L O W - L E V E L A N D HIGH-LEVEL MOTION PERCEPTION IN CHILDREN WITH U N I L A T E R A L A M B L Y O P I A by P A M E L A S. P A U L B.A., University of British Columbia, 1998 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS in THE F A C U L T Y OF G R A D U A T E STUDIES (Department of Psychology; Cognitive Systems) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA June 2001 © Pamela S. Paul, 2001 UBC Special Collections - Thesis Authorisation Form Page 1 of 1 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I further agree that permission f o r extensive copying of t h i s thesis f o r sc h o l a r l y purposes may be granted by the head of my department or by his or her representatives. It i s understood that copying or p u b l i c a t i o n of t h i s thesis f o r f i n a n c i a l gain s h a l l not be allowed without my written permission. The University of B r i t i s h Columbia Vancouver, Canada Date http://www.library.ubc.ca/spcoll/thesauth.html 7/25/01 11 Abstract It has been suggested that there are two motion systems: (1) a passive, low-level motion system that automatically signals motion and has been linked to the directionally selective neurons of primary visual cortex and the medial temporal area (MT) and (2) an active, high-level motion system that is engaged by tracking the visible features of a stimulus by actively attending to it (Cavanagh, 1992). This thesis tests the possibility that the high-level motion system is selectively disrupted in amblyopia. Amblyopia is a developmental visual disorder characterised by reduced visual acuity in an otherwise healthy, properly refracted eye. It is usually associated with deficits in spatial vision. Recent work suggests that visual attention may also be disrupted and the status of motion perception is an unresolved issue. The present study assessed 13 children with unilateral amblyopia and 24 age-matched controls on one low-level motion task and four high-level motion tasks. Children with amblyopia showed similar performance to controls in both eyes (the amblyopic eye and non-amblyopic, fellow eye) on a low-level motion coherence task and two high-level motion tasks: apparent motion and visual search. Performance on a single-object tracking task was depressed in the amblyopic eye. Children with amblyopia showed depressed performance in both eyes on a multiple-object tracking task. These results suggest that there is a preservation of low-level motion perception in amblyopia, while children with amblyopia have deficits at attentively tracking multiple targets. iii T A B L E OF CONTENTS Abstract 1 1 Table of Contents m ' List of Tables v List of Figures v l Acknowl edgem ents v u Introduction • 1 Cavanagh's Motion Systems 2 Evidence for Cavanagh's Motion Theory 4 Crowding and Attention 6 Attention and Amblyopia 8 Motion Perception and Amblyopia 11 Experiment 1 16 Method 16 Results and Discussion 19 Experiment 2 21 Method 21 Results and Discussion 23 Experiment 3 25 Method 25 Results and Discussion 27 Experiment 4 28 Method 28 iv Results and Discussion 29 Experiment 5 32 Method 32 Results and Discussion 33 General Discussion 36 References 43 Appendix A Visual acuity and performance correlations 47 Appendix B Stereopsis and performance correlations 48 Appendix C Visual Search Performance 49 Appendix D L L M and H L M correlations 51 LIST OF TABLES Table 1. Clinical diagnoses and subject data for 13 pediatric patients vi LIST OF FIGURES Figure 1. Wertheimer's display 3 Figure 2. Spatial resolution vs. Visual Attention 7 Figure 3. Results of Expt. 1 20 Figure 4. Schematic of stimulus displays used in Expt. 2 22 Figure 5. Results of Expt. 2 23 Figure 6. Schematic of stimulus used in Expt. 3 26 Figure 7. Results of Expt. 3 27 Figure 8. Results of Expt. 4 30 Figure 9. Ball(s) x Eye interaction 30 Figure 10. Schematic of stimulus used in Expt. 5 34 Figure 11. Results of Expt. 5 (Eye 1) 34 Figure 12. Results of Expt. 5 (Eye 2) 35 V l l A C K N O W L E D G E M E N T S I would like to thank my supervisor, Debbie Giaschi, for her constant guidance and suggestions; Patrick Cavanagh, for the idea on which my thesis is based; Veronica Edwards for sharing her extensive knowledge daily; Alan Kingstone and Geoff Hall for their statistical consulting; Amanda for being an excellent sounding board throughout my university career; Sandra, for her beautiful pictures; Carolyn, Ryan, Jessica and Timothy for being my usually cheerful and always competitive pilot data guinea pigs; KITH, for supplying me with my daily dose of laughter; and, of course, my parents for their endless faith and encouragement. Thanks for helping "everything turn out fine". 