Optometry and Vision Science

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This is the collection for the University of Waterloo's School of Optometry and Vision Science.

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The adaptive elements of disparity vergence: Dynamics and directional asymmetries.
(University of Waterloo, 2018-04-24) Erkelens, Ian; Bobier, William
Vergence eye movements alter the angle between the two visual axes, creating changes in binocular fixation distance. They are primarily stimulated by retinal image disparities, but can also be driven by inputs from ocular accommodation (accommodative-vergence) and perceived proximity (size) changes. Because of these diverse and complex sensory inputs, the neuro-motor substrates that sub-serve vergence control possess robust adaptive capabilities to manage the interactions with other oculomotor systems (accommodation). This adaptive plasticity in vergence allows for a high degree of precision in binocular alignment to be maintained throughout life in the face of constantly changing environmental demands. The precise alignment of each eyes’ fovea is a fundamental requirement for stereopsis and the perception of depth in 3 dimensions. In a significant portion of the ophthalmic clinical population, the adaptive capacities of vergence are reduced or dysfunctional, leading to difficulties focusing clearly and comfortably at near distances such as books, computer screens and other hand-held devices. Furthermore, new wearable technologies such as virtual and augmented reality increase the demand on the adaptive capacities of vergence by drastically altering the congruency of the sensory inputs to vergence. Currently, our understanding of the mechanisms that underlie this adaptive control and their behavioral limits are limited. This knowledge gap has led to conjecture in the literature regarding proper rehabilitative therapies for clinical dysfunctions of vergence control and in the optimal environmental design parameters that should provide comfortable and compelling user experiences in wearable technologies like VR and AR. The inward (convergence) and the outward (divergence) turning of the eyes in response to retinal disparities are controlled by two separate systems and demonstrate significant directional asymmetries in their reflexive response properties. In general, reflexive divergence responses tend to be slower and longer than their convergence counter-parts. It is unclear whether the adaptive mechanisms are influence by these reflexive asymmetries. It is also unknown whether similar directional differences exist in the different adaptive capacities possessed by vergence. The purpose of the following dissertation was to characterize the effects of stimulus direction on the adaptive behavior of disparity-driven vergence eye movements with an end goal aimed at improving rehabilitation therapies for clinical populations with vergence dysfunction and providing valuable insight for the design and future development of wearable technologies like virtual and augmented reality environments. A series of 4 experiments were conducted in order to characterize the effect of stimulus direction and the physiological limits of the adaptive behavior within the two-main disparity vergence motor controllers, fast-phasic and slow-tonic. In each study, binocular viewing conditions were dichoptic, which allowed retinal disparity to be altered while the accommodative and proximity cues were clamped. Such designs create incongruencies between the sensory stimuli to vergence and thus elicits a much stronger adaptive response for observation than would normally occur when viewing real-world objects. Eye movements were monitored binocularly with a video-based infrared eye-tracking system at 250Hz using the head-mounted EyeLink2 system. A total of 14 adult binocularly normal controls and 10 adult participants with dysfunctional convergence control (convergence insufficiency) were recruited for the main studies. 4 controls completed the first two studies, 10 additional controls completed the third and fourth studies while the 10 participants with convergence insufficiency completed the fourth study. The results of this dissertation make four significant contributions to the current scientific literature pertaining to vergence oculomotor control and plasticity. 1) Both fast-phasic and slow-tonic vergence controllers display directional asymmetries in their general behavior and adaptive responses. 2) Reflexive fast-phasic divergence responses in controls tend to saturate at lower disparity-stimulus amplitudes than convergence under specific viewing conditions. This saturation limit is defined when the primary vergence response amplitude and peak velocity are unable to increase when the stimulus amplitude increases, suggesting saturation in neural recruitment and firing rates. Saturated reflexive vergence responses instead recruit an increased response duration (neural firing time) in order to produce larger amplitude responses. 