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Vision is important; two projects were conducted to explore the enhancement of vision (multisensory combination, novel procedure to improve visual functioning). In the first project, audition was combined with vision, to examine if the conjunction of two sensory modalities benefits sensory decision-making in the context of looming - a potentially dangerous motion requiring quick responses (e.g., Neuhoff, 2001). Four behavioural experiments were conducted, testing responses towards auditory, visual, or audio-visual motion-in-depth signals (i.e., looming or receding). From behavioural data, the decision-making mechanism was explored in two separate analyses: 1) a comparative approach (e.g., Innes & Otto, 2019) which compared empirical data to predictions made using probability summation (Raab, 1962), and 2) a computational modelling approach which determined the best permutation of an interactive probability summation rule (Otto & Mamassian, 2012) to fit the empirical data. Altogether, the first project revealed that decision-making had a speed benefit when auditory and visual signals were both present, compared to either modality alone, and this benefit could be explained by a probability summation rule with simple audio-visual interactions. Not found was evidence of especially quick processing uniquely towards audio-visual looming signals. For the second research direction, a pilot study was conducted to test a novel short-term monocular patching technique which purportedly induces eye dominance via latent neuroplasticity (e.g., Lunghi, Burr, & Morrone, 2011), with the idea that it could become a procedure for improving amblyopic visual functioning. In one small-scale experiment (n=4), a significant dominance effect on the patched eye was found, but the effect was short-lived. Two single-participant experiments indicated that patching durations should not be too short (10 minutes) and stereopsis sensitivity may slightly change after patching. This pilot study offered a preliminary look into patching effects, on the back of which further experiments are suggested.

Inter-ocular suppression occurs when very different images are presented to each eye. Diplopia ensues if different images are superimposed and perceived. The brain removes this unfavourable viewing experience by suppressing one eye’s input to enable clear single vision. Inter-ocular suppression during visual development occurs in response to sufficiently disparate images caused by strabismus (misalignment of the visual axis) or anisometropia (uncorrected difference in refractive error), and if persistent may result in amblyopia. This is reduced visual sensitivity, usually in one eye, to a range of visual functions that cannot be corrected by refraction. Furthermore, binocular vision is reduced or absent. Depth and extent of suppression is measured across the central visual field in healthy participants with monocularly blurred vision, healthy participants with monocularly reduced luminance using neutral density (ND) filters, and participants with naturally disrupted binocular vision and/or amblyopia. Suppression of spatial stimuli defined by luminance (L) and luminancemodulated noise (LM) was compared to that measured for stimuli defined by contrast-modulated noise (CM), for which there is no change in mean luminance. For all stimuli suppression depth increased with increased imbalance of binocular input. Suppression was of a similar depth across the visual field with imposed blur and localised central suppression was found with ND filters. Microstrabismics showed central suppression, while strabismic amblyopes showed central in addition to hemifield suppression. Suppression for all participants was measured to be deeper for CM spatial stimuli than for LM spatial stimuli. This is suggested to be a result of CM stimuli engaging more binocular mechanisms of processing, than LM stimuli, thereby becoming more sensitive to disruptions of binocularity such as those produced in the participants in the present study. CM stimuli are therefore more sensitive to detecting suppression, which is associated with amblyopia.