1 Clinically, unilateral amblyopia is defined as reduced visual acuity in an otherwise healthy, properly refracted eye (the amblyopic eye). The other eye (the fellow eye) has normal acuity. Well-documented deficits found in amblyopia include aspects of spatial vision such as contrast sensitivity, spatial localization, position acuity, and crowding (Flom, Weymouth, & Kahneman, 1963; Levi, 1991). Amblyopia is often associated with an eye-turn (strabismus) or unequal refractive errors in the two eyes (anisometropia). These different causes of amblyopia coupled with the fact that different results are obtained on psychophysical tasks (e.g. position acuity, spatial perception) for each type of amblyopia suggest that different mechanisms may underlie strabismic and anisometropic amblyopia (Levi, 1991). It has also been suggested that amblyopia may involve a deficit in motion processing (Donahue, Wall, & Stanek, 1998; Hess, Demanins, & Bex, 1997; Schor & Levi, 1980). In the present research, motion processing in amblyopia is investigated within a dual-motion-processing-systems framework. There have been multiple reports suggesting the existence of two motion-perception systems (Anstis, 1980; Braddick, 1974, 1980; Cavanagh, 1992; Chubb & Sperling, 1989; Mather, Cavanagh, & Anstis, 1985). Researchers have proposed two motion systems based on two possibilities. First, the two different motion systems could be defined by the type of motion stimuli they respond to (system-equals-stimuli hypothesis). For instance, Braddick's (1974) short-range motion system responds to small spatial displacements (< 20 minutes of arc) and short interstimulus intervals (<100 ms), while his long-range motion system responds to larger spatial displacements (up to a few degrees of visual angle) and longer interstimulus intervals (<500 ms). A more recent example of a system-equals-stimuli hypothesis has been proposed by Chubb and Sperling (1988). The Fourier system responds to visual stimuli that have a Fourier power spectrum that predicts the direction of perceived motion. The Fourier power spectrum of 2 visual stimuli corresponding to the non-Fourier system, in contrast, does not predict the direction of perceived motion. Alternatively, the two motion systems might correspond to two different motion processing mechanisms. Unlike the system-equals-stimuli theories, system-equals-mechanism theories predict that both systems could respond to the same type of motion stimulus, but process it differently. This thesis will explore this second proposal, which will be discussed in detail shortly. The purpose of this thesis is two-fold: (1) to study motion perception in amblyopia in order to learn more about this developmental visual problem and (2) to consider the pattern of motion deficits in amblyopia from the perspective of the system-equals-mechanism theory proposed by Cavanagh (1992). Cavanagh's Motion Systems: Cavanagh (1992) has asserted that there is a passive (low-level) motion system that automatically signals motion and that has been linked to the directionally selective neurons of primary visual cortex and the "motion area" MT. Performance on low-level motion (LLM) is thus explained by the behaviour of single neurons. The high-level motion (HLM) system, in contrast, requires visual attention. What exactly does Cavanagh mean by "attention"? Cavanagh has asserted that the H L M system is engaged by "attentive tracking", tracking the visible features of a stimulus by actively attending to it without moving the eyes. For example, in one H L M task subjects are asked to track three of eight identical green discs moving randomly across a screen, while centrally fixating. Attention is moved spatio-temporally in that the subject must track or attend to moving objects across time and space. Some H L M tasks require filtering out irrelevant stimuli (distractor items), other tasks require shifts of attention, and some tasks require both. H L M task performance cannot be explained by the behaviour of single motion neurons. Although these two systems often work in tandem and give the same impression of motion, there are situations where the same stimulus may elicit different impressions of motion from each system. For instance, Werfheimer (1912/1961) discovered that if two intersecting lines (a cross) are alternated in time with a second cross that is rotated 45 degrees (degs) (Figure 1 a), passive viewing elicited the impression of back and forth motion (Figure lb). However, i f one chose to attend to one arm of the cross as it moves in a direction chosen by the observer, motion is perceived in the chosen direction (Figure lc). In other words, the impression of motion was different depending on whether one passively viewed the stimulus or attentively tracked the stimulus. \ T I \ M E (a) ^ (b) (without attention) (c) (with attention) Figure 1. (a) Display stimulus: One cross is alternated in time with a second cross rotated 45 degs. (b) Passive viewing resulted in the impression of back and forth motion, (c) Selectively attending to one of the arms of the stimulus resulted in the impression of continuous motion in one direction. (From Wertheimer cited in Verstraten, Cavanagh, & Labianca, 2000). Cavanagh (1989) has also asserted that there are two types of stimuli: first-order (luminance, colour) and second-order (motion, texture, and binocular disparity). Two areas of an 4 image differ in their first-order statistics if they differ in intensity (luminance) or spectral composition (colour). In contrast, two areas may have the same mean colour and luminance, but differ in their temporal, spatial, or ocular distributions (motion, texture, and binocular disparity, respectively). Both the L L M and H L M systems can respond to first-order or second-order stimuli, however the H L M system requires that the stimuli are visible and thus able to be tracked.1 In other words, the first-order/second-order