Browsing by Author "Stevenson, Scott B."
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Item A Comparison of Contour Interaction and Crowding(2018-08) Marten-Ellis, Stephanie; Bedell, Harold E.; Stevenson, Scott B.; Manny, Ruth E.Purpose: The ability to correctly identify a target is significantly reduced by the presence of flanking distractors. The term “crowding” is often used to encompass a large variety of target and flanker types. A major aim of this study was to enumerate the differences between the impairment of target identification produced by flanking bars (contour interaction, “CI”) and flanking letters (crowding, “CW”). Both the magnitude and extent of CI and CW were studied in the context of foveal and peripheral viewing, for various background luminances, and in color congruent versus incongruent conditions. Methods: Subjects (n=4) viewed Sloan letters (C, D, H, K, N, O, R, S, V, Z) and were asked to identify such target. Flanking objects were placed at various distances from the target to induce CI or CW. Subjects repeated this task for a variety of target eccentricities (foveal versus 1.25, 2.5, 5.0, and 10 degrees in the inferior visual field), background luminances (photopic, mesopic, high scotopic, and low scotopic), and color-congruent (flanker and target either matched or did not match in color) conditions. Average percent correct letter identification was plotted as a function of flanker-to-target separation (in arcmin). Results: A significant interaction effect was found in all conditions. In the fovea, a change from photopic to mesopic luminance produced a significant decrease in both CI and CW. In scotopic conditions, peripheral CI also was reduced. No significant effect of color congruency was found. The extent of foveal CW and CI was approximately 5 arcmin. During peripheral viewing the extent ranged from 6-40 arcmin for CI and from 10 to greater than 40 arcmin for CW, with the extent increasing as targets were viewed further from the fovea. Conclusion: This study suggests that the use of flanking bars versus letters should not both be subsumed within the term “crowding” and that future comparisons must consider flanker type. The effects of luminance are consistent with a center-surround receptive-field neural model for CI. The results regarding flanker type and color congruency raise further questions on the role of flanker complexity and “top-down” influences on CW and CI.Item Accommodative Stability in the Developing Visual System(2015-12) Roberts, Tawna L.; Anderson, Heather A.; Manny, Ruth E.; Stevenson, Scott B.; Hertle, Richard W.Purpose: The purpose of this work was to gain an understanding of the impact that extrinsic, intrinsic and cognitive factors have on accommodative accuracy (lag) and variability (RMS) in young children with uncorrected hyperopia. Individuals with greater amounts of hyperopia must exert a larger accommodative response to achieve the same level of accommodative accuracy as an individual with a lesser hyperopic refractive error. Given that accommodative responses are positively correlated with RMS, which in turn is positively correlated with depth-of-field (DOF), children with uncorrected hyperopia may experience greater RMS that may also impact their ability to detect blur. Additionally, the accommodative response has been shown to be influenced by the cognitive demand of the viewing task, which may impact the RMS observed in these children. Experiments investigating accommodative accuracy and variability under different viewing conditions were performed on both children with varying amounts of uncorrected hyperopia and adults for comparison with a mature visual system. Methods: Lag and RMS were measured over 3 experiments using photorefraction in children 3 to <10 years with a range of uncorrected hyperopia (+0.06D to +4.91D spherical equivalent (SE) of the most plus eye) and visually normal uncorrected adults (+0.08D to +1.51D SE). Experiment 1 investigated the impact blur and disparity cues have on lag and RMS by systematically removing these cues from the stimulus. Relationships between total accommodative response and lag and RMS were investigated. Experiment 2 measured blur detection thresholds and DOF psychophysically. Associations between blur detection thresholds with independent variables RMS, age, SE and lag and associations between DOF and RMS were investigated. Experiment 3 investigated the impact of cognitive effort on lag and RMS as functions of SE and the total accommodative response for both passive and active viewing conditions. Results: Experiment 1) In children, accommodation was most accurate and stable when blur and disparity cues were present in the stimulus. The removal of blur cues from the stimulus resulted in both greater lag and RMS than the removal of disparity cues. In adults, RMS was not impacted by the removal of blur or disparity while lag was significantly greater when blur-cues were removed. Experiment 2) In children, increased RMS, increased SE and younger age were significantly associated with increased blur detection thresholds. RMS and age were independently associated with blur detection thresholds when controlling for the other independent variables. Additionally, increased SE was significantly associated with increased RMS when controlling for age and lag. Experiment 3) In children, increased cognitive effort resulted in a smaller average accommodative lag and decreased RMS. Increased SE was significantly associated with increased lag and increased RMS in the passive viewing condition only. In adults, cognitive effort did not significantly affect lag, however, a small significant difference in RMS was detected with cognitive effort compared to passive viewing but the difference is unlikely to be clinically meaningful. Children vs. Adults) The children had significantly greater DOF and blur detection thresholds and RMS than the adult subjects throughout all experiments. Conclusions: Children do not have adult-like RMS, which may be secondary to children having larger blur detection thresholds and DOF than adults. Children appear to rely more heavily on both blur and disparity cues for accommodative accuracy and stability than adults. Additionally, cognitive effort appears to improve accommodative accuracy and stability in children more than adults, as measured responses in adults did not change with cognitive effort. Lastly, the impact of uncorrected hyperopia on lag and RMS was variable across conditions suggesting that while children with uncorrected hyperopia may experience greater lag and RMS at times, their accommodative behavior is not consistently poorer across near tasks.Item Autonomic Markers of Visual Awareness(2013-12) Li, Ziyang 1987-; Sheth, Bhavin R.; Ogmen, Haluk; Contreras-Vidal, Jose L.; Francis, David J.; Stevenson, Scott B.The mind–body problem in philosophy examines the relationship between mind and matter, and in particular, the relationship between consciousness and the brain. In order to provide a scientific footing to this centuries old philosophical problem, an investigation