Visual acuity is reduced when optotypes are viewed in the presence of surrounding contours. This reduction in acuity is known as the crowding effect and is thought to be caused by a varying combination of contour interaction, gaze instability and attention. Traditional studies have used single optotypes surrounded by flanking bars to investigate crowding. Such targets may not realistically replicate the crowding effect inherent in clinical vision charts. The aim of this thesis was to systematically investigate the effect of crowding on visual thresholds in subjects with normal vision and in subjects with amblyopia, using specially designed charts. In the 1st and 2nd experiment, contour interaction was assessed using a high (80 %) and low contrast (5.8%) Sheridan Gardiner repeat letter (SGRL) chart in subjects with normal vision. The effect of contour interaction was investigated by varying the inter-letter separation in the SGRL chart. Significant contour interaction was obtained at the abutting condition for both the contrast conditions. In the 3rd experiment the same protocol was repeated but in amblyopes. Significant contour interaction was obtained at 0.2 letter separation and the abutting condition for both the contrast conditions. The effect of contour interaction appears to be less for low contrast than for high contrast letters in normal, non-amblyopic and amblyopic eyes. Finally, in the 4th experiment a Sheridan Gardiner Complex Interaction (SGCI) chart that requires imposed gaze fixations was constructed to measure visual acuity in normal’s and amblyopes. The effect of any gaze instability on crowding was investigated by comparing SGRL thresholds to SGCI thresholds. The SGCI thresholds were higher than the SGRL thresholds at all the separations measured, suggesting an important effect of gaze instability on crowding. In conclusion, this research has shown that gaze instability is an important component of the crowding effect for letter chart acuity measurements. Visual acuity especially when screening for amblyopia should be measured using a whole optotype chart that requires optotype to optotype fixation.

Amblyopia is the leading cause of monocular visual impairment in children. Therapy for amblyopia is extremely beneficial in some children but ineffective in others. It is critical that the reasons for this discrepancy are understood. Emerging evidence indicates that current clinical protocols for the diagnosis of amblyopia may not be sufficiently sensitive in identifying individuals who, on more detailed examination, exhibit subtle structural defects of the eye. Presently, the magnitude of this problem is unknown. The aim of this study was to establish the prevalence of subtle retinal/optic nerve head defects in eyes diagnosed with amblyopia, to distinguish between possible explanations for the origin of such defects and to investigate the relationship between quantitative measures of retinal structure, retinal nerve fibre layer thickness and optic nerve head dimensions. Using the imaging technique of Optical Coherence Tomography (OCT) retinal structure has been investigated in detail, following the visual pathway across the retina from the fovea, via the paramacular bundle to the optic disc, where peripapillary retinal nerve fibre thickness has been imaged and subjected to detailed measures along with optic disc size and shape. The study formed two phases, the first imaging the eyes of visually normal adults and children, comparing them to amblyopes, both adults and children who had completed their treatment. The second phase, a longitudinal study, investigated retinal structure of amblyopic children undertaking occlusion therapy for the first time. By relating pre-therapy quantitative measures to the visual outcome the second phase of the study aimed to examine whether OCT imaging could identify children achieving a poor final outcome. The results show a clear picture of inter-ocular symmetry structure in all individuals, visually normal and amblyopic. Optic disc characteristics revealed no structural abnormalities in amblyopes, in any of the measured parameters, nor was there any association between the level of visual acuity and the measured structure. At the fovea differences were shown to occur in the presence of amblyopia, with thickening of the fovea and reduction of the foveal pit depth. The structural changes were found to be both bilateral and symmetrical with the fellow eye also affected. In the longitudinal phase of the study these changes were demonstrated to a greater extent in children who 'failed' to respond to treatment. This bilateral, symmetrical structural change found at the fovea, which has not been previously reported, cannot therefore be the primary cause of the visual loss which has been diagnosed as amblyopia.

In the past decade, considerable attention has been paid to examination of the impact of amblyopia, and strabismus, upon the lives of the individuals. Although an extensive amount of literature exists regarding amblyopia and its associated visual defects, little is known about the contribution of the amblyopic eye in the habitual viewing condition (i.e. both eyes viewing). The purpose of these studies was to determine whether amblyopes are disadvantaged in the performance of tasks under habitual viewing conditions, highlighting any functional differences which may exist as a consequence of amblyopia. Secondly, the work aimed to investigate whether the amblyopic eye contributes to the habitual performance of these tasks. A simple light detection task, in a dichoptic arrangement based upon blue/yellow stimuli viewed through yellow filters, was used to investigate the above two aims and investigate the degree of interocular suppression in amblyopic participants. Using a 3D motion analysis system performance was assessed for an obstacle crossing task (adaptive gait) and a task of reaching for and grasping of an isolated object and in a 'cluttered' environment. Fine motor skills were assessed in a threading a needle task. On the whole it was found that amblyopes are not disadvantaged under habitual viewing conditions, and in cases where differences were found to exist this appeared to be in tasks requiring speed and accuracy. Consistently across all studies it was found that the amblyopic eye contributed in a positive manner, thus, as in visual normals, two eyes are better than one.