distinction is independent of the passive (LLM)/active (HLM) distinction. Evidence for Cavanagh's Motion Theory: If these two motion systems exist and can be engaged by the same stimuli, it is possible that in the same stimulus situation each system would give a different perception of motion. Cavanagh (1992) found evidence that, with certain parameters, the same stimulus could give opposite perceptions of motion, depending on which motion system was engaged. Each subject was presented with two circular gratings (one luminance-defined, one colour-defined) rotating in opposite directions. The colour-defined (CD) grating was superimposed on the luminance-defined (LD) grating. Passive viewing of the gratings, which engaged the L L M system, resulted in an impression of motion in the direction of the LD grating, even though the subject could not see the LD grating. Engagement of the active motion system was achieved by asking each subject to attentively track one of the coloured bars in the CD grating. This condition resulted in an impression of motion in the direction of the CD grating, i.e., the opposite direction to the passive viewing condition. Therefore, attentive tracking can alter the perception of motion. The fact that an individual can perceive opposite motion for the same rotating stimulus by merely manipulating attention provides support for Cavanagh's H L M and L L M systems. 1 It may seem unlikely that perception is affected by stimuli that are not visible. Shortly however, it will be shown that a stimulus not visible to the observer (a LD grating) can determine the perceived direction of motion. 5 Further evidence of a passive L L M system and an active, attention-mediated H L M system would be i f an individual showed selective damage to one motion system. Battelli, Cavanagh, and Barton (2001) hypothesised that individuals with parietal lesions would show preserved L L M but a H L M deficit (due to the visual attention deficits associated with their parietal lesions). As expected, visual attention deficits (not specifically related to motion processing) were found in the visual field contralateral to the lesion in individuals with unilateral parietal lesions, and in both visual fields for individuals with bilateral parietal lesions. When motion processing was assessed in these subjects, they all showed normal L L M processing in both visual fields. This was established by asking the subject to identify whether a motion-defined rectangle was horizontal or vertical. H L M processing was measured using a multiple-object tracking task and an apparent motion task. In the multiple-object tracking task, the four unilateral parietal lesion patients showed normal performance in the visual field ipsilateral to their lesion (i.e., the field of visual space corresponding to their unaffected hemisphere) and depressed performance in the visual field contralateral to their lesion. The bilateral patients show depressed performance for both fields. For the apparent motion task, in contrast, the three unilateral lesion patients tested showed significantly worse performance than age-matched controls in both visual fields. Two of the three bilateral lesion patients showed significantly worse performance in one hemifield. Battelli et al. (2001) concluded that the different pattern of results for the two H L M tasks suggest that the two tasks may utilise attentional resources in different ways. In addition to asserting that L L M is automatically signaled, whereas H L M requires attentive tracking of the visible features of a stimulus in order to signal motion, Cavanagh stated that the passive (LLM)/active (HLM) distinction is independent of the first-order/second-order distinction. L D and CD stimuli are first-order stimuli, and texture-defined (TD), disparity-6 defined (DD), and motion-defined (MD) stimuli are second-order stimuli. Using motion nulling and motion plaid techniques, Cavanagh (1995) found evidence of L L M detectors for LD, CD, and TD stimuli, but not for DD or M D stimuli. Cavanagh also showed that attention was necessary to perceive motion for DD and M D stimuli. Subjects attentively tracked a rotating inner LD circular grating, while an outer circular grating was either M D , DD, CD, or TD. While tracking the inner luminance grating, subjects perceived the motion of the CD and TD gratings. However, subjects did not perceive the motion of the M D or DD grating. Cavanagh concluded that CD, LD, and TD stimuli can engage either the passive or active motion systems, whereas M D and DD stimuli only engage the active motion system. This represents a slight shift from the earlier view that any stimulus (first-order or second-order) can engage both systems. Although the evidence supporting Cavanagh's L L M and H L M systems has been accumulating, further investigation is necessary. This thesis tests the hypothesis that children with amblyopia have abnormal H L M and normal L L M perception. This hypothesis is based on the suggestion that amblyopia involves a deficit in visual attention that is reflected in a larger than normal crowding effect (Flom et. al, 1963; Levi, 2000). How may crowding be related to visual attention deficits in amblyopia? The crowding effect describes the phenomenon that measured visual