into the interaction between consciousness and the autonomic nervous system, which controls the internal viscera, is required. However, this issue has received scant attention to date. Here, I investigate the response of the autonomic system and its sympathetic and parasympathetic components, to visual awareness using classical paradigms of binocular rivalry and visual detection, using a combination of electrocardiography (ECG), impedance cardiography (ICG) and pupillometry to examine cardiac autonomic functions, namely heart rate, the high-frequency component of heart rate variability (HRV), pre-ejection period (PEP) and change in pupil area. My studies reveal that the parasympathetic component dominates the autonomic response to visual awareness; physical alternation of stimuli has effects on the autonomic activation that go above and beyond alternations in percept; and uncertainty of subjective judgment drives the dynamics of autonomic response. The present studies, from the autonomic pathway, demonstrate that “mind affects body in action”, which leads to a more integrative view of sensory awareness and suggests the involvement of structures in the nervous system above and beyond the cortex.Item Design of an Automated System for the Retrieval of Emotional Content in Natural Images(2018-05) Ambati, Saikiran; Sheth, Bhavin R.; Han, Zhu; Nguyen, Hien Van; Stevenson, Scott B.; Hernandez, Arturo E.Emotions often drive us and our behaviors. An ability to reliably and automatically estimate the degree and kind of emotion aroused in a typical individual has widespread applicability. Two dimensions of emotion are key: arousal, or the extent to which a scene excites/calms and its valence, i.e. the extent to which it is pleasant/unpleasant. Past attempts to automate emotion detection in images have failed. Here, we adopt a hybrid, integrated approach that broadly consists of two components: a front-end consisting of a bank of classifiers that recognize the presence of specific semantic content (e.g. person/animal/beach) in the image; a back-end that weights the importance of each semantic category to generate a discrete output of image valence and arousal. Model performance was compared with ground truth, i.e. ratings provided by human subjects: test accuracy was 96.0% on valence (chance=50%) and 92.0% on arousal classification (chance=33%) across the externally validated image set.Item Effect of Rapid Alternating Binocular Flicker on Vergence and Stereopsis(2023-08) Ramakrishnan, Bhavatharini; Stevenson, Scott B.; Manny, Ruth E.; Coates, Daniel R.Purpose: Rapid, alternating occlusion therapy is one of several binocular treatment options for amblyopia. Recent studies suggest it may improve stereoacuity and letter acuity in some amblyopic subjects. Little is known about its impacts on binocular functions like stereopsis and vergence. Here we examine the effect of alternating (out-of-phase) flicker and in phase flicker of different frequencies on stereopsis and vergence eye movements. Methodology: Stereoscopic disparity thresholds were measured using a 30 trial, 1 up 2 down staircase beginning at 10 arc minutes disparity. Five staircases were obtained from each of five healthy adult subjects for all combinations of contrast (100%, 50%, 25%) and flicker (2, 4, 8, 16 and 32Hz, in-phase and out of phase binocular flicker, and no flicker). The stimulus was a spatially filtered noise pattern (1 cpd, 1 octave bandwidth), presented for one second. Vergence eye movements were measured using a binocular dual Purkinje image eye tracker. The vergence target was a 30-degree diameter filtered noise pattern of spatial frequency either 0.25 or 2 c/deg with a bandwidth of 1 octave. Stimulus contrast ranged from 3% to 100%. The trials were 2 seconds long. Results: Flicker elevated stereoscopic thresholds in most subjects. The flicker did not abolish stereopsis, however: subjects were able to detect a few arc minutes of disparity at all but the lowest frequencies alternating phase flicker. Lower frequencies of flicker impaired but did not abolish stereoscopic thresholds in most subjects. Vergence responses did not show any difference at higher contrast stimuli for all the flicker frequencies. At lower contrasts, latency, velocity, and final amplitude accuracy were lower for lower frequency compared to high frequency flicker. Conclusion: When it comes to stereopsis, results show that thresholds are elevated by both alternate and in phase flicker but are mildly impacted only at the lowest temporal frequencies of 2 and 4Hz. The results of vergence measures show that there are minor differences in vergence responses between out of phase and in phase flicker and the vergence responses showed a trend of decrease in vergence performance compared to vergence responses with in-phase flicker.Item Environmental and Behavioral Factors in Childhood Myopia(2021-08) Bhandari, Khob R; Ostrin, Lisa A.; Richdale, Kathryn L.; Stevenson, Scott B.; Harb, Elise N.Purpose: Near work and light exposure are known risk factors for myopia. This dissertation validates the Clouclip, a rangefinding device, to assess its utility in measuring viewing distance and ambient illumination, and to use it along with an actigraph device and questionnaire to measure environmental and behavioral risk factors for myopia in four different studies. Methods: Study 1 validated the Clouclip for measuring distance and ambient light, and measured viewing behaviors during four near tasks in adults. Study 2 measured viewing behaviors in children during similar tasks as in study 1. Study 3 utilized the Clouclip and Actiwatch in participant’s habitual environment over a one week measurement period in adults. Study 4 used the Clouclip, Actiwatch, and a questionnaire to comprehensively quantify visual behaviors over one week in myopic and non-myopic schoolchildren. Results: Study 1) Clouclip measured distance and ambient light levels were comparable to actual values, and viewing distance in adults differed by type of near task but not by refraction. Study 2) Viewing distance and posture differed in children by type of near task 3) Clouclip- and Actiwatch-measured illumination were correlated, while questionnaire overestimated compared to objective sensors. Study 4) Objective measures showed that myopic children spent a greater amount of time on near + intermediate viewing and less time outdoors compared to non-myopic children. Questionnaire-derived electronic device use was 12.0 ± 0.7 hrs per day with no differences between refractive error groups. Conclusion: Wearable objective sensors allowed precise quantification of behavioral components related to myopia. Findings demonstrate that several factors differ in the visual environment between myopic and non-myopic children.Item Impact of eye movements on the visual crowding effect(2023-05-08) Prahalad, Krishnamachari; Coates, Daniel R.; Das, Vallabh E.; Stevenson, Scott B.; Hafed, ZiadTo study the role of eye movements of different magnitudes on the perception of a crowded stimulus. Given that eye movements are ubiquitous, and