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 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. The University of British 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 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).