acuity is higher when letters are presented one at a time (isolated-letter format) rather than in lines (line format). The crowding phenomenon is present in all eyes to some extent. The ratio of visual acuity for letters presented in isolated-letter format to letters presented in a line (e.g., the typical Snellen chart), is abnormally large in some amblyopic eyes, i.e., amblyopic eyes suffer from abnormally large crowding (Flom et al., 1963). In other words, the difference in acuity measurements obtained from isolated-letter cards and line format charts is relatively larger in amblyopic eyes compared to control eyes. The findings of He, Cavanagh, and Intriligator (1996) suggested that crowding (the lower acuity measurement for Snellen line charts) is not due to spatial resolution limits per se, but the limits placed on spatial resolution by the resolving power of visual attention. For example, while fixating on the cross in Figure 2 subjects with normal visual acuity can easily resolve the black bars - they are vertical, thin, and black. However, i f asked to selectively attend to, say, the fourth bar from fixation it is impossible to individuate and thus selectively attend to that bar (an attentional resolution task). This finding was interpreted as evidence that the "spotlight" or attention is larger than one's spatial resolution, and therefore it is difficult to selectively attend to one bar. Why is this so? As demonstrated by He et al. this inability to selectively attend to an item you can easily resolve may be due to the much coarser grain of visual attention placing a limit on spatial resolution. Figure 2. A demonstration that shows the difference between spatial resolution and visual attention. While fixating on the cross, the lines to the right are easily seen. However, while fixating on the cross it is difficult to select and attend to one line. Using an adaptation study, He et al. (1996) explored whether the crowding effect occurs before or at the level of VI (as suggested by the traditional contrast masking explanation of crowding), or at a level beyond V I . An example of the classic adaptation effect is that subsequent to viewing a grating of a particular orientation for an extended period of time, it is more difficult to detect the orientation (measured by the amount of contrast needed to correctly identify the orientation) of the adaptation grating than any other orientation. This increase in 8 detection threshold is presumably due to the fatigue of neurons tuned to the adapting orientation (Coren, Ward, & Enns, 1999). He et al. (1996) found such an adaptation effect whether the participant could discriminate the grating orientation during adaptation (a "single" condition where only 1 grating was present during adaptation) or the participant could not discriminate the grating orientation during adaptation (a "crowded" condition where the adaptation grating was flanked by four other gratings). He et al.'s dissociation of a VI process (orientation adaptation) and perceptual awareness (whether the participant could discriminate the target orientation during adaptation) suggests "that visual awareness, and the 'crowding' that blocks it, occur after orientation analysis in the visual information processing stream" (He et al., p. 335). Additional studies suggested that crowding is specifically due to the insufficient spatial resolution of visual attention. The fact that attentional resolution causes the crowding effect, and individuals with amblyopia show an abnormally large crowding effect, suggests that an additional characteristic of amblyopia might be a visual attention deficit. Three studies (Levi, 2000; Rohaly & Karsh, 1998; Sharma & Levi, 1999) have investigated visual attention in amblyopia. Attention and Amblyopia: Levi (2000) suggested that amblyopia may be accompanied by a visual attention deficit. He investigated crowding in the fovea of individuals with amblyopia and control subjects. The target stimulus was the letter " E " made of 17 Gabor or Gaussian patches (5 patches per side). Initially, he measured the contrast thresholds for identifying the orientation (up, down, left, or right) of the "E" . He then added flanks (patches identical to the target which were varied in distance from the target) that surrounded the target and measured contrast thresholds for identifying orientation. By varying the distance between the target and the flanks, Levi calculated the "critical distance": the distance between the flank and target at which the contrast 9 threshold was elevated by a criterion amount. Levi found that in normal foveal vision the critical distance varied proportionally with the size of the target. For instance, at a patch size of 0.5 arc min the critical distance was 1 arc min; at a patch size of 25 arc min, the critical distance was 43 arc min. In amblyopic foveal vision, however, the critical distance did not vary proportionally with target size. The critical distance was fixed over a large range of target sizes (25-45 arc min). Levi (2000) suggested that this increased extent of crowding found in strabismic amblyopia is due to "an increase in the spatial extent of the window of attention" in amblyopic vision. Sharma and Levi (1999) investigated whether the poor performance in judging