essential in helping us navigate our surroundings, understanding their implications in visual perception is a crucial step in understanding visual processing.Five experiments are described: (1) We characterized the oculomotor behavior of normally sighted individuals who were trained to use different artificial preferred retina loci (PRL) positioned in distinct locations of the visual field when performing a reading-like visual task. (2) We monitored microsaccadic behavior (a proposed attentional correlate) to understand how naturally occurring shifts in attention following the presentation of a crowded stimulus affect visual performance. (3) We investigated the role of microsaccadic preparation when crowded stimuli were presented at a future microsaccade goal 20 arc minutes from the center of gaze. (4) Using the same paradigm, we investigated the effect of microsaccade preparation on the shape of the crowding zone. (5) We further dissociated the microsaccadic goal and the stimulus location to understand the effect of microsaccadic preparation on performance at other locations within the foveola.(1) We observed that a right artificial PRL caused the spontaneous development of excessively large saccades and more direction switches. (2) The spatial extent of visual crowding was found to decrease by 26% on trials which contained incongruent microsaccades (proposed to indicate attentional capture). (3) The influence of microsaccade preparation on performance was variable between subjects, with the temporal pattern across subjects indicating a general enhancement across most target-flanker spacings tested. (4) When crowded stimuli were presented within 50 ms from microsaccadic onset an exclusive benefit in performance was observed when flankers were oriented along the tangential axis. (5) When the saccade goal differed from the stimulus location, the benefit in performance immediately before microsaccadic onset was no longer present.Different PRL positions lead to different oculomotor patterns, which we suggest may help explain biases in PRL selection amongst individuals with central visual loss. Fixational eye movements have diverse impacts on crowded stimuli at different eccentricities from the center of gaze.Item Investigating Gaze Orientation and Spatial Localization in Strabismus(2023-05-15) Karsolia, Apoorva; Das, Vallabh E.; Stevenson, Scott B.; Nurminen, Lauri; Tamber-Rosenau, Benjamin J.Purpose: Disruption of binocular vision during the critical period of development results in strabismus in 3-5% of the population. The visual system adapts to this decorrelation with the help of suppressive mechanisms that influence eye choice behavior. The overall goal of this research was to identify visual and non-visual factors that may impact gaze orientation and localization behavior in strabismus. These studies shed light on the mechanisms underlying binocular vision and spatial localization and provide insights into the temporal dynamics of visual suppression and its impact on eye-choice behavior. Methods: In Aim 1, development of horizontal and vertical ocular alignment was assessed in six prism-reared infant monkeys using Hirschberg photographic methods. In Aim 2, ten human subjects with normal ocular alignment localized briefly presented targets, presented to same or alternate eyes under dichoptic conditions, in an unreferenced environment. In Aim 3, eye movements were recorded in two adult exotropic monkeys while performing memory saccade tasks with variable delays to assess influence on persistence of visual information on fixation-preference. Results: Aim 1: Monkeys reared with prisms during infancy developed strabismus as early as 3 weeks of age (~3 months in humans) suggesting influence of both visual and non-visual mechanisms in development of normal alignment. Aim 2: Under conditions of binocular competition (dichoptic viewing), human subjects were unable to compensate for their inherent phoria and made greater errors as compared to same-eye viewing condition. Aim 3: Fixation preference behavior was observed in adult prism-reared monkeys during memory-guided saccades, similar to patterns observed during visually-guided saccades. Memory delays up to 800msec did not alter fixation preference behavior. Conclusion: Prism-reared monkeys mimic strabismus in humans and are a useful model to study its behavioral and neurophysiological implications including influence of oculomotor proprioception. Binocular dissociation in absence of visual cues, leads to inaccurate localization even in normal ocular alignment, indicating that extra-retinal eye position feedback in the form of oculomotor proprioception may be imprecise or derived from the wrong eye. Visual suppression in strabismus leads to long-lasting adaptations that influence eye choice behavior beyond the stimulus presentation time (800ms), as indicated by fixation patterns of localization.Item Neural Control of Vertical Vergence and Extraocular Muscles Compartmentalization(2022-08-08) Adade, Samuel; Das, Vallabh E.; Stevenson, Scott B.; Chino, Yuzo M.; May, Paul J.Purpose: Vertical vergence is an important eye movement used to adjust the vertical position of each eye to achieve and maintain binocular fusion. This dissertation investigated the neural circuitry that controls vertical vergence by determining the contribution of each cyclovertical extraocular muscle (EOM) to its generation using both behavior and neurophysiological approaches. We also tested for evidence for selective innervation of EOM compartments to explain MRI observations of horizontal recti compartments contributing to specific vertical eye movements and that not all compartments of the cyclovertical EOMs contribute to all vertical eye movements. Methods: Horizontal and vertical position of both eyes were recorded as two awake behaving non-human primates performed a dichoptic asymmetric vertical vergence task. We (1) measured and compared vertical fusion amplitude and vertical vergence at different horizontal gaze directions, (2) recorded single-unit activity of cyclovertical motoneurons and analyzed the relationship between their firing rate and vertical eye position during vertical vergence and (3) recorded the single-unit activity of horizontal motoneurons during vertical vergence and vertical smooth-pursuit to complement cyclovertical motoneuron recordings and test for compartmental innervation. Results: (1) Vertical fusion amplitude and vertical vergence differed at different orbital eye positions, but was idiosyncratic between the eyes and between animals. (2) The firing activity of neurons in all four cyclovertical motoneuron populations (n=149) correlated with vertical eye position during vertical vergence indicating the involvement of all cyclovertical EOMs. Moreover, we identified paradoxical behavior in that almost half of the cyclovertical motoneurons modulated with movement of either eye. (3) Only 2/56 of the tested horizontal motoneurons modulated their activity with vertical eye position, and 147/149 of the tested cyclovertical motoneurons modulated their activity during vertical vergence, indicating an absence of evidence for compartmental innervation. Conclusion: All cyclovertical EOMs contribute to the generation of vertical vergence, but their relative contribution may differ and also vary amongst different subjects. Paradoxical behavior suggests the existence of eye specific premotor control for vertical vergence, and so show some similarities to horizontal vergence control. The EOM compartments are not selectively innervated, and the observed differential behavior is likely not driven by oculomotor control.Item Perception and Eye Alignment Control: Do People Think the Same Way Their Brain Thinks?