PURPOSE: In the Vision in Preschoolers (VIP) Study, the SureSight wavefront autorefractor was shown to be one of the best screening tests for identifying preschoolers with one or more targeted vision conditions, however, it is no longer commercially available. The purpose of this study was to evaluate the ability of the QuickSee (QS) wavefront autorefractor to detect children at risk for targeted vision conditions (significant refractive error [RE], amblyopia, and strabismus).METHODS:Children ages 3 to 12 years presenting for comprehensive eye exams were recruited. Non-cycloplegic QS refraction (with and without +2 D (diopters) fogging lenses) and unaided binocular near visual acuity (NVA, HOTV) were measured. Exam findings, including cycloplegic retinoscopy, were used to determine the presence of RE, amblyopia (corrected distance visual acuity 20/40 or worse in one or both eyes or ≥2 lines inter-ocular difference and amblyogenic factor present), and constant strabismus. QS performance was summarized by area under the Receiver Operating Characteristic (ROC) curve (AUC), sensitivity, and specificity for various levels of RE for 1) all participating children (Tables 3 and 4), 2) for children ≥ 4 years (Table 8), and 3) for children ≥ 5 years (Table 9). Sensitivity and specificity of the QuickSee measurements to identify any significant refractive error were also determined using 1) AAPOS 2021 criteria (for children ≥ 4 years) and 2) modified Orinda criteria (for children ≥ 5 years). Similar analyses were performed for any significant vision condition (any significant refractive error or amblyopia or constant strabismus). Logistic regression analysis was used to evaluate the benefits of adding 1) binocular unaided NVA of 20/40 or worse or 2) global stereoacuity worse than 250 sec of arc to the model using only Quicksee for detecting various levels of significant hyperopia. Frequency tables were used to investigate the relationship between any significant vision condition as identified by QS referral criteria, and 1) binocular unaided NVA of 20/40 or worse or 2) global stereoacuity worse than 250 sec of arc. Paired t-test was used to compare QS with and without fogging lenses.RESULTS:Mean age was 8.15 (± 2.52) years (n=177). RE ranged from -9.25D to +8D sphere, up to 5.25D cylinder, and up to 3.5D anisometropia. Testability of the QS was 94.4% (167/177). Area under the curve (AUC) was ≥0.92 for each RE. QS referral criteria were chosen to maximize sensitivity at a specificity of approximately 85-90%. For detection of any vision condition, the sensitivity and specificity of QuickSee were 93% and 78.6%, respectively using AAPOS 2021 criteria and 83.3% and 87.1%, respectively using modified Orinda criteria. There was no significant benefit of adding NVA or stereoacuity to QS for detection of hyperopia (p=0.34 and p=0.8, respectively) or any targeted vision problem. There was no significant difference between QS measurement with or without fogging lens (p=0.51).