the number of highly visible objects in subjects with strabismic amblyopia had an attentional basis. In experiment 1, 6 subjects with strabismic amblyopia were presented with a display consisting of test Gabor patches (with vertical carrier orientation) and distractor Gabor patches (with horizontal carrier orientation). Test and distractor patches were presented in a 7 x 7 array. The subject's task was to estimate the number of test patches present (1 to 4 patches). In experiment 2, the subject estimated the number of test patches present and indicated where the test patches had appeared. In experiment 3, the subject was cued to a quadrant where a group of test patches had an 80% probability of appearing. As in experiments 1 and 2, the subject's task was to the report the number of test patches present in the display. The results of experiment 1 and 2 showed that the number of test patches present was significantly underestimated in the amblyopic eye relative to control eyes.2 In experiment 3, amblyopic and control eyes showed similar performance: performance slightly improved when the correct location was cued, and there was a reduction in accuracy when the incorrect location 2 Additional experiments showed that the amblyopic eyes' performance was not due to decreased visibility, crowding, or undersampling. _ 10 was cued. Sharma and Levi's (1999) results suggest that subjects with strabismic amblyopia are able to allocate attention (experiment 3), but their "attentional resolution is compromised." Rohaly and Karsh (1998) investigated the effects of divided attention in anisometropic amblyopia. Participants localised a peripheral target that could appear anywhere in a 30 deg radius semicircle while simultaneously performing a foveal task. Experimental difficulty was manipulated by varying the number of distractors in the search task and the difficulty of the foveal task. The peripheral and foveal tasks were either presented to different eyes (dichoptic viewing) or to the same eye (monocular viewing). Fellow eyes performed similarly to control eyes on the search task regardless of viewing condition: performance on the visual search task deteriorated as the number of distractors increased, as the target extended farther into the periphery, and as the workload of the foveal task increased. In other words, subjects performed equally well regardless of whether they performed both tasks with their fellow eye or they performed the search task with their fellow eye and the foveal task with their amblyopic eye. When they performed both tasks with the amblyopic eye, however, performance, was comparatively worse on the visual search task and (unlike the fellow eye) was not systematically influenced by target eccentricity or the foveal task workload. Rohaly and Karsh (1998) concluded that the results of the dominant eye in both viewing conditions suggested that individuals with amblyopia were as good at dividing their attention between both eyes as they are at dividing their attention in the fellow eye. It remains inconclusive whether the amblyopic eye has a visual attention deficit. The results of these three studies investigating visual attention in amblyopia show two things. Levi (2000) and Sharma and Levi (1999) suggest that individuals with strabismic amblyopia have a visual attention deficit. The results of Rohaly and Karsh (1998) are 11 inconclusive as to whether there is a visual attention deficit in the amblyopic eye of individuals with anisometropic amblyopia. Motion Perception in Amblyopia: Studies investigating whether there is a motion deficit in amblyopia have provided mixed results. Some studies have argued that there is a motion deficit in amblyopia (Donahue, Wall, & Stanek, 1998; Hess, Demanins, & Bex, 1997), while others argue that motion perception is preserved (Hess & Anderson, 1993; Kubova, Kuba, Juran, & Blakemore, 1996; Levi, Klein, & Aitsebaomo, 1984). These studies have investigated numerous aspects of motion perception such as motion aftereffects (MAE), direction discrimination, and brain activity elicited by moving stimuli. After viewing a moving object for an extended period of time, i f you look at a stationary object it will appear to move in the direction opposite to that of the moving object. This motion aftereffect (MAE) was studied in 8 individuals with strabismic amblyopia (4 had anisometropia as well) by comparing the length of time their M A E persisted in comparison to a group of 5 control subjects (Hess, Demanins, & Bex, 1997). Subjects-were adapted to two 1 cycle/deg (cpd) sinusoidal gratings drifting towards a central point of fixation. They then viewed a test grating that was either stationary or counterphasing at a temporal frequency of 1 Hz. The persistence of the M A E was not significantly different between control eyes and fellow eyes for either test condition; in contrast, seven of eight amblyopic eyes exhibited significantly reduced MAEs in both test conditions. Giaschi, Regan, Kraft, & Hong (1992) measured speed thresholds for recognising motion-defined dotted letters in the amblyopic and the fellow eye of 20 children with amblyopia, and the eyes of a control group that consisted of 30 children without any ocular disorders. Fifteen of the eighteen fellow eyes (9/9 eyes with strabismic