LOW-LEVEL AND HIGH-LEVEL MOTION PERCEPTION IN CHILDREN WITH
UNILATERAL AMBLYOPIA
by
PAMELA S. PAUL
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 FACULTY OF GRADUATE 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
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The University of British Columbia Vancouver, Canada
Date
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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
TABLE OF CONTENTS
Abstract 11
Table of Contents m'
List of Tables v
List of Figures vl
Acknowl edgem ents vu
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 LLM and HLM 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 Vll
ACKNOWLEDGEMENTS
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 HLM 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
HLM 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 HLM tasks require filtering
out irrelevant stimuli (distractor items), other tasks require shifts of attention, and some tasks
require both. HLM 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, if 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 LLM and HLM systems can respond to first-order or second-order stimuli, however the HLM 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 LLM 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 HLM and LLM 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 LLM system and an active, attention-mediated HLM system would be if an individual showed selective damage to one motion system. Battelli,
Cavanagh, and Barton (2001) hypothesised that individuals with parietal lesions would show preserved LLM but a HLM 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 LLM processing in both visual fields. This was established by asking the subject to identify whether a motion-defined rectangle was horizontal or vertical. HLM 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 HLM tasks suggest that the two tasks may utilise
attentional resources in different ways.
In addition to asserting that LLM is automatically signaled, whereas HLM 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. LD 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 LLM detectors for LD, CD, and TD stimuli, but not for DD or MD stimuli. Cavanagh also showed that attention was necessary to perceive motion for DD and MD stimuli. Subjects attentively tracked a rotating inner LD circular grating, while an outer circular grating was either MD, 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 MD or DD grating. Cavanagh concluded that CD, LD, and TD stimuli can engage either the passive or active motion systems, whereas
MD 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 LLM and HLM systems has been accumulating, further investigation is necessary. This thesis tests the hypothesis that children with amblyopia have abnormal HLM and normal LLM 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, if 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 VI. 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, if 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 MAE 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 MAE 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. All 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 MD 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
(N2) 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 N2 in children with amblyopia (fellow eye and amblyopic eye ~ 160 ms) was similar to that of the N2 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 if 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 LLM and HLM 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
LLM motion perception (because their direction discrimination is normal, and a direction
discrimination task is how we will measure LLM processing), and a deficit in HLM motion
perception. To test this hypothesis, I have tested 37 children (13 children with amblyopia, 24
age-matched controls) on one LLM task and four HLM 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 LLM
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 (Fi>35 = 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. All 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 HLM 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 LLM 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 ANOVA 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 ANOVA 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).
0.40 -,
0.35
0.30
O "|5 0.25
J3 H 4) 0.20 O C
0.05
0.00 Control 2 Amblyopic 2 Control 1 Fellow 1 Eye Figure 3. Results of Experiment 1. Coherence thresholds are plotted for each type of eye. The lower the threshold value, the better the performance. 21
Experiment 2: Classic 2-Dot Apparent Motion (HLM)
Two similar stimuli presented successively at an appropriate time and distance between each other will be perceived as one object in motion, rather than two similar objects presented successively. In such displays, real continuous motion is not present, yet an observer perceives motion. The successive stimuli in apparent motion (AM) do not fall within the receptive field of a single motion detector (and therefore do not activate the LLM system); thus higher cognitive processes are believed to be necessary to achieve this type of "classic" AM perception.
Wertheimer (1912/1961) first suggested how a higher cognitive process (attention) might be involved in the perception of AM. He studied AM by presenting one object (Ol) at position
PI and a second object (02) subsequently at position P2. Wertheimer (1912/1961) hypothesised that when 02 is presented, there is an involuntary dragging of attention (a type of attentional capture) from Ol to 02. This movement of attention from PI to P2 gives rise to the impression of continuous motion. More recent studies support the hypothesis that attention is involved in
AM when Ol and 02 are placed far enough apart to prevent them from stimulating the same receptive field and thus triggering a LLM response (Dick, Ullman, & Sagi, 1991; Horowitz &
Treisman, 1994). Battelli et al.'s (2001) study (reviewed earlier) also suggests that attention is involved in classic AM: patients with parietal lesions (and associated visual attention deficits)
show abnormal classic apparent motion perception, but normal low-level motion perception.