Sleep is thought to be a critical regulator of neural circuitry during neurodevelopment. Under conditions where postnatal neurodevelopment is modified - either by altered sensory experience, or in the context of genetically-mediated neurodevelopmental disorders - sleep is commonly altered or disrupted. However, the precise role sleep plays in promoting brain plasticity is still unknown, and it is unclear whether normalizing sleep could help normalize brain functions in atypical neurodevelopment. My dissertation work addresses this gap, by measuring how sleep loss affects recovery of disrupted visual cortex function in a mouse model of amblyopia (a form of vision loss caused by altered experience during postnatal development) and testing how normalizing sleep behavior affects memory disruptions in a model of Fragile X syndrome (FXS; a genetic disorder). My electrophysiological, behavioral, and molecular data suggest that sleep not only is critical for recovery mechanisms in disrupted neurodevelopment, but is also a useful target for therapeutic intervention. The first part of this dissertation (Chapter 2) examines the relationship between sleep and recovery of disrupted cortical dynamics in early neurodevelopment. Here, I examined the relative impacts of visual experience and sleep on recovery of cortical visual responses in a mouse model of amblyopia. Using monocular deprivation (MD) to disrupt vision for one eye during early postnatal development, we used in vivo single-unit recordings and immunohistochemistry (IHC) to measure how binocular and monocular visual experience influenced recovery of deprived eye responses after MD. We found that binocular experience during recovery from MD vastly improved restoration of visual responses to the deprived eye. Furthermore, this recovery was dependent on sleep. Together, both binocular visual experience and subsequent sleep could be important factors mediating recovery of visual cortical responses in amblyopia. The second part of this dissertation assesses the intersection of sleep and recovery of hippocampal-cognitive dysfunction in neurodevelopmental disorders. Here, I tested how a novel hypnotic, ML297, which acts via activation of G-protein inward rectifying potassium (GIRK) channels can rescue sleep loss and subsequent memory consolidation in a mouse model of FXS. First (Chapter 3), using C57BL/6 mice, we determined how ML297 affects memory consolidation using EEG/EMG recordings, contextual fear conditioning and IHC. We found that ML297 reversed reductions in NREM and REM spectral power, along with REM sleep amount and led to improved contextual fear memory. In a second study using Fmr1-/y mice (Chapter 4), we characterized sleep architecture and oscillatory activity for the first time in the FXS mouse model. We found deficits in NREM sleep architecture, spectral power, and inter-cortical coherence that were restored with ML297. Restoration of sleep via ML297 also showed improved cognition via fear learning and spatial memory tasks. Lastly, we found hippocampal activation patterns associated with memory formation change with ML297 to facilitate improved cognition. Overall, rescue of abnormal sleep could help with cognitive dysfunction in FXS and possibly other neurodevelopment disorders. Together, these findings enlighten us on the contributions of sleep to cortical development and hippocampal cognition in the context of neurodevelopment. While both have been extensively measured in past studies, the role of sleep remains vastly understudied. Further research into the underlying mechanisms by which sleep facilities recovery in these domains may lead to future and targeted therapeutic intervention for the diverse number of developmental and neurological disorders where sleep loss impacts plasticity and cognition.

Advances in technology allow us to track gaze across multiple environments, from the laboratory to the natural world as well as in virtual reality. This has far reaching basic and clinical implications. This dissertation explores both. Our eyes are constantly in motion, helping us piece together a coherent visual perception of the natural world around us. Even during fixation, when the eyes are focused on an object of interest, tiny fixational eye movements shift the position of the image landing on the retina. Our oculomotor system is adapted to the regularities in our natural world, and as a result our visual system functions quickly, comfortably, and accurately in the natural environment. Previous work has shown oculomotor behavior is abnormal in visual disorders such as amblyopia. Chapter 2 of this dissertation describes a study to measure fixational stability in children with amblyopia currently undergoing treatment. This work demonstrates that fixational eye movements can be used as a nonverbal metric for the recovery of visual function in amblyopia. Chapter 3 measures the statistics of fixation and binocular disparity in the natural environment as well as virtual reality (VR) headsets. This work suggests that there are key differences in the statistics of fixation and disparity between the natural and VR environments. The vertical horopter and natural disparity statistics show a top back pitch in the natural environment. This top-back pattern is lacking in the disparity statistics of the virtual environment, and the differences are quite dramatic (in the lower visual field the natural environment has disparities that are much nearer than in VR, an effect of around 900 arcsec!). We additionally found that gaze is biased towards straight ahead viewing and farther fixation distances in VR, as compared to the natural environment. Chapter 4 follows up on the findings from the third chapter, and tests whether observers prefer content that is congruous with the statistics of the natural world in virtual reality. This work found that scene content that violates the top-back pitch pattern of the natural world leads to more discomfort. This suggests that the mismatch between the statistics of the natural and VR-environment documented in chapter 3, is leading to conflict and discomfort in VR headsets.