amblyopia, 4/6 eyes with 12 anisometropic amblyopia, 2/3 eyes with strabismic and anisometropic amblyopia) showed degraded ability to recognise motion-defined letters. A l l of the fellow eyes had normal high-contrast and low-contrast acuity. Eighteen of the nineteen amblyopic eyes exhibited abnormal thresholds for motion-defined letter recognition. It is not clear whether this deficit found in both the fellow and amblyopic eye is a form or a motion deficit. Donahue, Wall, and Stanek (1998) have suggested that there are motion detection abnormalities in individuals with anisometropic amblyopia. They presented motion defined (MD) circles of various sizes (0.25-21 deg) to 44 locations in the visual field (both central and peripheral) of 10 individuals with amblyopia and 15 age-matched controls. After each trial, subjects were asked to indicate where the center of the circle had been in order to ensure that they had detected the target. Using the staircase method, they estimated for each location how large the M D circle had to be in order to be detected. Donahue et al. (1998) found that the amblyopic eye's size threshold was larger than the fellow eye's threshold for 39 of the 44 tested thresholds. The average size threshold of the 44 locations was larger in the amblyopic eyes (2.77 +- 0.09 pixels) compared to the control group eyes (2.59 + - 0.09) and the fellow eyes (2.6 + -0.04) (p< .03). Since there is a significant difference between the three types of eyes (amblyopic, control, and fellow), Donahue et al. suggested that anisometropic amblyopia is associated with an abnormality in motion detection that extends into the peripheral visual field. Do these findings necessarily suggest that there is a motion detection abnormality in individuals with anisometropic amblyopia? Each fellow and control eye had vision that was 20/20 or better, whereas the acuity of the amblyopic eyes varied from 20/25 to "count fingers".3 Perhaps the elevated size threshold in the amblyopic eyes was due to an inability to see the target 13 rather than a motion processing problem. In fact, Donahue et al. (1998) found a significant negative correlation between acuity and size thresholds in amblyopic eyes (r= -0.73). In other words, the poorer the vision in an eye, the larger the circles needed to be in order to be detected. Furthermore, a study that did take the reduced visual acuity of the amblyopic eye into account (Levi, Klein, & Aitsebaomo, 1984) found that motion detection is normal in individuals with amblyopia. Hess & Anderson (1993) also found that motion detection is normal in individuals with amblyopia. Levi, Klein, and Aitsebaomo (1984) measured motion detection thresholds and direction discrimination thresholds for gratings of different spatial frequencies in 9 individuals with amblyopia (3 strabismic, 3 anisometropic, 3 mixed) and 2 control subjects. Two vertical gratings (of identical spatial frequency within a trial) were presented in succession with the second grating displaced horizontally by one of three distances. This created the perception of a moving grating. A fourth type of trial consisted of no displacement of the second grating. For each trial, the participant rated the displacement (if any) and direction of motion (left or right) by choosing one of six numbers (-3, -2, -1, 0, +1, + 2, +3) with the sign indicating the direction of motion and the number indicating the amount of motion. Levi et al. (1984) found a one to one relationship between grating acuity and displacement detection threshold for all subjects, i.e., subjects' detection threshold for gratings was at their resolution acuity. In general, each subject could discriminate the direction of motion at detection threshold, except at gratings of spatial frequency near the visual resolution limit. Near the visual resolution limit, the motion discrimination threshold was elevated relative to the detection 3 "Count fingers" is an alternative method for measuring visual acuity in individuals whose vision is so poor that it cannot be estimated using a letter chart. Individuals with "count fingers" acuity can tell how many fingers the 14 threshold for all the subjects. The results of this study suggest that motion detection and direction discrimination are normal in amblyopia if the reduced visual acuity of the amblyopic eye is considered. Hess and Anderson (1993) measured contrast thresholds for detection and direction discrimination of vertical sine-wave gratings in 8 individuals with amblyopia (7 strabismic, 1 anisometropic). No control subjects were used. In experiment 1, monocular contrast thresholds for both detection and direction discrimination were obtained using a two-alternative forced-choice (2-AFC) staircase method. For the detection task, each subject was presented with two frames in succession. Their task was to indicate which frame contained a moving grating. For the . discrimination task, each subject was presented with a single frame containing a moving grating. Their task was to indicate the direction of motion (right or left). In addition to varying the contrast of the gratings, Hess and Anderson also varied the spatial frequency and temporal frequency. They found that one subject with strabismic amblyopia and one with anisometropic amblyopia were