Method
Apparatus As shown in Figure 4, two types of stimulus displays were used: AM and
flickering dots. The AM display was created by alternating two visual frames: in frame 1 two
white dots (0.5 deg; 62 cd/m ) were arrayed on diagonally opposed vertices of a square
(measuring 3 deg by 3 deg), and in frame 2 the dots were arrayed on the opposite pair of vertices
(see Figure 4a). The flickering dots display was also created by alternating two visual frames: in 22 frame 1 four white dots were presented, and in frame 2 no dots were presented (Figure 4b). The white dots and central fixation dot were presented on a gray background (16 cd/m"). This distance from the fixation dot to a white dot was 2 deg. For this experiment and each subsequent experiment, observers viewed the displays at a distance of 57 cm in a dimly lit room.
(a) AM (b) Flicker
Figure 4. Schematic of the stimulus displays used in Experiment 2: (a) Apparent Motion display (b) Flickering Dots display. Frame TI and Frame T2 were alternated in time. Cycle length (ms) was varied across trials.
For both displays, the cycle length (the time from the onset of frame 1 to the offset of frame 2) was varied. For example, a cycle length of 140 ms would correspond to presenting frame 1 for 70 ms and frame 2 for 70 ms. There were eight cycle lengths generated: 26.67,
45.33, 65.33, 84.00, 102.67, 121.33, 141.33, 160 ms.
Procedure In this method of constant stimuli 2-AFC task, the subject reported whether they saw two dots moving back and forth or four dots flashing on and off while they stared at the fixation dot. The subject initially performed 16 practice trials. Subsequently, each subject performed one block of 64 test trials (4 trials per cycle length for each display type). Each trial was 1000 ms long.
For each cycle length the average percent correct was plotted. The threshold cycle length
was taken as the point on the psychometric function at which the subject correctly distinguished
motion from flicker 75% of the time. If the subject achieved 75% accuracy for more than one 23 cycle length, the cycle lengths were averaged to give a threshold value. In summary, the cycle length threshold expressed the shortest cycle length at which the subject could discriminate between the AM and Flickering dots stimulus displays.
Results and Discussion As illustrated in Figure 5, a one-between (Group: control, amblyopes) x one-within (Eye: first eye tested, second eye tested) factor ANOVA showed no main effect of Group (Fi 35 = .727, p=.40) or Eye (F135 = 3.217, p=.08), nor a significant Group x
Eye interaction (Fi,35 = .65 8, p=.42). These findings suggest that the perception of classic AM is normal in both the amblyopic and fellow eye of children with unilateral amblyopia. An ANOVA that split the children with amblyopia into two groups (those with associated strabismus and those without) showed the same pattern of results as the initial ANOVA.
120
o 100 2 "0 80
H •B-C 60 c ID —J «U 40
o 20
Control 1 Fellow 1 Control 2 Amblyopic 2 Eye Figure 5. Results of Experiment 2. The minimum cycle length require to correctly identify motion vs. flicker is plotted for each type of eye. Shorter cycle length thresholds correspond to better performance.
Task performance was not significantly correlated with visual acuity or stereopsis 24
(see Appendices A and B, respectively). Although Cavanagh et al. (2001) have shown that individuals with visual attention deficits show deficits for this AM task, it is not apparent what the specific role of attention is in the perception of AM. Whether AM is due to an involuntary tracking of attention remains to be determined. To further investigate whether children with amblyopia have an attentive tracking deficit, performance on a task that definitively requires the use of voluntary attentive tracking, single-object tracking, was measured. 25
Experiment 3: Single-Object Tracking (HLM)
In one type of tracking task, the display consists of a central fixation dot and a set of identical discs moving around the fixation dot in a circular motion. While the subject maintains fixation, they must track the target disc (indicated by a brief colour change at the beginning of the trial). At the end of the trial, one of the discs briefly changes colour and the subject must indicate whether this disc is the target disc i.e. the disc that initially changed colour. This is an attention-based motion task because the subject cannot follow the target with their eyes while maintaining fixation but must attend to the target as it moves around the fixation dot. The subject, therefore, must selectively attend to the target disc, while filtering out irrelevant stimuli
(distractor discs).
Method
Apparatus As illustrated in Figure 6a, three arrays of four discs were alternated in space and time to create the perception of four white discs (size: 0.8 deg; 62 cd/m2) rotating around a central bull's-eye (size: 2 deg; 37 cd/m"). Each disc completed 12 "steps" in one revolution
(Figure 6b). The bull's-eye and white discs were presented on a gray background (16 cd/m2).
The distance from the bull's-eye to a white disc was 9 deg. We varied how fast the discs rotated around the bull's-eye. Eight speed values were generated: 0.05, 0.114, 0.179, 0.243, 0.307,
0.371, 0.436, 0.5 revolutions/s.
