Browsing by Author "Bedell, Harold E."
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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 A system for cursor control by eye position(1985) Iyer, Madhusudan; Batten, George W., Jr.; Glover, John R., Jr.; Bedell, Harold E.This thesis describes the design and development of a workstation intended for totally incapacitated persons. Among such people are individuals who can control only the motion of their eyes. The workstation described in this thesis uses a dedicated microcomputer system and eye-position sensors to control the cursor on the screen of a computer terminal. The cursor can be used to point out items on a menu, thereby allowing the user to communicate with the computer, hence with any system connected to the computer. The following are the salient features of the prototype system: - eye-position sensing for display control; - menu driven software;- a means for communicating with a remote host computer; - an application program allowing the user to produce "typewritten" documents using only the eyes. The system was tested and it performed satisfactorily.Item Bottlenecks of Motion Processing during a Visual Glance: The Leaky Flask Model(PLoS ONE, 2013-12-31) Öğmen, Haluk; Ekiz, Onur; Huynh, Duong; Bedell, Harold E.; Tripathy, Srimant P.Where do the bottlenecks for information and attention lie when our visual system processes incoming stimuli? The human visual system encodes the incoming stimulus and transfers its contents into three major memory systems with increasing time scales, viz., sensory (or iconic) memory, visual short-term memory (VSTM), and long-term memory (LTM). It is commonly believed that the major bottleneck of information processing resides in VSTM. In contrast to this view, we show major bottlenecks for motion processing prior to VSTM. In the first experiment, we examined bottlenecks at the stimulus encoding stage through a partial-report technique by delivering the cue immediately at the end of the stimulus presentation. In the second experiment, we varied the cue delay to investigate sensory memory and VSTM. Performance decayed exponentially as a function of cue delay and we used the time-constant of the exponential-decay to demarcate sensory memory from VSTM. We then decomposed performance in terms of quality and quantity measures to analyze bottlenecks along these dimensions. In terms of the quality of information, two thirds to three quarters of the motion-processing bottleneck occurs in stimulus encoding rather than memory stages. In terms of the quantity of information, the motion-processing bottleneck is distributed, with the stimulus-encoding stage accounting for one third of the bottleneck. The bottleneck for the stimulus-encoding stage is dominated by the selection compared to the filtering function of attention. We also found that the filtering function of attention is operating mainly at the sensory memory stage in a specific manner, i.e., influencing only quantity and sparing quality. These results provide a novel and more complete understanding of information processing and storage bottlenecks for motion processing.Item Capacity, Precision, and Representations of Sensory and Mnemonic Processing(2016-05) Huynh, Duong Le Vu; Ogmen, Haluk; Jansen, Ben H.; Sheth, Bhavin R.; Bedell, Harold E.; Tripathy, SrimantMost of our mental processes rely on memorization, and dysfunctions of memory systems can lead to severe perceptual, cognitive, and motor deficits. Although decades of research has accumulated a large knowledge base on human memory, several critical issues and contentions remain to be resolved. In this dissertation, we have addressed three such problems. The first study aimed to determine the bottlenecks of information processing that constrain the capacity of visual memory. We measured observers’ psychophysical performance in a direction-of-motion recalling task, and through statistical modeling, analyzed the quantity and quality of information along early stages of mnemonic processing. In contrast to the long-standing view targeting visual short-term memory as the only major bottleneck of processing, we find significant loss in both the quantity and quality of information also during the initial encoding and sensory memory stages. The second study aimed to characterize the representation format for visual information in terms of different featural dimensions. Specifically, we examined the roles of three different features, viz., position, color, and direction-of-motion, in the construction and maintenance of object representations. Using a cross-cuing paradigm, we showed that features are organized in bound forms, in which any feature can serve as an effective cue to retrieve another. However, the pattern of binding strength and cuing effectiveness is asymmetric and exhibits stream-specificity (parvocellular/magnocellular) of features. This study further showed that the distribution of information loss across processing stages found in the first study for direction-of-motion holds true for all features. The third study aimed to characterize the representation format for visual information in terms of different reference frames, viz., retina-based, world-based, or mixed. Using a motion-vector decomposition approach, we analyzed observers’ behavior and considered models with different assumptions about the contribution