Levi D.M.,University of California at Berkeley |
McKee S.P.,Smith Kettlewell Eye Research Institute |
Movshon J.A.,New York University
Vision Research | Year: 2011
Amblyopia is usually associated with the presence of anisometropia, strabismus or both early in life. We set out to explore quantitative relationships between the degree of anisometropia and the loss of visual function, and to examine how the presence of strabismus affects visual function in observers with anisometropia. We measured optotype acuity, Pelli-Robson contrast sensitivity and stereoacuity in 84 persons with anisometropia and compared their results with those of 27 persons with high bilateral refractive error (isoametropia) and 101 persons with both strabismus and anisometropia. All subjects participated in a large-scale study of amblyopia (McKee et al., 2003). We found no consistent visual abnormalities in the strong eye, and therefore report only on vision in the weaker, defined as the eye with lower acuity. LogMAR acuity falls off markedly with increasing anisometropia in non-strabismic anisometropes, while contrast sensitivity is much less affected. Acuity degrades rapidly with increases in both hyperopic and myopic anisometropia, but the risk of amblyopia is about twice as great in hyperopic than myopic anisometropes of comparable refractive imbalance. For a given degree of refractive imbalance, strabismic anisometropes perform considerably worse than anisometropes without strabismus - visual acuity for strabismics was on average 2.5 times worse than for non-strabismics with similar anisometropia. For observers with equal refractive error in the two eyes there is very little change in acuity or sensitivity with increasing (bilateral) refractive error except for one extreme individual (bilaterally refractive error of -15 D). Most pure anisometropes with interocular differences less than 4 D retain some stereopsis, and the degree is correlated with the acuity of the weak eye. We conclude that even modest interocular differences in refractive error can influence visual function. © 2010 Elsevier Ltd.
Morash V.S.,Smith Kettlewell Eye Research Institute
IEEE World Haptics Conference, WHC 2015 | Year: 2015
Movement strategies were investigated in a one-handed haptic search task where blindfolded sighted participants used either one or five fingers to find a landmark on an unstructured tactile map. Search theory predicts that systematic strategies, such as parallel sweeps and spirals, should be more prevalent when the searcher's detection radius is small (one finger) than when the detection radius is large (five fingers). Movement patterns were classified as either non-systematic or systematic. As predicted by search theory, systematic strategies were more common in one-finger than five-finger searches. Overall, these results indicate that systematic haptic search strategies are used and modulated by detection radius for untrained sighted participants. © 2015 IEEE.
Song J.-H.,Smith Kettlewell Eye Research Institute |
McPeek R.M.,New York University
Journal of Neurophysiology | Year: 2010
Most visual scenes are complex and crowded, with several different objects competing for attention and action. Thus a complete understanding of the production of goal-directed actions must incorporate the higher-level process of target selection. To examine the neural substrates of target selection for visually guided reaching, we recorded the activity of isolated neurons in the dorsal premotor area (PMd) of monkeys performing a reaction-time visual search task. In this task, monkeys reached to an odd-colored target presented with three distractors. We found that PMd neurons typically discriminate the target before movement onset, ∼150-200 ms after the appearance of the search array. In one subset of neurons, discrimination occurred at a consistent time after search array onset regardless of when the reaching movement occurred, suggesting that these neurons are involved in target selection. In a second group of neurons, discrimination time depended on reach reaction time, consistent with involvement in movement production but not in target selection. To look for physiological corroboration of these two functionally defined groups, we analyzed the extracellular spike waveforms of recorded neurons. This analysis showed a population of neurons with narrow action potentials that carried signals related to target selection. A second population with broader action potentials was more heterogeneous, with some neurons showing activity related to target selection and others showing only movement production activity. These results suggest that PMd contains signals related to target selection and movement execution and that different signals are carried by distinct neural subpopulations. Copyright © 2010 The American Physiological Society.
Cottereau B.R.,Stanford University |
McKee S.P.,Smith Kettlewell Eye Research Institute |
Norcia A.M.,Stanford University
Journal of Neurophysiology | Year: 2012
The human stereoscopic system is remarkable in its ability to utilize widely separated features as references to support fine depth discrimination. In a search for possible neural substrates of this ability, we recorded high-density EEG and used a distributed inverse technique to estimate population-level disparity responses in five regions of interest (ROIs): V1, V3A, hMT+, V4, and lateral occipital complex (LOC). The stimulus was a central modulating disk surrounded by a correlated "reference" annulus presented in the fixation plane. We varied a gap separating the disk from the annulus parametrically from 0 to 5.5° as a test of long-range disparity integration. In the V1, LOC, and hMT+ ROIs, the responses with gaps >0.5° were equal to those obtained in a control condition where the surround was composed of uncorrelated noise (no reference). By contrast, in the V4 and V3A ROIs, responses with gaps as large as 5.5° were still significantly higher than the control. As a test of the spatial distribution of the disparity reference information, we manipulated the properties of the stimulus by placing noise between the center and the surround or throughout the surround. The V3A ROI was particularly sensitive to disparity noise between the center and annulus regions, suggesting an important contribution of disparity edge detectors in this ROI. © 2012 the American Physiological Society.
Colenbrander A.,Smith Kettlewell Eye Research Institute
Acta Ophthalmologica | Year: 2010
This article, based on a report prepared for the International Council of Ophthalmology (ICO) and the International Society for Low Vision Research and Rehabilitation (ISLRR), explores the assessment of various aspects of visual functioning as needed to document the outcomes of vision rehabilitation. Documenting patient abilities and functional vision (how the person functions) is distinct from the measurement of visual functions (how the eye functions) and also from the assessment of quality of life. All three areas are important, but their assessment should not be mixed. Observation of task performance offers the most objective measure of functional vision, but it is time-consuming and not feasible for many tasks. Where possible, timing and error rates provide an easy score. Patient response questionnaires provide an alternative. They may save time and can cover a wider area, but the responses are subjective and proper scoring presents problems. Simple Likert scoring still predominates but Rasch analysis, needed to provide better result scales, is gaining ground. Selection of questions is another problem. If the range of difficulties does not match the range of patient abilities, and if the difficulties are not distributed evenly, the results are not optimal. This may be an argument to use different outcome questions for different conditions. Generic questionnaires are appropriate for the assessment of generic quality of life, but not for specific rehabilitation outcomes. Different questionnaires are also needed for screening, intake and outcomes. Intake questions must be relevant to actual needs to allow prioritization of rehabilitation goals; the activity inventory presents a prototype. Outcome questions should be targeted at predefined rehabilitation goals. The Appendix cites some promising examples. The Low Vision Intervention Trial (LOVIT) is an example of a properly designed randomized control study, and has demonstrated the remarkable effectiveness of vision rehabilitation. It is hoped that further similar studies will follow. © 2009 Acta Ophthalmol.