Procedure In this method of constant stimuli, 2-AFC paradigm, the observer's task was to track one white disc, while maintaining fixation on the central bull's-eye. The importance of
maintaining fixation was emphasized to each subject, and the experimenter monitored fixation
throughout the task. A trial began when the target disc was identified by turning red for 2000 ms
(see figure 6). Next, the target disc turned white again and the subject attentively tracked the 26
target disc for 1500 ms. Last, one of the discs turned red and the subject indicated whether it was
the disc they were tracking or a different disc. Each trial was 3500 ms.
o
M
Figure 6. Schematic of the stimulus used in Experiment 3. (a) Three arrays of four discs were alternated in space and time to create the perception of four discs rotating around a bull's-eye. (b) Each disc took 12 "steps" around the bull's-eye.
Each subject performed 16 practice trials. Subsequently, they performed one block of 64
test trials (8 trials per speed value). For each speed value the average percent correct was plotted.
The speed threshold was taken as the point on this psychometric function at which the subject
correctly identified the target 75% of the time. If the subject achieved 75% accuracy for more
than one speed value, the speed values were averaged to give a threshold value. The threshold
values express the maximum speed at which attentive tracking was possible. 27
Results and Discussion Figure 7 shows a Group x Eye interaction of the intersection type
(Gaito, 1973). In such a situation, the simple effect of Group for each eye was analysed, as suggested by Gaito (1973). A one-factor (Group: controls, amblyopes) ANOVA showed no difference between the eyes tested first: Control Eye 1 and Fellow Eye 1 (Fi,3s = 1.049, p=.31).
As evident in Figure 7, a one-factor ANOVA revealed a difference between the tracking ability
of Control Eye 2 and Amblyopic Eye 2 (Fi,35 = 4.182, p=.0484). This result suggests that the speed of the target disc had to be slower in the amblyopic eye compared to Control Eye 2 for successful tracking.
0 Control • Ainblyope
Eye 1 Eye 2
Figure 7. Results of Experiment 3. Average speed thresholds for each type of eye is plotted. Higher threshold values correspond to better performance.
Task performance was not significantly correlated with visual acuity or stereopsis (see
Appendices A and B, respectively). Attentive tracking ability was further investigated in
Experiment 4 by varying the number of objects tracked. 28
Experiment 4: Multiple-Object Tracking: "Bouncing Balls" (HLM)
In a typical multiple-object tracking trial, a subject is first presented with eight identical moving discs. The subject would then be asked to track, say, two moving green discs (that are deemed targets by briefly changing colour at the beginning of the trial) amongst a field of 6 identical moving green discs. At the end of the trial, the subject indicates which discs were the targets. This is an attentive tracking task because rather than following the target(s) with one's eyes (which is impossible once you must track more than one target anyway) the subject must fixate on the centre of the screen and thus "pay attention" to where each target is moving. Like the single-object tracking test, the subject must selectively attend (and in cases with more than one target, divide their attention) to the target disc(s), while filtering out the distractor discs.
While this task requires intact LLM (to perceive the motion of each target), it is an HLM because it adds the divided attentional component of keeping track of the targets as they move. Past research suggests that individuals can track up to four or five targets if they use concentrated effort (Pylyshyn & Storm, 1988).
Method
Apparatus The display was a 14 by 14 deg dark gray square (0.8 cd/m2) in which 8 identical green discs (1 deg; 53 cd/m2) moved in a semi-random fashion. Every 45 ms, each
disc's trajectory was subject to random variations, which resulted in unpredictable paths. The
discs "bounced" off the edge of the square and each other; thus, the balls never occluded each
other or collided with each other. The velocity of the discs was a constant 6 deg/s.
Procedure The subject's task was to track 1, 2, 3, or 4 disc(s), while maintaining fixation
on a central fixation dot. Once again, the subject was told it was very important to maintain
fixation, and the experimenter monitored fixation. A trial began when the target disc(s) turned 29 red (20 cd/m2)for 1200 ms (see figure 8). Subsequently, the discs turned green and the subjects attentively tracked the target disc(s) for 5000 ms. After the 5000 ms of tracking, the disc(s) stopped moving and the subject used a mouse to click on the target disc(s). Each trial lasted 6200 ms. Each subject performed 10 practice trials. Following practice, 40 test trials (10 trials for each tracking condition) were completed. It should be noted that the more balls as subject is asked to tracked, the higher the probability that they will correctly guess a target ball.