3) Saturation in the fast-phasic divergence mechanism leads to saturation in the speed slow-tonic vergence adaptation. The function of the underlying reflexive fast-phasic response was found to be associated with the adaptive behavior of the slow-tonic mechanism, suggesting one drives the other, which is consistent with model predictions. 4) Convergence responses from individuals with convergence insufficiency are generally indistinguishable from that of the slower divergence responses of controls. These impaired convergence responses lead to impairment of the adaptive mechanisms underlying each fast-phasic and slow-tonic controller. Clinically, these results suggest that rehabilitative therapies for vergence control dysfunctions should primarily target the performance of the fast-phasic reflexive vergence mechanism. This work also suggests that improvements in adaptive capacities of vergence, known to be the mechanism under-pinning symptom reduction in these patient populations, should follow when reflexive fast-phasic responses are normalized. In terms of wearable technology, the generally limited adaptive plasticity demonstrated within divergence responses when compared to convergence in controls, provides a behavioral explanation for the increase in symptoms of discomfort when viewing distant objects in virtual reality environments. Future investigations should seek to determine the effects of other disparity stimulus parameters, such as contrast and spatial frequency on the adaptive behaviors of both fast-phasic and slow-tonic mechanism. Finally, the cerebellum is known to be central to the adaptation of almost every motor system and yet its role in the different adaptive capacities of disparity-vergence control remain unclear. Future studies should aim to characterize these neural structures role in the different vergence oculomotor adaptive mechanisms described here.
Binocular vision and fixational eye movements
(University of Waterloo, 2017-07-19) Nallour Raveendran, Rajkumar; Bobier, William; Thompson, Benjamin
Introduction In observers with amblyopia, abnormal patterns of amblyopic eye (AME) fixational eye movements (FEM) have been associated with monocular (reduced amblyopic eye visual acuity) and binocular sensory deficits (e.g. suppression) of amblyopia. However, it is unknown whether sensory deficits associated with amblyopia cause the FEM abnormalities. The overall goal of this thesis was to investigate the effect of monocular and binocular sensory function on FEM characteristics in observers with normal vision and observers with amblyopia. The specific objectives of this thesis were four-fold. The first objective was to investigate the effect of reduced visual acuity on FEM in observers with normal vision and in amblyopia. The remaining three objectives were experiment specific and were to understand the effect of binocular interaction on FEM in observers with normal vision and amblyopia. In all experiments, participants were instructed to fixate a target that was presented either dichoptically using haploscope or non-dichoptically. Then, the measured FEMs were analyzed in 3 different ways: 1) fixational stability (global bivariate contour ellipse area – BCEA), 2) characteristics of microsaccades and 3) fast Fourier transformation (FFT). Experiment-I Monocular visual acuity (VA) of controls was varied from 20/20 to 20/100 using plus lenses. The amblyopia group completed three monocular conditions; a) AME fixating, b) fellow eye (FFE) fixating and c) FFE fixating with VA matched to the AME using plus lenses. The results showed that the AME had significantly less stable fixation than the FFE even when visual acuity was matched between the two eyes. Similar results were noted for microsaccadic amplitude as well. Reduced VA also had no effect on fixational stability and microsaccadic amplitude in controls. Therefore, impaired AME fixational stability could not be explained on the basis of reduced VA. Experiment-II The objective of Experiment – II was to study whether is there any advantage of binocular fixation over monocular fixation and if there is any advantage, whether is it noted at all contrast levels. Fixation target contrast was varied from 0% to 100% while control participants fixated monocularly (fellow eye occluded) and binocularly. The results showed that the fixational stability was significantly improved during binocular fixation compared to monocular fixation for all contrast levels. FEMs were less stable when the stimulus contrast was 0%, (no central fixation target) during monocular as well as binocular fixation. Though FS was found to be significantly improved during binocular fixation, microsaccades were found to be not different between monocular and binocular viewing conditions. Experiment-III The objective of this experiment was to investigate the effect of binocular interaction on