Amblyopia is the most common visual disorder in children and is potentially curable if detected early and treated properly in the first few years of life. Amblyopia is the leading cause of monocular vision loss in children (Bradfield, 2013). It is a developmental neuroplasticity which derives from birth causing structural and functional changes in the eye and brain. With this structural and functional disruption, visual blur occurs due to refractive amblyopia, strabismic amblyopia, cataracts (form-deprivation amblyopia), or a combination of any of these (Solebo, Cumberland, & Rahi, 2015). Refractive errors related to amblyopia can also occur. The purpose of this evidence-based practice project was to determine if screening a pediatric population ages 9 months, 24 months, 36 months, and 48 months using an automated visual screener would affect the number of refractive errors detected. The Stetler Model’s stepwise process for gathering sound evidence was used to guide this evidence-based practice project at a busy Midwest pediatric clinic. Anyone that failed the screening was referred to ophthalmology for further testing. Post-intervention group data were collected on patients from the designated age groups receiving visual screening during a well-child check-up by two designated providers over a three-month period. Pre-intervention group data were collected from electronic health records for patients in the same designated age group receiving a well-child check-up by the same two providers as post-intervention data over a three-month period. Data were analyzed using Pearson’s Chi-Square Goodness of Fit test in an effort to show the sensitivity and specificity of the automated visual screener to screen for amblyopic risk factors. Of the total sample size (N = 322), there were 161 in the pre-implementation group and 161 in the post-implementation group. Results supported the PlusOptix™ S12 vision screener in identifying more refractive errors than traditional visual exams performed during routine well child check-ups (χ2 = 20.184a, p < 0.001, 99% CI).

Amblyopia is a developmental visual disorder resulting in sensory, motor and attentional deficits, including delays in both saccadic and manual reaction time. It is unclear whether this delay is due to differences in sensory processing of the stimulus, or the processes required to dis-engage/shift/re-engage attention when moving the eye from fixation to a saccadic target. In the first experiment we compare asymptotic saccadic and manual reaction times between the two eyes, using equivalent stimulus strength to account for differences in sensory processing. In a follow-up study, we modulate RT by removing the fixation dot, which is thought to release spatial attention at the fovea, and reduces reaction time in normal observers. Finally, we discuss the implications for these findings on future amblyopic treatment, specifically dichoptic video game playing. Playing videogames may help engage the attentional network, leading to greater improvements than traditional treatment of patching the non- amblyopic eye. Further, when treatment involves both eyes, fixation stability may be improved during the therapeutic intervention, yielding a better outcome than just playing a video game with a patch over the non-amblyopic eye.

Purpose: Vision-related quality of life (VRQoL) of children with and without amblyopia is poorly understood from the children's perspective. The aims of the work presented in this thesis were to understand aspects of VRQoL in children with and without amblyopia from their perspectives and to examine parents’ awareness of their children’s VRQoL. Methods: A number of studies were conducted. Firstly, an existing instrument designed to assess VRQoL in children with visual impairment was tested for validity to assess amblyopic children using Rasch analysis. Secondly, semi-structured interviews with amblyopic children and their carers were qualitatively analysed. Thirdly, items generated from the qualitative study and the literature were tested in amblyopic and control children and developed into a unidimensional instrument designed to assess vision-related activity limitation (VRAL), a subset of VRQoL according to the Rasch model. Construct validity of the newly developed instrument was then tested by comparing amblyopes at different stages of treatment and controls. Finally, parental awareness of their children’s VRAL was tested using the newly developed instrument. Results: An existing instrument designed to assess VRQoL in children with visual impairment was shown to be poorly targeted for amblyopic children supporting the argument for a need of a new instrument to assess VRQoL in amblyopic children. Qualitative interviews indicated that many amblyopic children reported negative social and emotional experiences and problems doing activities attributable to their amblyopia. Subsequently, 43 items were developed into an instrument which was piloted in a total of 101 amblyopic and control children. A VRAL questionnaire was formed which fit the Rasch model but a valid subscale from the items relating to social and emotional wellbeing could not be formed. The VRAL scores were significantly lower in amblyopic children than in those without amblyopia. Parents were mostly aware of the difficulties experienced by their children based on the qualitative data and the VRAL individual item scores. Conclusions: Vision-related activity limitation in amblyopic children was found to be poorer than in controls based on the VRAL questionnaire. Further, social and emotional well being may be negatively impacted. Parental knowledge of their children’s VRAL did not depend on whether their child was amblyopic or not in this sample.