Tracking accuracy for each subject was calculated for each tracking condition (1 disc, 2 discs, 3 discs, 4 discs). Tracking accuracy was defined as the total number of balls tracked for that condition divided by the total number of target balls for the condition, multiplied by 100.
For instance, in the "track two" set size condition, there are a total of 20 balls to be tracked (10 trials x 2 balls/trial). If the subject correctly tracked 18 balls in the "track two" condition, their accuracy would be 90%. This type of accuracy calculation takes into account the increasing level of difficulty with set size; therefore, each tracking condition is equated for difficulty.
Results and Discussion A one-between (Group: controls, amblyopes) two-within (Eye: first eye, second eye; Balls Tracked: one, two, three, four) factor ANOVA revealed a main effect of Group (Fijs = 4.368, p=.04) (Figure 8). As is evident in Figure 8, the amblyopic and fellow eye performed worse on this task than both control eyes. This suggests a general deficit in
attentive tracking in amblyopia that affects both eyes. There was also a significant Eye x Balls
Tracked interaction (F3jos = 3.523, p=.02) (Figure 9). Performance was better in the first eye
tested when tracking multiple items, but when only one item was tracked performance was better
with the second eye tested. No other main effects or interactions reached significance. The same
pattern of results was found when the children with amblyopia were split into two groups (those
with strabismus and those with pure anisometropia). This indicates that the initial group 30 difference found (amblyopes vs. controls) is not due to a difference between one type amblyopia and controls, but between both types of amblyopia and controls.
Control Amblyope
Group
Figure 8. Results of Experiment 4. Tracking accuracy is plotted for each type of eye.
two three four
Number of Ball(s) Tracked
Figure 9. A significant Ball(s) tracked x Eye interaction: Eye 1 was better at tracking multiple balls, but Eye 2 was better at tracking one ball.
The previous three HLM tasks are presumed by Cavanagh to require some sort of
attentive tracking: involuntary tracking in the case of the AM task, and voluntary tracking in the
two object tracking tasks. The last task we used (a serial visual search task) requires visual
attention, but does not require attentive tracking. This task allowed investigation of whether the 31 type of visual attention deficit found in amblyopes (attentive tracking) extends to tasks that
require different aspects of visual attention (e.g. shifts of attention required for a serial search
task). 32
Experiment 5: Visual Search (HLM)
Johansson (1976) placed 10 to 12 lights on an individual's joints (a "point light walker") and filmed the individual moving in the dark. When the point light walker was stationary an observer reported seeing a meaningless pattern of lights; however, once the point light walker starting walking the observer could tell it was a human form. This phenomenon was termed
"biological motion". Cavanagh, Labianca, and Thornton (2001) explored whether the perception of biological motion required attention by creating a visual search task with biological motion walkers. The effect of set size on reaction time was used to classify the walker search task as parallel or serial. A parallel search task is assumed to require little visual attention because the number of distractors present does not change the time needed to find the target. In contrast, the use of focused attention in a serial search task is reflected in the increase in target detection times as the number of distractors increases (Treisman & Gelade, 1980).
Cavanagh et al. (2001) found that detecting the absence or presence of a target (a biological motion walker with a rightward or a leftward gait) in a field of distractors (walkers with the opposite gait) was a serial search, suggesting the involvement of visual attention in the task. Unlike the previous HLM tasks, this task requires shifts of attention as the subject shifts their visual attention from walker to walker instead of attentive tracking of the stimulus.
Method
Apparatus The display was presented on a 15 inch Apple Multiple Scan monitor. A biological motion walker's profile was simulated using 11 white dots near the walker's head,
shoulder, both elbows, both wrists, both knees, and both ankles (Figure 10). The walker either
had a rightward or leftward gait, and did not move across the screen but walked in place. The
distance from the central fixation cross to the middle circle of the walker was approximately 4 33 deg of visual angle. Each walker was 4 deg high, and each black dot making up the walker had a diameter of 0.2 deg (0.2 cd/m2).5 The gray background's luminance was 3.5 cd/m2. The walker's stride cycle was 1.3 s. Each walker's position around the fixation cross and the starting phase of the walker's stride was selected randomly for each trial. When more than one walker was present, the starting phase of the stride for each walker was selected randomly, and the walkers were spaced equally around the fixation cross. The set size ranged from 1 to 4 walker(s). The viewing distance was 57 cm
Procedure In this 2-AFC visual search task, the subject had to indicate the presence or absence of the target as quickly and accurately as possible while staring at the fixation cross. The experimenter monitored fixation. The subject had a maximum of 5 s to indicate the presence or absence of the target by pressing a key. The subject completed 80 trials (40 target absent, 40 target present). The independent variables were the number of walkers displayed and whether the target was present or absent, and the dependent variable was the time required to correctly answer (RT). Half of the subjects' targets had a rightward gait, while half of the subjects' targets had a leftward gait.