observers with normal vision by introducing a different form of binocular interactions such as binocular rivalry and monocular stimulation. FEMs were measured under three dichoptic viewing conditions; 1) binocular rivalry (orthogonal sinusoidal gratings), 2) monocular stimulation (left eye was presented with a grating and the right eye with a blank mean luminance screen), 3) dichoptic fusion (similarly oriented pair of gratings) and one non-dichoptic viewing condition (single grating presented to both eyes). The results showed that except during monocular stimulation viewing condition, there was no significant difference in fixational stability between the right eye and the left eye. Experiment-IV The objective of this experiment is to investigate the effect of binocular interaction on observers with normal vision and amblyopia by varying the interocular contrast level. FEMs were measured for both eyes simultaneously while interocular contrast was varied by reducing stimulus contrast to one eye whilst keeping it constant at 100% for the other eye. In controls, fixation stability was unaffected by interocular contrast except for when one eye viewed 0% contrast (no central fixation stimulus). In this case, the eye viewing 0% contrast had less stable fixation than the eye viewing 100% contrast. In observers with anisometropic amblyopia, interocular contrast had no effect for any condition. However, the amblyopia group had less stable fixation than the control group for all conditions. The results suggested that, in amblyopia, AME FEM were consensually controlled by the FFE under dichoptic conditions. However, in controls, the two eyes could behave independently. Conclusion Thus, the results of the thesis suggested that monocular sensory deficit (impaired VA) did not influence FEM. However, the relationship between AME VA and AME fixational stability during monocular fixation implied 2 possibilities, 1) abnormal FEM could contribute to impaired VA, and 2) an independent third factor such as positional uncertainty, cortical deficits could mediate both impaired VA and impaired FEM. Similarly, the results of this thesis also suggested binocular sensory deficit (suppression) did not influence FEM. During binocular fixation, AME fixation was consensually controlled by FFE. However, lack of fixation target influenced FEM which suggested positional uncertainty could have resulted in impaired FEM in AME.
Investigating the enhancement of visual cortex plasticity through non-invasive brain stimulation
(University of Waterloo, 2024-01-02) Chen, Xiaoxin; Thompson, Benjamin; Bobier, William
Purpose: (1) To investigate the effects of various non-invasive brain stimulation (NIBS) modalities, including high-frequency transcranial random noise stimulation (hf-tRNS), anodal transcranial direct current stimulation (a-tDCS), and repetitive transcranial magnetic stimulation (rTMS), on short-term ocular dominance plasticity in adults with normal vision; (2) To probe the neural mechanisms underlying short-term ocular dominance plasticity using NIBS techniques; (3) To explore the state-dependency of NIBS within the visual cortex; (4) To evaluate the efficacy of a novel ocular dominance test (the letter-polarity test) as a tool of measuring ocular dominance shifts following monocular deprivation (MD). Methods: Three studies using hf-tRNS, a-tDCS and rTMS were conducted. NIBS was delivered to V1 during MD. The primary outcome was ocular dominance shift, measured through two ocular dominance tests, a traditional binocular rivalry test and the letter-polarity test, before and after the interventions. Secondary outcomes included mixed percept durations and alternation rates as provided by the binocular rivalry test. The reliability of the letter-polarity test was evaluated in comparison to the binocular rivalry test through a comprehensive set of analyses. Results: (1) In three studies, short-term ocular dominance plasticity was observed as a shift in ocular dominance towards the deprived eye. (2) No significant effects of NIBS were observed on the primary and secondary outcome measures. (3) By comparing the effect of 120-minute MD and 30-minute MD, we observed a significantly smaller magnitude of ocular dominance shifts with 30-minute MD. (4) The reliability of the letter-polarity test was similar to that of the binocular rivalry test. Conclusions: These experiments suggest that the neural mechanisms underlying short-term ocular dominance plasticity in adults with normal vision may be more complex than a simple reduction in cortical inhibition. It may be necessary to reconsider the cortical site responsible for this plasticity and the neuromodulatory effects of NIBS on visual cortex activity. Our null findings of NIBS effects may also be explained by a different cortical activation state induced by MD. These findings provide valuable reference points for future studies investigating the enhancement of visual cortex plasticity.