Amblyopia occurs in up to 5% of the normal population, and is a unilateral or (rarely) bilateral visual disorder that occurs during the critical period of brain development and produces deficits in visual function such as acuity and stereopsis, plus abnormalities in eye movement (EM) and eye-hand coordination (EHC). Previous approaches to the assessment of EM and EHC are limited and poorly developed for clinical use and research. In particular, they are subjective, require a high degree of skill, are time consuming or are costly. The aim of this study was to develop novel tests of EM and EHC using simple computer-based objective tasks. These two tests (the Lee EM Test and Lee-Ryan EHC Test) were validated and used to examine differences in performance in normal, untreated amblyopic and treated-amblyopic vision under monocular and binocular viewing conditions in children and teenager/young adults. The Lee EM Test was based on the â Northeastern State University College of Optometry (NSUCO) Testâ . In the Lee EM test, video recordings were made of individuals making saccadic and pursuit movements. Recordings were analysed regarding ability and accuracy of EM plus head and body movements on 5-point scales. The Lee-Ryan EHC Test comprised an i-Pad® app and the involved the use of a stylus pen to trace 13 shapes of different degrees of difficulty. For each shape, time taken to complete the trace, number of errors, mean speed, x/y axis deviation and mean deviation are measured and automatically saved by a program on the iPad platform. The EM testing revealed significant differences between normal, amblyopic and treated-amblyopic vision in both saccadic (p