(2023-08) Pham, Matthew Q; Stevenson, Scott B.; Das, Vallabh E.; Coates, Daniel R.Purpose: This thesis compares two distinct but related binocular disparity processing phenomena— stereoscopic depth perception and reflexive disparity vergence. Stereoscopic thresholds typically improve with increasing target contrast, but when only one eye’s target contrast increases, the stereoscopic thresholds are worse than for equal low contrast. A similar effect occurs for motion and vernier thresholds when the target components being compared have different contrast. The effect occurs especially for middle and low spatial frequencies. This study compares observers’ stereoscopic depth perception with vertical disparity vergence to understand if this ‘contrast paradox’ affects reflexive eye alignment control. Methods: Four binocular contrast combinations were tested, (OD/OS= 0.125/0.125 ‘low/low’, 0.125/0.5 ‘low /high’, 0.5/0.125 ‘high/low’, and 0.5/0.5 ‘high/high’), at 0.5cpd and at 1.0cpd spatial frequencies. Left/right image pairs were fused with the aid of a four-mirror haploscope and nonius lines were superimposed on the image to allow the subject to monitor eye alignment. Responses to imposed vertical disparity were detected indirectly as observers reported the direction of shifts in nonius alignment. Stereoscopic thresholds were measured by detection of a difference in horizontal disparity between upper and lower halves of the stereogram. A staircase method was used to find the disparity that produced 75% correct performance in each condition, taken as the mean of the last five staircase trials. Results: Results from 5 subjects show strikingly similar effects for stereoscopic depth and vertical eye alignment, including the detrimental effect of mixed contrast. Conclusion: These results imply that both stereopsis and eye alignment may be impaired for individuals with unequal retinal image contrast, as for example in unilateral cataract or monovision prescription.Item Perceptual Filling-in and Reading with Central Scotomas(2012-08) Pratt, Joshua; Bedell, Harold E.; Stevenson, Scott B.; Chino, Yuzo M.; Woo, Stanley Y.; Timberlake, George T.Purpose: Macular degeneration can be severely disabling as patients frequently develop central blind areas (scotomas) which impair their ability to read, drive, and recognize faces. To compensate for the loss of central vision, patients with absolute central scotomas must learn to use their peripheral retina to perform tasks normally performed with the fovea/parafovea. Because of perceptual filling-in, patients perceive characteristics of a scene, such as its color or texture, within the area corresponding to complete vision loss and generally are unaware of the scotoma border and location. I hypothesized that perceptual filling-in makes it more difficult for patients with central scotomas to read effectively using peripheral vision and that making the boundaries of the scotoma visibile would be beneficial as a training mechanism. The specific purpose of this dissertation was to investigate the effects of perceptual filling-in on fixation and reading speed in patients with bilateral central scotomas. Methods: In experiment 1, I investigated the retinal locus used and the stability of fixation in patients with bilateral central scotomas for six targets, three expected to fill-in and three with letters. In experiments 2 and 3, I examined whether the reading speed of normal subjects is affected by the visibility and information content of a simulated central scotoma. In experiment 4, I tested whether perceptually delineating the visual field location of the scotoma improves reading speed in patients with bilateral central scotomas. A gaze contingent display was used, first to map the scotoma, and then to display the scotoma location continuously as a high-contrast polygon while patients read computer-presented text. Results: Eleven of twelve subjects in experiment 1 used a retinal location closer to the vestigial fovea to fixate targets expected to fill-in, compared to letters. Target type produced no overall significant difference in fixation stability, which was measured as bivariate contour ellipse area (BCEA). However, for some individual subjects, fixation on letter targets tended to be more stable. In experiments 2 and 3, elapsed reading times were longer for simulated scotomas that were less visible and contained more linguistic information. Subjects adopted different eye-movement strategies for the more and less visible scotoma types, and the improvement in reading speed for the least visible scotoma type was associated with a decrease in saccadic amplitude and an upward shift of the mean fixation locus. Older subjects had more difficulty than younger subjects reading with a simulated central scotoma, especially the scotomas that were less visible and contained more linguistic information. In experiment 4, reading speed improved in all but one of the patients with central field loss after a brief period reading while viewing a polygon that marked the scotoma location. After experience with the overlaid polygon, five of seven patients shifted their fixation location to position the scotoma further from the center of the text, thereby, imaging more of the text on viable retina. Conclusions: The results of experiment 1 suggest that in patients with central field loss, letter targets generate more consistent fixation behavior than fill-in targets. The results also indicate that fixation using fill-in targets does not allow clinicians to estimate the visual field location of a central scotoma reliably. The data obtained from experiments 2 – 4 suggest that enhancing scotoma visibility has the potential to improve reading speed and to train effective eccentric viewing in patients with central scotomas. The vertical shift of the mean fixation locus demonstrated by normal subjects and by most patients with central field loss suggests that upward eccentric viewing is an adaptive oculomotor strategy in the presence of a central scotoma.Item Predicting Driving Fitness in a Low Vision Clinic: Correlating Two Tasks(2017-05) Johns, Katherine; Modi, Swati C.; Bedell, Harold E.; Harwerth, Ronald S.; Stevenson, Scott B.Purpose: The Useful Field of View (UFOV) computer task is a visual processing assessment tool used to aid in predicting driving fitness in low vision patients. The Dynavision 2000 board tests visuomotor reaction time, and also has been