Understanding the sensory and motor behavior of accommodation in progressive myopic children
(University of Waterloo, 2017-09-27) Labhishetty, Vivek; Bobier, William
Introduction: Accommodation and vergence, in unison, help in maintaining a clear and single binocular vision, a linchpin for normal vision development. Under natural viewing conditions, accommodation and vergence systems mutually interact with each other through a unique cross-link mechanism. Based on the empirical data from visually normal adults, several control theory models were proposed to predict the behavior of accommodation and vergence. Progressive myopic children, however, exhibit a transient but consistent abnormal accommodative behavior which is not predicted by the currently accepted models. Progressive myopes exhibit high accommodative adaptation, elevated AC output coupled with high accommodative lags. Several predictions, both sensory and motor origin, have been proposed to explain this behavior however they failed to predict this behavior completely. Furthermore, empirical conclusions on the impact of myopia over accommodation and blur sensitivity were mostly based on data obtained from adult population. Accordingly, this dissertation examined the sensory and motor mechanism of accommodation in young children with and without progressive myopia. Data from children were also compared to adults. Methods: 12 children (8-13 years), 6 myopic and 6 non-myopic, and 6 naïve adults (25-33 years) were recruited for the purpose of this dissertation. Varying demands of accommodation were stimulated using a Badal optical system and recorded using custom-built dynamic photorefraction system (DPRS). Theoretical and empirical designs were developed to identify the ideal sampling rate necessary to measure a dynamic accommodative response. First and second order main sequence relationship and strength of the cross-link (CA) response was tested in the first experiment. In the second experiment, blur discrimination thresholds were tested psychophysically using simulated blur targets at two different stimulus demands. Furthermore, objective DOF and sensitivity of accommodative system to blur were examined using known demands of lens defocus and compared across different subject groups. Results: The new custom built high speed photorefractor (DPRS) was found to be superior in measuring dynamic accommodation compared to the commercially available Power Refractor (p <0.001). The current dissertation makes four major contributions to the field of accommodation and myopia: first, a motor deficit such as a sluggish or rigid plant cannot explain the abnormal behavior of accommodation in myopes. This was established based on the findings of a similar first and second order main sequence relationship between myopes and non-myopes (p > 0.05). Also, CA/C measures were not found to be attenuated as would be expected with a rigid plant. Novel simulations with reduced blur sensitivity coupled with a motor recalibration of AC gain did predict the myopic behavior. Second, atypical response patterns of accommodation such as the dynamic overshoots and double step responses were reported to exist with blur-driven accommodation. These patterns were suggested to occur due to an incorrect initiation of the response based on the similar main sequence findings between typical and atypical responses. Refractive error had no influence on the frequency of the atypical response in children. However, the influence of age reflects a developmental pattern in these dynamic atypical responses. Third, young progressive myopes showed large DOF and reduced accommodative sensitivity to lens defocus compared to non-myopes and adults. However, the ability to detect blur was similar between myopes and non myopes. Finally, depth of focus was found to increase with an increase in the stimulus demand. This change in the depth of focus was strongly correlated to a change in the microfluctuations but was independent of changes in the pupil size. Discussion: These findings confirm that school aged myopic children exhibit an altered pattern of accommodation. The pattern appears to be associated to progressive myopia but cannot be readily explained by current motor models of accommodation and vergence. Empirical data and model simulations suggest that a sluggish or rigid motor plant (lens and ciliary muscle) cannot predict this altered behavior completely. The current dissertation speculates a reduced sensory gain (or reduced blur sensitivity) as the prime factor coupled with a compensatory motor recalibration in the accommodative convergence cross link. The large lags of accommodation reflect a large objective depth of focus in myopes. However, the capacity to detect blur was found to be independent of the retinal defocus present in the myopic eyes. The elongating eye of a progressive myopes appears to compensate the persistent retinal defocus on the retina by some form of a perceptual adaptation.

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