of processing in each reference-frame. With some exceptions, the world-based account was found to best describe the data. Taken together, we propose that the visual system encodes and stores information with quantitative and qualitative loss that occurs at multiple processing stages; this loss is mainly tuned to the metrics of the external world and its magnitude varies according to stimulus characteristics and task demands.Item Change in visual acuity is well correlated with change in image-quality metrics for both normal and keratoconic wavefront errors(Journal of Vision, 2013-11) Ravikumar, Ayeswarya; Marsack, Jason D.; Bedell, Harold E.; Shi, Yue; Applegate, Raymond A.We determined the degree to which change in visual acuity (VA) correlates with change in optical quality using image-quality (IQ) metrics for both normal and keratoconic wavefront errors (WFEs). VA was recorded for five normal subjects reading simulated, logMAR acuity charts generated from the scaled WFEs of 15 normal and seven keratoconic eyes. We examined the correlations over a large range of acuity loss (up to 11 lines) and a smaller, more clinically relevant range (up to four lines). Nine IQ metrics were well correlated for both ranges. Over the smaller range of primary interest, eight were also accurate and precise in estimating the variations in logMAR acuity in both normal and keratoconic WFEs. The accuracy for these eight best metrics in estimating the mean change in logMAR acuity ranged between ±0.0065 to ±0.017 logMAR (all less than one letter), and the precision ranged between ±0.10 to ±0.14 logMAR (all less than seven letters).Item Contrast sensitivity of the amblyopic eye in central and peripheral retina with variation of stimulus field size-implications for its neural basis(1982) Katz, Laurence M.; Levi, Dennis M.; Loshin, David S.; Bedell, Harold E.Functional amblyopia is often assumed to be restricted to the central foveal region. The purpose of the present study was to establish the extent of the amblyopic deficit at 0°, 10°, and 20° retinal eccentricity using contrast sensitivity measures for sine wave gratings subtending various field sizes. Contrast sensitivity functions were shown to vary with stimulus field size, retinal location and degree of amblyopia. Foveally, the peak contrast sensitivity of the amblyopic eyes increased markedly with increasing field size, to reach the same magnitude as that of the contralateral non-amblyopic eyes for large fields. In the retinal periphery a significant difference in peak contrast sensitivity was found between the amblyopic and non-amblyopic eyes. High spatial frequency cut-offs were reduced both centrally and peripherally with all field sizes in the amblyopic eyes. The effect of increasing field size on the cut-off acuity was found to be smaller than the effect on peak contrast sensitivity. The most significant conclusions drawn from the results are that: 1) The amblyopic deficit in terms of both peak contrast sensitivity and acuity is not restricted to the central foveal region, and 2) Amblyopic eyes benefit to a greater extent from increased stimulus field size than non-amblyopic eyes in terms of peak contrast sensitivity. An hypothesis is proposed which suggests that the results obtained are due to fewer and/or less sensitive cortical neurons being driven by the amblyopic eye in humans with naturally occurring amblyopia.Item Differences in balance and visual characteristics in a group of brain injured patients(1982) Winkler, Patricia Ann; Rosner, Jerome; Bedell, Harold E.; Abplanalp, Paul; Littell, Elizabeth H.Postural stability was measured using a biostereometric sensor in 20 brain-injured subjects and 10 normals. Measurements of stability were recorded for two minutes with eyes open, eyes closed and one eye open in both sitting and standing positions. Based on results of postural measures with both eyes open or closed, brain-injured subjects could be divided into two groups; those whose balance is better with eyes open (E.O. group) and those whose balance is better with eyes closed (E.C. group). Thus, this study documents the clinical impression that some brain-injured subject's static balance is destabilized by vision. A battery of visual sensory, oculomotor and perceptual functions were then examined to identify the extent to which differences in balance in the E.O. and E.C. groups could be explained by differences in one or more of these functions. The E.C. group performed worse on all visual tests as a whole, having significantly worse scores on tests of binocular vision (fine gross) and eye alignment. Many brain-injured subjects may be more readily rehabilitated using monocular vision or without vision. Further investigation of the role of binocular vision in normal postural control may clarify how abnormal binocular vision in brain-injured subjects can result in poor balance.Item Displacement sensitivity in amblyopia(1983) Katz, Beverley J.; Bedell, Harold E.; Allen, Jane L.; Levi, Dennis M.Although defined as reduced visual acuity, spatial uncertainty and distortion have been proposed as the basic anomaly in functional amblyopia. The primary purpose of this study was to assess spatial abnormalities within the centralmost 1 degree of the amblyope's visual field (functionally critical for acuity) by determining displacement thresholds for a small target in the absence and