investigated for training skills pertinent to driving. However, very few studies to date have investigated Dynavision scores with relation to criteria for determining driving fitness, and standard normative scores have not yet been established. This study investigated the correlation between performance on the UFOV and Dynavision tasks, as the tasks could be used interchangeably if the correlation were strong. Performance data on both tasks were gathered for normal-vision adults and low vision patients of driving age. The effect of age and gender on subject performance on each task also was analyzed. Methods: Fifty-one normal-vision adults and 17 low vision patients participated on the UFOV and the Dynavision Mode A tasks. Each subject completed one UFOV computer trial, which measures visual processing speed (ms) as flashed images must be identified and located on the screen, and three 60-s trials on Dynavision Mode A, which determines visuomotor reaction times as a random sequence of illuminated buttons are struck on a large wall-mounted board. The UFOV Selective Attention task (Task 3) was analyzed in isolation because most normal subjects achieved the optimal threshold for Task 1, Processing speed (100% normal vision vs. 59% low vision) and Task 2, Divided Attention (84% normal vision vs. 6% low vision). Results: UFOV and Dynavision scores correlated positively and significantly (p<0.05) in normal adult (r2 = 0.392) and low vision subjects (r2 = 0.479). Increasing age was associated with poorer scores on both the UFOV and Dynavision tasks in the normal adult and low vision cohorts. Gender effect on performance was significant only for Dynavision performance in normal vision subjects, as males performed with shorter reaction times. Gender did not significantly affect performance on UFOV or on Dynavision for low vision patients. Conclusion: Performance on the UFOV and Dynavision tasks correlate moderately in both low vision and normal vision adult subjects. The amount of variability on one task accounted for by the other (39% for normal-vision adults and 48% for low vision patients) is not enough to support the interchangeability of the two tasks clinically. Dynavision should be further and more specifically investigated for its individual cutoffs, sensitivity, and specificity for identifying driving candidates and correlating to crash risk. If found to be related to driving capability, the results of both UFOV and Dynavision tasks could be combined to improve the recommendations concerning driving by low vision patients.Item Prediction of Accommodative Optical Response in Young and Pre-Presbiopic Human Eyes Using Ultrasound Biomicroscopy(2014-12) Ramasubramanian, Viswanathan; Glasser, Adrian; Stevenson, Scott B.; Porter, Jason; Anderson, Heather A.; Schwiegerling, JamesPurpose: Clinical accommodation testing involves measuring either the accommodative optical response (AOR) or the accommodative biometric changes in the ocular anterior segment. Currently, it is not possible to measure both with a single instrument. Measuring the AOR and the accommodative biometric changes are important for evaluating accommodation restoration concepts. The specific goals of this research are: 1) to perform automated, objective measurements of accommodative biometric changes from ultrasound biomicroscopy (UBM) images in young phakic eyes; 2) to measure the static AOR using a Grand Seiko (GS) autorefractor and infra-red photorefraction (PR) in young eyes and to predict the AOR from UBM measured biometric changes; 3) to measure the AOR using GS and PR and the biometric changes using UBM in pre-presbyopes to predict the AOR; 4) to calculate and correct the spatial and optical distortion in Visante optical coherence tomography corneal images; 5) to construct accommodative schematic eye models for individual eyes for each of the young and pre-presbyopic subjects and calculate refraction and AOR from the schematic eye models. Methods: Experiments were 1) Accommodative anterior segment biometric changes were measured in response to 0 D to 6 D accommodative stimuli in 1 D steps in 26 young human subjects using a 35 MHz UBM and an A-scan ultrasound. 2) Static AOR to the same stimulus demand were measured with GS and PR in the same group of young subjects. AOR was predicted from UBM measured biometry parameters using linear regression, 95% confidence intervals and 95% prediction intervals. 3) Static AOR to 0 D to maximal stimulus demand in at least 0.25 D steps was measured with GS and PR in 25 pre-presbyopic human subjects. Accommodative anterior segment biometric changes were measured using UBM and A-scan ultrasound. AOR was predicted from UBM measured biometry parameters as described in experiment 2. 4) Five contact lenses of known front and back surface radii of curvature and central thicknesses were imaged using the Visante to calculate spatial and optical distortion corrections which were then applied to corneal images captured from the young and pre-presbyopic subjects. 5) Ocular biometry parameters (Visante, A-scan and UBM) from Experiments 1, 3 and 4 in young and pre-presbyopic subjects were used to construct paraxial schematic eye models for each individual subject for each accommodative stimulus demand. Results: 1) Standard deviations of UBM measured parameters were smaller than A-scan measures. 2) Mean prediction errors of AOR using linear regression in young subjects for various biometry parameters ranged from 0.56 D to 0.91 D. 3) Mean prediction errors of AOR using linear regression in pre-presbyopic eyes ranged from 0.41 D to 0.62 D. 4) Root mean square (RMS) error of the power of the contact lens surfaces after distortion correction was 0.18 D for the front and 0.11 D for the back surfaces, respectively. 5) Mean ± SD of prediction errors of AOR from individual schematic eyes for the young and pre-presbyopic subjects were 0.50 ± 0.39 D and 0.50 ± 0.37 D, respectively. Conclusions: The results show: 1) the utility of automated image analysis to get accurate, rapid and objective measurements of anterior segment biometry from UBM images; 2) how spatial distortion in UBM images can be corrected to get accurate measurements and the ability of each UBM measured biometry parameter to predict the AOR; 3) how UBM, despite having low axial resolution, can predict AOR in pre-presbyopic eyes with low accommodative amplitudes; 4) how spatial and optical distortions in Visante images can be corrected to get accurate corneal biometry that can be used for schematic eye modeling; 5) how individual schematic eye predictions of the AOR are better than predictions using LR from individual UBM measured biometry parameters.Item Relative Contributions of Eye-Position and Retinal Information to Perceived Egocentric Visual Direction(2011-05) Sridhar, Deepika; Bedell, Harold E.; Das, Vallabh E.; Manny, Ruth E.; Stevenson, Scott B.; Patel, Saumil S.Purpose: Visual directions of object(s) in space are judged with reference either to the self (egocentric), or to the eye (oculocentric). Perceived egocentric direction (EVD) is based on the combination