Results and Discussion A one-between (Group: amblyopes, controls) three-within (Eye: first eye, second eye; Set Size: one, two, three, four walkers; Target: absent, present) revealed a significant main effect of Eye (F1.35 - 6.022, p=.02). The average reaction time for eye 2
(1887.04 ms) was significantly faster than the average reaction time for eye 1 (1977.70 ms).
There was also a significant main effect of Target (Fij35 = 52.570, p=.0001): subjects responded faster when the target was present (1827.64 ms) compared to when the target was absent
(2037.10 ms). A main effect of Set Size (F3,ios = 150.604, p=.0001) revealed that subjects
5 Although these individuals dots are very small, Hyle, Raggio, Miller, and Owens (2001) findings suggest that the perception of biological motion is independent of acuity. 34 responded slower as set size increased (set size 1: 1538.06 ms; set size 2: 1805.78 ms; set size 3:
2086.57 ms; set size 4: 2299.07 ms). No other main effects or interactions attained significance.
The lack of a group effect indicates that individuals with amblyopia performed this task as well as control subjects. Performance for each eye is shown in Figures 11 and 12. Accuracy data
(Appendix C) suggests that there was no speed-accuracy trade-off. Performing an ANOVA with the amblyopes split into two groups showed a similar pattern of results to the initial ANOVA. • • • • • • • • • • • • • • • • • • • • • + • • • • • • • • • . •• • • • • • • • • •
Figure 10. Schematic of a trial used in Experiment 5. The target walker (bottom left corner) looks like he is "walking" to the left once the black dots that comprise his figure are set in motion.
3000 Target Present 2800 Target Absent 2600 o Control Eye 2400 Fellow Eye 2200
P 2000 C 1800 o at 1600 1400
1200
Set Size Figure 11. Results of Experiment 5. Reaction time is plotted as a function of set size for Control Eye 1 and Fellow Eye 1. Figure 12. Results of Experiment 5. Reaction time plotted as a function of set size for Control Eye 2 and Amblyopic Eye 2. 36
GENERAL DISCUSSION
Summary of findings
Individuals with amblyopia showed normal performance in their fellow and amblyopic eyes on the LLM coherence task and two HLM tasks: AM and visual search. The fellow eyes of individuals with amblyopia showed normal performance on the single-object tracking task, while the amblyopic eyes showed depressed performance on this task. The fellow and amblyopic eyes of individuals with amblyopia showed significantly worse performance than controls on the multiple-object tracking task.
How do the results of my study compare with other studies?
Two of the tasks I used have been used in amblyopia research in the past (coherence:
Giaschi, Boden, Dougherty, Lyons, & Cline, 1999; visual search: Rohaly & Karsh, 1998).
Rohaly and Karsh (1998) found that the fellow eye performed similarly to controls in a visual search task, whereas the amblyopic eye performed significantly worse than controls on a visual search task. In contrast, the present paper found that both the amblyopic eye and fellow eye performed similarly to control eyes. There are three potential reasons why these two studies have different results. First, Rohaly and Karsh's task was a divided attention task in which the amblyopic eye had to perform two tasks simultaneously. In my study, the amblyopic eye performed only one task. Second, Rohaly and Karsh's target and distractors were thirty degrees from central fixation, while our target and distractors were 4 degrees from fixation. It is possible that the reduced visual acuity of the amblyopic eye depressed their visual search performance: if target and distractor are both blurred it is easier to make an identification mistake. In my study visual acuity was not correlated with task performance (Appendix A). Third, the attentional deficit in amblyopia may be in peripheral but not central vision. 37
/
Levi (2001) claimed that foveal crowding is due to simple contrast masking, but peripheral crowding reflects limitations imposed by the resolution of attention in normals. If attentional resolution is a limiting factor in the peripheral vision of individuals with amblyopia but not central vision as well, it is possible that individuals with amblyopia will do worse on
HLM tasks with stimuli presented to peripheral vision versus stimuli presented to central vision.