presence of stationary reference contours. With reference contours absent, 3 of 4 amblyopes demonstrated 1) elevated displacement thresholds over a range of target movement durations and 2) substantial constant errors in discriminating rightward from leftward displacements. Conversely to normals, displacement thresholds for severe amblyopes apparently were determined by target velocity for movenent durations up to about 0.5 sec and by target displacement for longer durations. With reference contours, responses were less abnormal for the 2 amblyopes tested. Amblyopes' elevated thresholds and constant errors for displacement found in this study are interpreted as resulting from both their spatial abnormalities and poor eye movements.Item Effect of optical defocus on motion sensitivity(1988) Lavallée, Dary G.; Bedell, Harold E.; Barbeito, Raphael; Levi, Dennis M.Motion thresholds are known to be lower in the presence of reference cues than without reference cues, and lower at the fovea than in the periphery. Since the peripheral retina usually receives out-of-focus images, and also since some prescribing techniques involve the defocus of one eye (e.g., monovision), it is important to know how defocus affects motion sensitivity in the fovea and periphery. While some literature exists which bears on this issue, no systematic study has been reported, and existing results do not provide a clear answer to this question. [...]Item Effects of chronic optical defocus on the kitten's refractive status(1988) Ni, Jinren; Smith, Earl L., III; Bedell, Harold E.; Grosvenor, Theodore P.Lid closure initiated early in life consistently produces axial myopia in a variety of species. It is not known what aspects of the anomalous visual experience associated with lid closure disrupt the emmetropization process and cause abnormal axial elongation. This study was designed to determine if a degradation in the quality of the spatial characteristics of the retinal image alone is sufficient to produce an experimental myopia. Optical rearing procedures were employed to defocus retinal images in one eye of developing kittens from three to fourteen weeks of age. Retinoscopic and ultrasonic procedures were used to evaluate the kitten's refractive status and ocular axial length. The types of defocusing lenses, the nature of the pretreatment visual experience, and the magnitude of the lens power were used to evaluate variables that may have confounded previous investigations. The major finding of the present study is that early chronic optical defocus resulted in axial myopia in kittens, irrespective of the pretreatment visual experience. The frequency and magnitude of the induced myopia were dependent on the amount of optical defocus, but they were not affected by the sign of the defocusing lens or its type (goggles or soft contact lens). The results demonstrate that a clear retinal image is important for the normal regulation of ocular growth and the maintenance of an emmetropic refractive state.Item Fixation Stability in Monkeys with Strabismus(2013-12) Pirdankar, Onkar Harishchandra; Das, Vallabh E.; Chino, Yuzo M.; Bedell, Harold E.Purpose: To assess the effect of target parameters on fixation stability in strabismic monkeys. Methods: Eye movements were recorded in one normal and three strabismic monkeys during 72 fixation conditions (4 shapes; 3 sizes; 2 backgrounds; OD, OS or OU viewing), each repeated 5 times. Fixation stability was quantified using the Bivariate Contour Ellipse Area (BCEA). Influence of target parameters was assessed using 4-way ANOVA. Results: BCEA was greater in the strabismic monkeys compared to the normal. In strabismus, BCEA of the deviated eye was significantly greater than BCEA in the fixating eye. Target shape and size significantly influenced fixation stability in both normal and strabismic monkeys. Background effects were idiosyncratic. Conclusions: Target parameters that influence fixation stability in a normal, also affects fixation stability in disease conditions such as strabismus. Target parameter influences likely function via conjugate mechanisms since proportional effects were observed in both viewing and covered eyes.Item High capacity, transient retention of direction-of-motion information for multiple objects(Journal of Vision, 2010-06) Shooner, Christopher; Tripathy, Srimant P.; Bedell, Harold E.; Öğmen, HalukThe multiple-object tracking paradigm (MOT) has been used extensively for studying dynamic visual attention, but the basic mechanisms which subserve this capability are as yet unknown. Among the unresolved issues surrounding MOT are the relative importance of motion (as opposed to positional) information and the role of various memory mechanisms. We sought to quantify the capacity and dynamics for retention of direction-of-motion information when viewing a multiple-object motion stimulus similar to those used in MOT. Observers viewed three to nine objects in random linear motion and then reported motion direction after motion ended. Using a partial-report paradigm and varying the parameters of set size and time of retention, we found evidence for two complementary memory systems, one transient with high capacity and a second sustained system with low capacity. For the transient high-capacity memory, retention capacity was equally high whether object motion lasted several seconds or a fraction of a