of sensed eye-position in the orbits, provided by proprioception and efference copies, and retinal information provided by local sign mechanisms. This dissertation examined whether (1) the eye-position and the retinal information covary within the same subjects, (2) the eye-position information from the non-viewing eye contributes to perceived EVD during complete absence of retinal information from one eye, (3) the contribution of eye-position information varies similarly in the presence and absence of foveal suppression, (4) vergence eye movements contribute to perceived EVD, and (5) differences in eye velocities can account for differences in perceived EVD for different viewing conditions. Methods: The relative contributions of eye-position information from the two eyes during different viewing conditions were determined by open-loop pointing responses to an isolated target presented at one of several horizontal locations on a completely black background. These targets were presented binocularly with either asymmetric or symmetric vergence demands, and monocularly during heterophoria. The targets were either static, or were ramped with a constant asymmetric vergence velocity of 0.75 or 1.5 deg/s. The effect of foveal suppression on the eye-position information was determined using targets with 1.5D of anisometropic blur. The relative contributions of retinal information from each eye were estimated by an alignment task, for contrast-varying targets with horizontal or vertical disparity, and for luminance-varying targets with horizontal disparity. Comparisons of relative contributions of eye-position information during asymmetric vergence and retinal information for targets of unequal contrast or luminance in the two eyes indicated if the two sources of information covary within the same subjects. Comparisons of the contributions of eye-position information during binocular viewing and asymmetric vergence with monocular viewing, suppression of foveal information, and different eye-velocities indicated whether the contribution of eye-position to perceived EVD remains similar during different viewing conditions. Results: Between-eye differences exist in the contributions of eye-position and retinal information in some subjects. Across subjects, the inferred weighting of eye-position and retinal information covaries quantitatively (p < 0.05). During monocular viewing, the contribution of eye-position information from the non-viewing eye varies idiosyncratically. The contribution of eye-position information from a foveally suppressed eye is less than when no suppression occurs. In subjects who weight the position information from the two eyes unequally, changes in perceived direction were noted during symmetric vergence. No statistically significant differences in the contributions of eye-position information occur for different eye velocities. Conclusions: The contributions of eye-position and retinal information to perceived EVD covary similarly. However, the contribution of eye-position information depends to some extent on the retinal information, and/or suppression. Vergence eye movements contribute to perceived EVD. Differences in eye velocity cannot account for differences in the contributions of eye-position information observed under the different viewing conditions. The combination of eye-position and retinal information could occur at a common neural locus.Item Safety Evaluation of Light Levels in Ophthalmic Instruments and Devices(2016-05) Chen, Dong; Porter, Jason; Stevenson, Scott B.; Patel, Nimesh B.PURPOSE: There is an absence of information regarding safe levels of light exposure in present ophthalmic devices, and data on the highest continuous usage of these devices is unavailable. Studies that explored similar safety parameters were last conducted over 35 years ago using the American National Standard Institute (ANSI) Z136.1-1976 Standard. The maximum permissible exposure (MPE) identified in the Standard has been periodically updated based on new findings in studies examining light safety and damage thresholds in the eye. Furthermore, evolving imaging techniques have given rise to a number of new ophthalmic devices that did not exist previously, such as optical coherence tomography (OCT). The purposes of this research were to assess the light exposure levels of current ophthalmic devices and to provide a quantitative safety time limit for each instrument using the most recent ANSI Z136.1-2014 American National Standard for Safe Use of Lasers. METHODS: The spectral distribution, radiant power, pulse characteristic, and visual angle of the light emitted from 15 different ophthalmic devices at the University Eye Institute were measured using a spectroradiometer and a calibrated power meter. The examined instruments included optical biometers, corneal topographers, autorefractors, wavefront aberrometers, OCT instruments, fundus cameras, slit lamps, and binocular indirect ophthalmoscopes. A combination of a custom MATLAB program and an Excel Spreadsheet with Excel Visual Basic for Applications (VBA) was developed based on the guidelines written in the 2014 ANSI standard and used to analyze all measurements. Spectral weighting functions were applied following an adjustment guideline for using the ANSI Standards to estimate the weighted MPEs of ophthalmic devices with broadband sources. RESULTS: The ophthalmic instruments and devices evaluated were measured to be safe for typical, clinically relevant usage times. Instruments that were calculated to have the shortest exposure times before exceeding the MPEs were slit lamps when used for direct illumination (~100-400 seconds for continuous usage). Exposure times were much less hazardous when using a condensing lens with a slit lamp for indirect illumination of a larger retinal area. CONCLUSIONS: All examined instruments were calculated to be light safe for normal eyes using our methods. However, caution should be exercised when using slit lamps for direct illumination as exposure levels for these devices can most quickly reach the MPEs within achievable durations. In particular, caution should be exercised when practicing with and training in the use of these instruments, as longer exposure times are expected compared with a typical clinical examination. The reason that slit lamps with direct illumination more quickly reached the MPEs is the visual angle subtended by the source is much smaller. The reason that slit lamps and BIOs with LED sources more quickly reach the MPE limits could be due to the presence of large amounts of short wavelength light in their illumination spectrum. Based on these findings, as well as the uncertainty with which these standards apply to diseased eyes with retinas that are already compromised, it is suggested that manufacturers of ophthalmic instruments reduce the recommended exposure times for instruments that emit light with wavelengths between 400 and 600 nm, as well as increase the transparency of device specifications with regards to exposure parameters and safety.Item Signals of eye-muscle proprioception