In the present study children with amblyopia performed similarly to controls on the two HLM tasks with stimuli presented to central vision. The AM task stimuli were 2 deg from fixation, and the visual search task stimuli were 4 deg from fixation. The fellow eye showed normal performance on the single-object tracking task, while the amblyopic eye showed depressed performance. The disc tracked in the single-object tracking task was 8.5 deg from fixation. The balls in the multiple-object tracking moved in a 7 x 7 deg square: thus, the largest distance between two balls is 9.9 deg. Perhaps this task showed a deficit in children with amblyopia because they had to divide their attention over a large distance (relative to the other three HLM tasks).
Giaschi et al. (1999) assessed coherence thresholds in the fellow eye of 28 children with unilateral amblyopia and 120 control children. The performance of all patients with strabismic amblyopia was within normal limits relative to the age-matched control group. Three of the 18 patients with anisometropic amblyopia had abnormally high coherence thresholds (more than 2
standard deviations above the control-group mean). The amblyopic eyes of these patients were not tested. In the present study, the mean coherence thresholds for fellow and amblyopic eyes
did not differ from that of the control group. When the results for individual patients were
compared to the control group, one patient with anisometropic amblyopia showed an elevated
coherence threshold in both eyes. The probability of this occurring by chance is .02. It appears
that a small proportion of children with anisometropic amblyopia may have an LLM deficit. 38
Is performance on HLM tasks correlated with performance on the LLM task?
Because it could be argued that one of the children with amblyopia has a LLM deficit, I
wanted to investigate whether LLM and HLM task performance was correlated. A significant
correlation might suggest that the differences found between amblyopic and control groups on
HLM tasks was due to amblyopes with compromised LLM perception.6 As shown in Appendix
D, LLM task performance (motion coherence) was not significantly correlated with any of the
HLM tasks.
Is motion coherence an adequate measure of LLM perception?
Although it is generally agreed that motion coherence is a low-level task, a recent study
conducted by Raymond, O'Donnell, and Tipper (1998) suggests a potential role for attention in a
motion coherence task. In Raymond et al.'s study, each trial consisted of a prime and a probe.
Before each trial, subjects were instructed to attend to a particular direction of motion (the target
prime) and ignore the other direction of motion (the distractor prime) during the dual prime
display. In the dual prime condition, fifty percent of moving dots moved in one direction (e.g.
up) while the remaining fifty percent of moving dots moved in an orthogonal direction (e.g. left).
Subjects were then presented with a typical motion coherence task (a pattern of moving dots that
varied in the percentage of dots moving coherently across trials) in which they indicated the
direction of motion (the probe condition).
Raymond et al. (1998) found that motion coherence thresholds were significantly higher
(i.e., decreased motion sensitivity) when the target prime direction matched the probe direction,
and significantly lower thresholds (i.e., increased motion sensitivity) when the distractor prime
matched the probe direction. They concluded that changes in attentional state could alter 39 subsequent sensitivity to motion. This study, however, does not interfere with the present use of a motion coherence task as a measure of low-level motion processing. Although Raymond et al. showed that priming attention to a particular direction of motion results in decreased sensitivity to motion in that direction, their finding does not suggest that the coherence task itself requires attention.
Does the amblyopic eye truly show depressed performance for the single-object tracking task?
The analysis of the single-object tracking task revealed that Fellow Eye 1 performed similarly to Control Eye 1. Amblyopic Eye 2 performed significantly worse than Control Eye 2.
Taken together, these two results seem to indicate that the amblyopic eye exhibits an attentive tracking deficit while the fellow eye is normal at tracking. A closer look at Figure 7, however, suggests that the amblyopic eye does not have an attentive tracking deficit. The difference between Amblyopic Eye 2 and Control Eye 2 is not due to the fact that the amblyopic eye's speed threshold is lower than fellow and control eyes. It is due to the fact that the speed threshold of Control Eye 2 is higher than that of Control Eye 1. In fact, if we compare
Amblyopic Eye 2 with Control Eye 1, tracking performance is very similar. Moreover, the presumably "unaffected" fellow eye has a speed threshold that is similar to the "affected" amblyopic eye.
Why does the speed threshold improve between Control Eye 1 and Control Eye 2, but not between Fellow Eye 1 and Amblyopic Eye 2? It has been suggested that the improvement in performance between the control eyes, and the lack of improvement between the amblyopic and
fellow eye may be due to a "sensory practice effect" operating in the controls but not in the
amblyopes (Crewther & Crewther, 2001, personal communication).