able to discriminate direction of motion at detection threshold over the entire spatio-temporal range tested in both eyes. The remaining 6 individuals with strabismic amblyopia were able to discriminate direction at detection threshold over most of the spatio-temporal range. For gratings of high spatial frequency and low temporal frequency, the direction discrimination threshold was elevated relative to detection. Hess and Anderson argue that a direction discrimination threshold elevation has been found in normals near their visual resolution limit and is likely to be mediated by non-motion mechanisms. Again these results suggest that motion detection and discrimination are normal in amblyopia. Kubova, Kuba, Juran, and Blakemore (1996) measured motion processing in 37 children with amblyopia (20 anisometropic, 7 strabismic, 10 mixed) by measuring visual evoked ophthalmologist is holding up. 15 potentials (VEPs) associated with the onset of motion. It has been found that in normal subjects the onset of steady linear motion elicits a characteristic VEP dominated by a negative component (N 2) that reaches a peak between 160-200 ms (Kubova et al., 1996). Kubova et al. had subjects view a full-field black and white checkerboard (check size: 35 min of arc) moving horizontally at 6 deg/s. The motion onset VEP (N2) was not significantly different between the fellow and amblyopic eye. In addition, the latency of N 2 in children with amblyopia (fellow eye and amblyopic eye ~ 160 ms) was similar to that of the N 2 elicited in normal subjects (~160-200 ms). Kubova et al. argued that these results suggest that the motion pathway is relatively spared in amblyopia. Most of the studies on motion perception in amblyopia suggest that motion processing is normal i f visual acuity is taken into account. One study, however, (Hess, Demanins, & Bex, 1997) suggests that their may be a motion deficit in individuals with strabismic amblyopia. Giaschi et al. (1992) suggests that there may be a motion deficit in the fellow and amblyopic eye of individuals with strabismic, anisometropic, and strabismic and anisometropic amblyopia. This thesis aims to further investigate motion processing in amblyopia. Specifically, it will investigate the L L M and H L M motion systems proposed by Cavanagh (1992). Based on previous research regarding motion perception in amblyopia, visual attention in amblyopia, and crowding in amblyopia, it is hypothesised that children with amblyopia will show preserved L L M motion perception (because their direction discrimination is normal, and a direction discrimination task is how we will measure L L M processing), and a deficit in H L M motion perception. To test this hypothesis, I have tested 37 children (13 children with amblyopia, 24 age-matched controls) on one L L M task and four H L M tasks. 16 Experiment 1: Motion Coherence (LLM) Newsome and Pare (1988) provided the first evidence that the middle temporal (MT) area of monkeys (which is comprised of directionally selective neurons with large receptive fields) is important for the perception of motion in a motion coherence task. In a typical coherence task, a subject is asked to indicate the direction a pattern of dots moved. The displacement of each dot between successive frames is small enough to fall within the receptive field of one cell. A motion threshold is obtained by varying the percentage of dots moving coherently i.e. in the direction to be identified. For instance, in a 100% coherence condition all the dots move in the same direction, while in a 40% coherence condition, 40% of dots move in the same direction while the remaining 60% of dots move in random directions. Newsome and Pare (1988) trained 2 rhesus monkeys to perform this type of direction discrimination task. Using a 2-AFC procedure they compared motion coherence thresholds before and after a selective lesion to area MT in one hemisphere. Before the lesion, the direction of motion could be discriminated when less than 2% of the dots were moving coherently. After the lesion, however, coherence thresholds for displays presented in the visual field contralateral to the lesion were increased by 400-800%). This finding suggests that motion coherence is a L L M task because it is a task that can be explained based on the behaviour of neurons (motion detectors) in area MT. Method Participants The control group consisted of 24 children with normal or corrected to normal visual acuity (Regan high-contrast letter chart) and normal stereoacuity (Randot circles test). None had a history of ocular disorders. At the time of testing, the children's ages ranged from 106 to 188 months (mean, 146.71 months; standard deviation, 23.56 months). The patient group consisted of 13 children with a history of amblyopia due to anisometropia, strabismus, or both anisometropia and strabismus (Table 1). They were free from any other ocular disorders. At the time of testing, the children's ages ranged from 108 to 200 months (mean, 148.54 months; standard deviation, 34.57 months). The mean age of the control and patient group was not significantly different (F i > 3 5 = 0.036, p = .85). Patients were attending or had attended the Ophthalmology Department at B.C.'s Children's Hospital under the care of Dr. Roy Cline or Dr. Christopher