modulate perceived motion smear(Journal of Vision, 2008-10) Tong, Jianliang; Stevenson, Scott B.; Bedell, Harold E.Achieving clear perception during eye movements is one of the major challenges that the human visual system has to face every day. Like most light sensitive mechanisms, the human visual system has a finite integration time that may cause moving images to appear smeared. By comparing the perceived motion smear during ongoing eye movements and fixation, previous studies indicated that smear is reduced by a neural compensation mechanism that uses “extra-retinal information” about eye movements. However, it is not clear whether eye-muscle proprioception (afferent input), internal copies of efferent oculomotor commands (efference copy), or both contribute to the smear reduction. The present study found that similar reductions of perceived motion smear occur during passive eye movement (which is signaled only by eye-muscle proprioception) and during active pursuit tracking (for which efference copy signals exist as well). These results reveal a novel neural contribution for maintaining visual clarity and stand in contrast to previous reports that eye-muscle proprioception makes only a minor contribution to visual perception.Item The effect of a temporary absence of target velocity information on visual tracking(Journal of Eye Movement Research, 2010) Cisarik, Patricia M.; Kasthurirangan, Sanjeev; Visco, Frank E., Jr.; Bedell, Harold E.; Stevenson, Scott B.; Raghunandan, AveshExperiments with the Rashbass ‘step-ramp’ paradigm have revealed that the initial catchup saccade that occurs near pursuit onset uses target velocity as well as position information in its programming. Information about both position and motion also influences smooth pursuit. To investigate the timing of velocity sampling near the initiation of saccades and smooth pursuit, we analyzed the eye movements made in nine ‘step-ramp’ conditions, produced by combining –2, 0 and +2 deg steps with –8, 0 and +8 deg/s ramps. Each trial had either no temporal gap or a 50-ms gap during which the laser target was extinguished, beginning 25, 50, 75 or 100 ms after the step. Six subjects repeated each of the resulting 45 conditions 25 times. With no temporal gap, saccades were larger in the step-ramp-away’ than the ‘step-only’ condition, confirming that saccade programming incorporates ramp velocity information. A temporal gap had no effect on the accuracy of saccades on ‘step-only’ trials, but often caused undershoots in ‘step-ramp’ trials. A 50-ms gap within the first 100 ms also increased the latency of the initial saccade. Although initial pursuit velocity was unaffected by a temporal gap, a gap that started at 25 ms reliably delayed pursuit onset for ramp motion of the target toward the fovea. Later gaps had a minimal effect on initial pursuit latency. The similar timing of the temporal gaps in target motion information that affect the initiation of saccades and pursuit provides further behavioral evidence that the two types of eye movements share pre-motor neural mechanisms.Item The Effect of Visual Attention on Eye Movements(2018-08) Mahadevan, Madhumitha S.; Stevenson, Scott B.; Bedell, Harold E.; Verghese, Preeti; Ogmen, HalukPurpose: Visual attention along with eye movements help scan the visual world by processing only the relevant information and filtering the extraneous information. Attention could enhance eye movement responses in more than one way. This dissertation aims to understand how endogenous visual attention affects eye movement responses. Methods: Experiments were designed to study the interaction between attention and saccades during fixation as well as pursuit. The dual Purkinje image eye tracking system sampling at either 120Hz or 358Hz was used to track saccades and pursuit respectively. To understand how saccadic reaction times (SRTs) are shortened with endogenous attention, two dual-task experiments were performed to assess psychophysical and oculomotor responses to non-foveal targets (3° or 6° eccentricity) of various contrasts for different spatial-attention-cueing conditions. Cues were either: valid, an arrow at fixation pointing in the direction of the upcoming target; invalid, an arrow pointing in a different direction from the target; or neutral, a small circle instead of an arrow. In both experiments, subjects were instructed to make a saccade to the location of a subsequent, briefly flashed target and make psychophysical judgements (2 alternative force choice) on contrast detection and contrast matching. It has been suggested that spatial attention leads pursuit as inferred from SRTs to targets presented at multiple locations during pursuit. These findings were revisited to investigate if saccadic amplitude influenced the difference in SRTs between forward and backward saccades during ongoing pursuit. The pursuit target was a letter from the set, C,D,H,K,N,O,R,S,V,Z,E, that traversed the screen horizontally in sinusoidal motion. Each time the letter crossed the straight-ahead position, it jumped either forward or backward with different amplitudes (2.25o to 4o). After confirming that it did not, another experiment was designed to determine the amount of equivalent contrast to explain the forward-backward difference in saccadic latency during pursuit. In the last experiment, scleral search coil eye tracking system at a sampling rate of 500Hz was used to track torsional eye rotations (cycloversion and cyclovergence). Subjects wore red–green anaglyph glasses and were asked to hold their head and gaze steady on the central fixation dot and pay attention to targets rotating in a sinusoidal fashion with different temporal frequencies (0.25Hz and 0.50Hz) at different spatial locations. Fourier analysis was used to determine the torsional tracking amplitude at each frequency for each condition. Results and Conclusion :Saccades exhibit a robust, ~40-50 milliseconds (ms) reduction of SRTs for attended compared to unattended targets. Cueing effects on contrast detection (0.08 log units) and matching were small and inconsistent across subjects. Hence, the observed decrease in SRTs could not be accounted for fully by an enhancement in the target’s effective contrast due to attention, as attended and unattended targets that were equally detectable or were perceived to have the same suprathreshold contrast showed substantial differences in SRT. Saccades elicited during pursuit had an average latency 30-40 ms less for target jumps in the direction of pursuit compared to targets that jumped in the opposite direction. This was observed at all contrast levels. This difference could not be explained by a difference in the saccade amplitudes of forward vs. backward saccades. The magnitude of equivalent contrast that accounts for the asymmetry in SRTs during pursuit is greater than a log unit. These results suggest that attention influences SRTs in more than one way. In the last experiment scleral search coil method of eye tracking was used to track torsional eye responses at a sampling rate of 500 Hz. Consistent responses