Both functional and physical properties related to vision are known to be abnormal in amblyopia. The former include spatial and temporal visual deficits such as reduced letter acuity and temporal sensitivity, crowding, and decreased position acuity; the latter refers to eccentric and unstable fixational eye movements in strabismic amblyopia. This dissertation represents parallel studies that aim to characterize amblyopic vision from these angles using different approaches. In the first series of experiments, we systematically explored the size and spacing requirements for identifying a letter among other letters by measuring flanked and unflanked letter acuities in normals and amblyopes, centrally and peripherally, while fixing the medium contrast and brief presentation duration. At fixation, for normals and non-strabismic amblyopes, no crowding has been found: legibility of a flanked letter is limited by overlap masking or acuity depending on the letter spacing. In normal peripheral vision and strabismic amblyopia, legibility is limited by crowding unless the letter spacing is very loose. We show that strabismic and non-strabismic amblyopia are much like normal vision at increased eccentricity or with added blur, respectively, as characterized by the crowding-acuity ratio. For clinical screening tests for strabismic and non-strabismic amblyopia, we recommend measuring both unflanked letter acuity and flanked letter acuity with tight spacing.In the second series of experiments, we explored the effects of various factors on both flanked and unflanked visual acuities in normal observers. Specifically, we varied polarity, contrast, and duration of the letter stimuli, as well as the pupil size of the observer by adding a pinhole. We find that the critical spacing for crowding in the fovea is 0.09 ~ 0.1 deg. The main effects of and interactions between the above factors provide the context and supplement to the results from the above experiments, based on which we extend the letter legibility model developed from the above experiments by parameterization of each legibility limit to account for a wide range of conditions.In the third series of experiments, the first experiment estimated the spatiotemporal mechanism for detecting a luminance increment of a bright bar embedded in spatiotemporal noise for normal and amblyopic observers. The normal template is characterized by a temporal summation zone surrounded by symmetric spatial inhibition zones and followed by a temporal inhibition zone. The abnormal amblyopic template lacks inhibition but with normal temporal summation. Neither blurring the stimuli in space and time nor varying the signal-to-noise ratio caused any significant change in the normal or amblyopic template. However, decreasing the fundamental frequency of the stimuli restored the normal template in the amblyopic eye. Furthermore, the normal periphery shares spatial properties with amblyopia but the temporal properties may be different. The second experiment mapped the dynamic of spatiotemporal interactions of crowded stimulus orientations in normal and amblyopic eyes. For normal eyes, crowding occurs at locations adjacent to the target in space and around or before the target presentation in time. Different types of cues may affect the spatiotemporal interaction map differently, but the strongest crowding never coincides with the target presentation. The amblyopic spatiotemporal interaction map is not substantially different from that of the normal eyes, except that in general the spatiotemporal interaction is more widely distributed around and after the target presentation. "Anti-crowding" - repulsion of perceived orientation that is induced by farther flankers is documented in both normal and amblyopic eyes.In the fourth series of experiments, an adaptive optics scanning laser ophthalmoscope (AOSLO) was used to record fixational eye movements, allowing direct viewing of retinal movements and fixation locations. Fixational eye movements were modeled as a stochastic process governed by a potential function assumed to have the form of a quadratic polynomial for both normal and strabismic amblyopic observers. Our results confirm that microsaccades, on average, correct for fixation inaccuracy. Microsaccades occur more frequently and tend to move faster towards the target at relatively large displacements. Drift is modeled as a Brownian motion with constant rate over time plus an error-correcting component initially following a microsaccade. The unstable eccentric fixational eye movements in strabismic amblyopia are characterized by frequent intrusive saccades with large amplitudes and high speeds. The strabismic amblyopic fixation pattern on multiple loci is task dependent, with less eccentric but unstable fixation induced by challenging tasks, and relatively stable fixation by small highly visible static targets.

The aim of this study was to add to our understanding of interocular interactions in normally sighted children (Group I, N=20), normal adults (Group II, N=20) and adults with anisometropic amblyopia (N=12) by investigating responses to a range of visual functions under three kinds of viewing condition. Visual functions tested were visual acuity, contrast sensitivity and alignment sensitivity. Stimuli were generated on a Cambridge VSG card driving a high resolution monitor and FE liquid crystal goggles, enabling three kinds of viewing conditions: 1. Monocular (non-tested eye occluded), used as a baseline for most functions. 2. Dichoptic (uniform field presented to the non-tested eye but with a binocular fusion lock). 3. Binocular. In general, binocular performance was better than monocular (binocular summation) but so too was dichoptic performance (dichoptic advantage). However there was much variation within individuals (the three functions showing different summation/advantage pattern) and between individuals. Significant conclusions include: (a) Maturational windows for interocular interactions differ for different spatial visual functions. (b) Interpretations of results from one visual function cannot be applied automatically to other functions. (c) Care must be taken in interpreting results based on 5 or fewer subjects.

Blood oxygen level dependent (BOLD) functional magnetic resonance imaging (FMRI) was implemented at 1.5 Tesla to characterize visual cortex activation patterns in human amblyopia. The dissertation consists of three main components. First, the voxel-wise repeatability of visual cortex FMRI results was assessed using a mixed-effects ANOVA group analysis. Two stimuli were presented monocularly and binocularly to ten subjects in two independent scanning sessions. Visual cortex activation maps were found to be repeatable regardless of the stimulus type or eye stimulated. The second study investigated interocular differences in the level and extent of activation in amblyopic and normal subjects. Five amblyopes and six normals were recruited. Stimuli included a homogeneous flickering field (8 Hz) and counterphasing (8 Hz) vertical sinusoidal gratings at 0.5, 1 and 2 cpd. Amblyopes exhibited a larger (P