Lyons. A l l 37 subjects participated in all subsequent experiments. Subjects were tested on the vocabulary subtest of the WISC-III. Individuals with a scaled score of 7 or less (i.e. more than one standard deviation from the mean) were excluded. One child with amblyopia was excluded due to this criterion. Informed consent was obtained from a parent or a guardian of each child after the nature of the procedures had been fully explained. Table 1 Clinical diagnoses* and subject data for 13 pediatric (age in months) patients Patient Diagnosis Age Sex Decimal Visual Decimal Visual Stereopsis WISC Acuity - Acuity - Vocab Amblyopic Eye Fellow Eye Score 1 A 129 F 0.40 1.03 30 12 2 A 164 F 0.68 1.20 30 11 3 A 170 M 0.30 1.30 20 10 4 A 123 F 0.65 1.03 30 14 5 A 124 F 0.88 1.30 400 13 6 A 186 M 0.50 1.08 30 11 7 A 196 M 1.23 1.48 70 10 8 S 108 F 0.83 1.20 100 11 9 s 116 M 0.50 1.00 70 13 10 S + A 124 F 0.83 0.83 25 12 11 S + A 111 F 1.05 1.43 40 13 12 S + A 200 M 1.28 1.30 400 9 13 S + A 180 F 0.63 1.03 30 8 *A, anisometropic amblyopia; S, strabismic amblyopia; S + A, strabismic plus anisometropic amblyopia. 18 Apparatus Stimuli were displayed on a Macintosh 8500 computer for the first four experiments. A pattern of moving dots was created as follows. Each high-contrast white dot (size: .013 deg; 75 cd/m2)4 was displaced 0.127 deg between each of four successively presented frames. For example, a dot moving to the right would move 0.127 deg to the right between frame 1 and frame 2, 0.127 deg to the right between frame 2 and frame 3, and 0.127 deg to the right between frame 3 and frame 4. The black background's luminance was 0.2 cd/m . Each frame of dots was presented for 107 ms, resulting in a total trial length of 428 ms. The dot density was 32 dots/deg2. Observers viewed the display in a dim room at a distance of 140 cm. Procedure Each subject performed each of the five tasks one eye at a time (monocular testing). The non-amblyopic, fellow eye was always tested first in the patient group. The eye with higher visual acuity was always tested first in the control group. The first eye tested in controls will be referred to as Control Eye 1, and the second eye tested will be referred to as Control Eye 2. The first eye tested in amblyopes (the non-amblyopic, fellow eye) will be referred to as Fellow Eye 1, and the second eye tested (the amblyopic eye) will be referred to as Amblyopic Eye 2. The number following the eye designation simply indicates whether the eye was tested first or second. 4 Luminance values were obtained using a CS-100 Minolta photometer. 19 Best-corrected visual acuity was assessed before testing using a high-contrast Regan letter chart. Patients' visual acuity scores are listed in Table 1. Control subjects had 20/20 vision or better in each eye. The stimuli presented in each H L M game were larger than .21 deg . An object that is .21 deg is large enough to be seen by an observer with a decimal visual acuity (DVA) of 0.4 (i.e., a visual acuity of 20/50). Although the individual dots presented in the L L M coherence task were smaller than .21 deg, preliminary data in our lab suggest that coherence thresholds are not adversely affected by blurred vision. In the motion coherence task, the percentage of dots moving in one direction ranged from 100 to 0.5. In this 2-AFC procedure, the subject indicated whether the dots moved to the right or to the left on each trial. When noise dots (dots moving in a direction other than the primary direction of motion) were added, the subject indicated whether most of the dots moved to the right or to the left. The first trial consisted of 100% of the dots moving in the same direction. Coherence level was reduced according to a staircase algorithm with a 2 down - 1 up rule. A run ended after 40 trials or 10 response reversals. Coherence level was plotted against percentage correct direction discrimination, and a Weibull function was fit to the data to determine the coherence threshold. This threshold was taken as the coherence level at which the direction of motion was correctly identified on 82% of trials. Results and Discussion Mean coherence thresholds for each eye of the amblyopic and control groups are displayed in Figure 3. A one-between (Group: control, amblyopes) x one-within (Eye: first eye tested, second eye tested) factor analysis of A N O V A showed no main effect of Group (F135 = .436, p=.51), or Eye (F1.35 = 1.227, p=.28), nor a significant Group X Eye interaction (Fps = .0002, p=.99). These results suggest that, as hypothesised, there is no deficit in direction discrimination in either eye of children with unilateral amblyopia. 20 Individuals with anisometropic amblyopia sometimes perform differently on tasks than individuals with amblyopia associated with strabismus (Levi, 1991). To examine this possibility the amblyopic group was split into two groups'based on their clinical presentation: (1) pure anisometropic amblyopia (2) amblyopia associated with strabismus. An A N O V A performed on the data for the three groups revealed the same pattern of result: no significant main effect of Group or Eye, and no significant interaction. Task performance was not significantly correlated with visual acuity or stereopsis (see Appendices A and B, respectively). J 3 H 4) O C