to both cyclovergence and cycloversion stimuli were observed in all subjects for both frequencies, with tracking gain ranging from 0.05 to 0.1. Enhanced cycloversion responses were observed with attention. None of the subjects showed a change in cyclovergence responses with attention suggesting that the mechanisms controlling cyclovergence are outside the influence of attentional enhancement. Attention appears to primarily impact voluntary eye responses with advanced oculo-motor planning.Item The Preferred Retinal Locus in Macular Degeneration: Relating Structure and Function(2016-05) Krishnan, Arun Kumar; Bedell, Harold E.; Harwerth, Ronald S.; Stevenson, Scott B.; Patel, Nimesh B.; Chung, SusanaPurpose: Central field loss (CFL) that ensues from macular degeneration can impact many activities of daily living, including reading, in both younger (as in Stargardt disease, STGD) and older (age-related macular degeneration, AMD) subjects. Subjects with CFL typically choose a non-central retinal location, called the preferred retinal locus (PRL) for fixation. This dissertation aims to understand and relate functional and structural changes within the PRL. Methods: Preliminary studies determined the effectiveness of the MP-1 microperimeter (a) to compensate for excessively unstable fixational eye movements (FEMs), such as occur in subjects with CFL, and (b) to accurately register the retinal test locations on baseline and subsequent automated follow-up testing. Subsequently, the following functional measures were obtained for 29 subjects with bilateral CFL: (a) reading performance using hand-held MNRead charts and LCD-displayed MP1 Read charts, (b) contrast-detection thresholds using the Freiburg acuity test, (c) fixation stability on 3-letter words, measured as bivariate contour ellipse areas (BCEAs) with the MP-1, (d) sensitivity in the central visual field determined with a standard 10-2 threshold grid, and (e) fine-grained sensitivity within the word-fixation PRL for supra-threshold 13x13 arc min spots. Spectral-domain optical coherence tomography (SD-OCT) was used to assess structural characteristics of the PRL, specifically, thickness ratios for the retinal pigment epithelium - Bruch’s membrane complex (RPE-BM), the photoreceptor and outer nuclear layer (PL), and the total retina layers (TRL) between PRL locations where test spots were and were not consistently detected. Finally, 8 younger (< 35 years) and 8 older (>50 years) naive subjects with normal vision read high and low contrast sentences presented one word at a time at the fovea and 5 and 10° in the inferior field. Random 13x13 arc min blocks corresponding to 0-78% of the text area were set to the background luminance to simulate retinal micro-scotomas (MSs) and a staircase algorithm estimated the threshold reading rate. Results: The MP-1 compensated ~90% of the experimentally induced increase in FEMs and the average registration error was ~8 arc min. The maximum reading speed of subjects with CFL correlated poorly with contrast thresholds, BCEA, PRL eccentricity, median sensitivity around the PRL and all retinal thickness ratios. Twenty-two of 29 subjects with CFL (AMD: 8/10 subjects; STGD: 10/12 subjects) exhibited one or more MSs, defined as local regions of insensitivity for supra-threshold targets within the PRL. Although the average percentage of MSs was similar in the cohorts with AMD (25.4%) and STGD (20.3%), reading speed was significantly faster in STGD than AMD subjects. Average thickness ratios for RPE-BM, PL and TRL were 0.97, 0.84 and 0.86 respectively in the AMD cohort and 0.97, 0.77 and 0.89 respectively in the STGD cohort. Only TRL in subjects with AMD differed significantly from 1. In normally-sighted subjects, log reading rate decreased significantly with decreasing contrast and increasing age, eccentricity, and density of element-deletions. For a given eccentricity and contrast, a higher density of element-deletions maximally affected the older subjects. Conclusion: The compensation of the MP-1 for excessive FEMs and the registration between retinal test locations during baseline and follow-up testing are sufficient to assess functional changes within local retinal regions in subjects with CFL. MSs exist within the PRL of a high proportion of subjects with CFL, but are not strongly associated with structural changes determined using SD-OCT. Based on a simulation in normally-sighted subjects, we expect impact of MSs on reading to be greater for older than younger subjects with CFL.Item The Visual Control of Vertical Eye Alignment(2023-08) Dhungel, Deepa; Stevenson, Scott B.; Das, Vallabh E.; Coates, Daniel R.; Read, Jenny C. A.Purpose: Maintaining proper binocular vision requires correcting misalignments of the eyes in the horizontal, vertical, and torsional directions. Although horizontal vergence has been extensively studied, a significant knowledge gap exists regarding the visual input and neurological pathway of the vertical eye alignment reflex. Our studies examined the role of spatiotemporal frequency, contrast, and visual periphery in the control of vertical eye alignment. Methods: We measured the contrast sensitivity of the vertical eye alignment reflex to sinewave gratings and bandwidth filtered noise patterns at various spatial frequencies [0.125-16 cycles per degree (cpd)] and contrast levels [0-64%] using an eye-tracking device and compared with psychophysical contrast sensitivity. We also measured the contrast sensitivity of vertical disparity-driven vergence eye movements in response to bandwidth filtered steady and 6 Hz counterphase flickering noise in a nonius task and compared with psychophysical contrast detection sensitivity. We tested seven spatial frequencies [0.25–16 cpd] and three vertical disparities [5, 10, and 30 arcmin] to obtain the contrast sensitivity functions for each disparity. Additionally, we determined the vertical disparity detection thresholds at various eccentricities up to 30 degrees of eccentricity using spatially scaled sinewave grating patches as stimuli in a nonius task. Lastly, we compared the contrast sensitivity of the eye alignment reflex near the center and periphery and compared with psychophysical contrast sensitivity. Results and Conclusion: The contrast sensitivity of the vertical eye alignment reflex is robust for the middle spatial frequency (0.50 to 4 cpd). However, the response was weaker for the lower and higher spatial frequencies. The addition of counterphase flicker did not enhance the contrast sensitivity of the eye alignment reflex, while the perceptual system showed a significant improvement. The spatial frequency showing the highest contrast sensitivity increased as the disparity decreased, consistent with the size disparity correlation of disparity processing. The vertical eye alignment reflex showed comparable disparity and contrast thresholds in the center and periphery but lower overall contrast sensitivity than the psychophysical method. These results suggest that eye alignment uses the same neural substrate as disparity processing for depth perception.