6 Recall that HLM tasks require an intact LLM system in addition to an intact HLM system. For example, in the multiple-object tracking task subjects must be able to perceive the motion of the discs in order to attentively tracked 40
Because testing is monocular, it is possible that practice in Eye 1 enhances performance in Eye 2 through the binocular neurons of VI (a sensory practice effect). In essence, a "message" from Eye 1 is passed to Eye 2 through the neurons that they are both connected to. Animal studies have found that amblyopia may result in significantly fewer binocular neurons in VI.
Thus, this sensory effect may occur in controls (and thus enhance performance in their second eye) but not in individuals with amblyopia (due to a lack of binocular neurons to carry the
"message"). In other words, a sensory practice effect linked to binocular neurons may not be present in individuals with amblyopia because they lack the vessel of the practice effect
(binocular neurons).
One way to investigate the status of binocular neurons in children using psychophysical methods is to measure stereopsis. Stereopsis is the ability to perceive depth based on the differences in the images on the retinas of the two eyes (retinal disparity). In the present study, stereopsis was assessed by measuring stereoacuity (the smallest degree of retinal disparity that can be perceived as depth. A score of 70 or less is considered normal. Ten of the thirteen amblyopes had normal stereopsis (Table 1). Thus it seems likely that most of my subjects had some binocular neurons and should be able to exhibit a sensory practice effect similar to controls.
Stereopsis did not seem to affect whether a patient's performance improved between eyes. In fact, both subjects with a stereoacuity score of 400 showed improved performance between their fellow eye and amblyopic eye. In conclusion, the results shown in Figure 7 are probably not due to a sensory practice effect only operating in control subjects.
There are two other possible explanations for the results of Figure 7. First, the controls
may demonstrate a cognitive practice effect for performing this task, while children with
them. 41 amblyopia do not. In contrast to a sensory practice effect, a cognitive practice effect does not rely on interocular transfer, but merely refers to the fact that one may perform better on a task as they get practice at doing it. This situation suggests that the amblyopic eye is fine at attentive tracking. Second, both the controls and the amblyopes may have a practice effect, but because the amblyopic eye is worse at tracking than the fellow eye, the effect of practice is not evident.
The results of my study cannot distinguish between these two possibilities. A future study that tests the amblyopic eye first could answer whether the amblyopic eye has depressed performance for attentive tracking of a single object. The amblyopic and fellow eye did exhibit attentive tracking deficits for multiple objects.
What do the deficits in the multiple-object tracking task suggest about amblyopia?
An fMRI study conducted by Culham, Brandt, Cavanagh, Kanwisher, Dale, and Tootell
(1998) investigated the brain areas active while attentively tracking three of nine target discs.
Comparison of the patterns of activation associated with passive viewing of the stimulus display and attentively tracking the target discs suggests that the attentive tracking is associated with areas of the parietal cortex (bilateral activation of the intraparietal sulcus, postcentral sulcus, superior parietal lobule, and precuneus). The attentive tracking deficits exhibited by children with amblyopia suggest their visual attention deficit is associated with the areas of the parietal lobe found by Culham et al.
What do my results suggest about the fellow eye of individuals with amblyopia?
Some people assume that because the fellow eye has normal acuity, this eye is "normal".
The fellow eye is even sometimes used as the control eye for the amblyopic eye (eg. Hess &
Anderson, 1993). The depressed performance exhibited by the fellow eye on the multiple-object
tracking task suggests that amblyopia does not just influence the amblyopic eye, and the fellow 42 eye is not an adequate control eye. Other studies (Giaschi, Regan, Kraft, & Hong, 1992; Leguire,
Rogers, & Bremer, 1990) also suggest that the fellow eye is not normal.
While some studies have found a difference between individuals with strabismic and anisometropic amblyopia on some psychophysical tasks, the present study found that individuals with both types of amblyopia show a deficit in the fellow and amblyopic eye.
What is going on in amblyopia?
This study's findings agree with past studies that suggest there is a visual attention deficit in individuals with strabismic amblyopia (Levi, 2000; Sharma and Levi, 1999). This study also suggests that there is a visual attention deficit in individuals with anisometropic amblyopia.