second. Also, a graded deterioration in performance with increased set size lends support to a flexible-capacity theory of MOT.Item Induced changes in oculocentric visual direction when saccades are recalibrated(1986) Moidell, Bonnie G.; Bedell, Harold E.; Barbeitio, Raphael; Levi, Dennis M.Classically, Oculocentric visual direction (the angular direction associated with a particular retinal locus) has been considered innately specified and immutable. This study examined whether, in contrast to the classical viewpoint, a target's Oculocentric visual direction is affected uhen the magnitude of saccades to that target is changed. For normal adults, saccades to a monocularly viewed target, initially 8.5 deg right of fixation, were made artificially inaccurate by Surreptitiously shifting the target 2.15 deg inward or outward during saccades to it. On separate trials the visual direction of the unshifted target was specified by matching its direction (relative to a fixated stimulus) with a target in the left field. When the right field target was shifted during saccades, each subject showed an appropriate adjustment of saccadic magnitude. Statistically significant but smaller changes occurred in the visual direction of the left field matching target. The changes in visual direction occurred more slowly than the adjustment of saccadic magnitude. Although induced changes of saccadic magnitude were larger than induced changes in the target's Oculocentric direction, changes were comparable when normalized to their respective variabilities. The results show that small changes of a peripheral target's Oculocentric direction can occur in normal adults.Item Investigation of accommodative tracking under monochromatic conditions(1988) Sun, Rong; Loshin, David S.; Bedell, Harold E.; Klein, Stanley A.The purpose of this reseach was to examine the role of chromatic aberration in supplying accommodative information to the brain. This was accomplished by measuring the latency and the initial directional errors of accommodation under different stimulus conditions. An automated infrared optometer was designed and built to monitor monocular changes of accommodation dynamically with a sensitivity of +- 0.1 D over a range of about 10 D. Three interference filters (500 nm, 570 nm, 640 nm) were used to minimize chromatic aberration; accommodative responses to these essentially monochromatic stimuli were compared to responses to purple (peaks at 435 nm and >700 nm) and white light with and without an achromatizing lens. The results from nine subjects show that in most cases accommodative latencies and initial errors increased under monochromatic conditions. When the stimulus to decrease accommodation was presented some subjects showed initial accommodative directional errors of 30% to 50% for monochromatic sources and fewer than 10% for white light. When the stimulus to increase accommodation was presented, most subjects showed fewer initial accommodative errors in both monochromatic and white light, suggesting that the accommodative system may have a natural bias toward increasing accommodation. This was verified by a supplementary experiment. A relationship was found between latencies and initial errors. The longer latencies associated with more directional errors. In total these results imply that when other directional accommodative cues are eliminated, chromatic aberration may provide information to the brain as to the appropriate direction the accommodation should take,i.e. increase or decrease.Item Large crowding zones in peripheral vision for briefly presented stimuli(Journal of Vision, 2014-12) Tripathy, Srimant P.; Cavanagh, Patrick; Bedell, Harold E.When a target is flanked by distractors, it becomes more difficult to identify. In the periphery, this crowding effect extends over a wide range of target-flanker separations, called the spatial extent of interaction (EoI). A recent study showed that the EoI dramatically increases in size for short presentation durations (Chung & Mansfield, 2009). Here we investigate this duration-EoI relation in greater detail and show that (a) it holds even when visibility of the unflanked target is equated for different durations, (b) the function saturates for durations shorter than 30 to 80 ms, and (c) the largest EoIs represent a critical spacing greater than 50% of eccentricity. We also investigated the effect of same or different polarity for targets and flankers across different presentation durations. We found that EoIs for target and flankers having opposite polarity (one white, the other black) show the same temporal pattern as for same polarity stimuli, but are smaller at all durations by 29% to 44%. The observed saturation of the EoI for short-duration stimuli suggests that crowding follows the locus of temporal integration. Overall, the results constrain theories that map crowding zones to fixed spatial extents or to lateral connections of fixed length in the cortex.Item Magnitude of lateral chromatic aberration across the retina of the human eye(1983) Ogboso, Youmay Ume; Bedell, Harold E.; Harwerth, Ronald S.; Loshin, David S.In this study, the lateral chromatic aberration of the human eye was quantified in order to assess its potential contribution to color vision in the peripheral retina. Lateral chromatic aberration was measured in the right eye of 4 adult observers as the physical misalignment