05. The three-way interact ion was examined b y l ook ing at the s imple interact ion of Distractor Side x Digit-Response M a p p i n g for Congruent and Incongruent tr ials. The s imple interact ion approached signif icance on l y on the incongruent tr ials, F (1 , 52) = 2.97, p < .10. The s imple m a i n effect of Distractor Side was signi f icant on ly for a top-toe & bot tom-heel m a p p i n g assignment, F (1 , 26) = 19.20, p <.01. Error CEs were larger overal l on 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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