Although the specific aspects of visual attention compromised in amblyopia remain to be further investigated, the present study suggests which aspects of visual attention may be compromised in amblyopia, and which aspects of visual attention appear to be intact. Involuntary capture of attention (as measured by the AM task) and shifting of attention (as measured by the visual search task) appear to be normal in children with amblyopia. The deficits for the multiple- tracking task suggest that children with amblyopia may have problems in the attentional pursuit of several moving objects over time and/or selectively attending to a few items while filtering out distractor items. An alternative interpretation is that the attentional deficit in amblyopia is in peripheral vision, but not central vision. 43
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Appendix A
Correlations between Visual Acuity and performance on each motion task*
TASK EYE CORRELATION (r) P-value Coherence Control Eyel -.121 .573 Control Eye 2 -.003 .990 Fellow Eye 1 -.248 .415 Amblyopic Eye 2 -.207 .498 Apparent Motion
Control Eye 1 -.095 .657
Control Eye 2 +.024 .911 Fellow Eye 1 -.277 .360 Amblyopic Eye 2 -.144 .639 Single-Object Tracking Control Eye 1 +.341 .341 Control Eye 2 -.139 .517 Fellow Eye 1 +.405 .170 Amblyopic Eye 2 +.304 .312 Multiple Object Tracking Control Eye 1 +.201 .346 Control Eye 2 -.119 .580 Fellow Eye 1 -.059 .848 Amblyopic Eye 2 -.504 .079 Visual Search (Target Absent) Control Eye 1 -.151 .482 Control Eye 2 +.060 .782 Fellow Eye 1 +.259 .394 Amblyopic Eye 2 +.205 .501 Visual Search (Target Present) Control Eye 1 -.270 .202 Control Eye 2 -.222 .297 Fellow Eye 1 +.236 .437 Amblyopic Eye 2 -.290 .336
* These correlations were also done for anisometropic amblyopes and strabismic amblyopes. None of the correlations attained significance. Appendix B
Correlations between stereopsis (Randot circles) and performance on each motion task*
TASK EYE CORRELATION (r) P-value Coherence Control Eyel -.017 .938 Control Eye 2 -.188 .378 Fellow Eye 1 -.298 .323 Amblyopic Eye 2 -.055 .858 Apparent Motion
Control Eye 1 -.007 .976
Control Eye 2 +.038 .861 Fellow Eye 1 -.065 .834 Amblyopic Eye 2 +.260 .391 Single-Object Tracking Control Eye 1 +.034 .876 Control Eye 2 +.073 .735 Fellow Eye 1 +.118 .701 Amblyopic Eye 2 +.456 .118 Multiple-Object Tracking Control Eye 1 +.188 .378 Control Eye 2 -.057 .790 Fellow Eye 1 -.409 .165 Amblyopic Eye 2 -.186 .544 Visual Search (Target Absent) Control Eye 1 +.112 .601 Control Eye 2 +.123 .567 Fellow Eye 1 +.535 .535 Amblyopic Eye 2 +.313 .299 Visual Search (Target Present) Control Eye 1 -.017 .935 Control Eye 2 +.130 .544 Fellow Eye 1 +.068 .825 Amblyopic Eye 2 +.142 .644
* These correlations were also done for anisometropic amblyopes and strabismic amblyopes. None of the correlations attained significance. Appendix C
r^~n control Eye 1 tmsm Fellow Eye 1
Set Size
Figured. Accuracy as a function of set size on Target Absent Trials for Control Eye 1 and Fellow Eye 1.
Control Eye 1 ESUjjj] Fellow Eye 1
12 3 4
Set Size
Figure C2. Accuracy as a function of set size on Target Present Trials for Control Eye 1 and Fellow Eye 1. 50
r^l Control Eye 2 Amblyopic Eye 2
.x. 1 ! 1 1
20 j 0 I
Set Size
Figure C3. Accuracy as a function of set size on Target Absent Trials for Control Eye 2 and Amblyopic Eye 2.
F^^l Control Eye 2 ftmm Amblyopic Eye 2
Set Si;
Figure C4. Accuracy as a function of set size on Target Present Trials for Control Eye 2 and Amblyopic Eye 2. Appendix D
Correlation between performance on the LLM (Coherence) task and each HLM task for the amblyopic subjects
TASK EYE CORRELATION (r) P-value Apparent Motion Fellow Eye 1 +.572 .041 Amblyopic Eye 2 +.263 .386
Single-Object Tracking Fellow Eye 1 -.122 .691
Amblyopic Eye 2 +.316 .293
Multiple Object Tracking Fellow Eye 1 -.074 .811 Amblyopic Eye 2 +.199 .514
Visual Search (Target Absent) Fellow Eye 1 -.189 .577 Amblyopic Eye 2 -.069 .824
Visual Search (Target Present) Fellow Eye 1 -.161 .636 Amblyopic Eye 2 +.324 .280
*Note. The overall alpha level is .05. A Bonferroni correction resulted in a critical alpha value of .005 for each correlation. None of the correlations attained signficance.