between perceptually aligned short- and long-wavelength targets. Results were compared with predictions from a computer-simulated wide angle model eye. Lateral chromatic aberration generally increased with retinal eccentricity, but remained less than 10 min arc within 40 degrees of the fovea. At 60 deg, lateral chromatic aberration increased to approximately 30 min arc, but was reducible by refractive correction in the two observers retested. The results are consistent with previous reports of a sizeable region of reasonably good optical quality extending into the retinal midperiphery. Within this region, lateral chromatic aberration approximates the average spacing between adjacent retinal cones, indicating that it does not substantially limit peripheral color vision.Item Monocular microsaccades are visual-task related(Journal of Vision, 2016-02) Gautier, Josselin; Bedell, Harold E.; Siderov, John; Waugh, Sarah J.During visual fixation, we constantly move our eyes. These microscopic eye movements are composed of tremor, drift, and microsaccades. Early studies concluded that microsaccades, like larger saccades, are binocular and conjugate, as expected from Hering's law of equal innervation. Here, we document the existence of monocular microsaccades during both fixation and a discrimination task, reporting the location of the gap in a foveal, low-contrast letter C. Monocular microsaccades differ in frequency, amplitude, and peak velocity from binocular microsaccades. Our analyses show that these differences are robust to different velocity and duration criteria that have been used previously to identify microsaccades. Also, the frequency of monocular microsaccades differs systematically according to the task: monocular microsaccades occur more frequently during fixation than discrimination, the opposite of their binocular equivalents. However, during discrimination, monocular microsaccades occur more often around the discrimination threshold, particularly for each subject's dominant eye and in case of successful discrimination. We suggest that monocular microsaccades play a functional role in the production of fine corrections of eye position and vergence during demanding visual tasks.Item Onset and progression of with-the-rule astigmatism in children with infantile nystagmus syndrome(Investigative Ophthalmology & Visual Science, 2010-01) Wang, Jingyun; Wyatt, Lauren M.; Felius, Joost; Stager, David R., Jr.; Stager, David R., Sr.; Birch, Eileen E.; Bedell, Harold E.Purpose: The purpose of this study was to examine the onset and progression of with-the-rule (WTR) astigmatism during the first 8 years of life in children with idiopathic infantile nystagmus syndrome (INS) or INS associated with albinism and to compare their development with that of normal children. Also explored was whether early WTR astigmatism influences emmetropization in children with INS and whether there is evidence of meridional emmetropization. Methods: Cycloplegic refractions culled from medical records were converted into power vector components: M (spherical equivalent), J 0 (positive J 0 indicates WTR astigmatism), and J 45 (oblique astigmatism). Two diagnostic groups (idiopathic, n = 106; albinism, n = 95) were evaluated and compared with a reference normal group (n = 495). Four age subgroups were evaluated: age≤0.5 year, 0.5Item Optimizing wavefront-guided corrections for highly aberrated eyes in the presence of registration uncertainty(Journal of Vision, 2013-06) Shi, Yue; Queener, Hope M.; Marsack, Jason D.; Ravikumar, Ayeswarya; Bedell, Harold E.; Applegate, Raymond A.Dynamic registration uncertainty of a wavefront-guided correction with respect to underlying wavefront error (WFE) inevitably decreases retinal image quality. A partial correction may improve average retinal image quality and visual acuity in the presence of registration uncertainties. The purpose of this paper is to (a) develop an algorithm to optimize wavefront-guided correction that improves visual acuity given registration uncertainty and (b) test the hypothesis that these corrections provide improved visual performance in the presence of these uncertainties as compared to a full-magnitude correction or a correction by Guirao, Cox, and Williams (2002). A stochastic parallel gradient descent (SPGD) algorithm was used to optimize the partial-magnitude correction for three keratoconic eyes based on measured scleral contact lens movement. Given its high correlation with logMAR acuity, the retinal image quality metric log visual Strehl was used as a predictor of visual acuity. Predicted values of visual acuity with the optimized corrections were validated by regressing measured acuity loss against predicted loss. Measured loss was obtained from normal subjects viewing acuity charts that were degraded by the residual aberrations generated by the movement of the full-magnitude correction, the correction by Guirao, and optimized SPGD correction. Partial-magnitude corrections optimized with an SPGD algorithm provide at least one line improvement of average visual acuity over the full magnitude and the correction by Guirao given the registration uncertainty. This study demonstrates that it is possible to improve the average visual acuity by optimizing wavefront-guided correction in the presence of registration uncertainty.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.