Feature search in persons with severe visual impairment.

Feature search performance was measured in visually impaired (VI) and age-matched controls with normal vision (NV). All VI subjects were legally blind. The task was to search for a 2 degrees x 2 degrees square target among smaller 1 degrees x1 degrees distracters. Targets and distracters were white and presented on a dark background that subtended 69 degrees by 58 degrees . Three field-sizes (10 degrees , 20 degrees , and 40 degrees ) and three set sizes (8-, 16-, and 32-items) were tested. The VI subjects searched more slowly than the NV subjects, but the reaction time of both groups of subjects did not rise with increasing number of items. The latter is consistent with a parallel search. Both groups searched more slowly when field-size increased, but the VI group was affected more by the increase than the NV group.

The contrast-response of the periphery effect.

This report examines the effect of varying the contrast of a flickering remote surround on thresholds for flicker detection, and color detection in rapidly flickering red and green foveal test targets, presented on a steady white background. Flicker in the surround reduced flicker sensitivity for the foveal test stimuli and yielded a periphery effect (PE), whereas it had no effect on color sensitivity (no PE). The magnitude of the PE increased non-linearly as a function of increasing surround flicker contrast. Much of the increase took place at low contrasts (< 0.20) and half-saturation of the PE occurred at 0.16 and 0.29 contrast for the red and green targets, respectively.

Visual correlates of mobility in real world settings in older adults with low vision.

PURPOSE: To determine the effects of reducing light level from photopic to mesopic on performance of real world mobility tasks and how performance of these tasks relates to measures of visual sensory and perceptual function. METHODS: The visual functions, acuity, peak letter contrast sensitivity, visual field extent, glare disability, color confusion, motion sensitivity, spatio-temporal contrast sensitivity, scanning ability, and figure-ground discrimination were measured to determine their ability to predict mobility performance of visually impaired adults on indoor hallway and outdoor residential travel routes under photopic and mesopic lighting conditions. RESULTS: Time to complete routes and number of mobility incidents were significantly increased under mesopic conditions. Depending on the task, lighting conditions, and performance measure, predictive models consisting of 4 vision variables were able to account for 30 to 42% of the variance in overall performance. The two most important variables in these models were visual field extent and scanning ability, followed by color confusion, grating contrast sensitivity, or spatial resolution. CONCLUSIONS: Reducing illumination levels from photopic to mesopic has an adverse effect upon mobility in older visually impaired adults. The aspects of vision which best predict performance include measures of sensory and perceptual visual function. The results compare well with those obtained under controlled laboratory conditions.

Visual correlates of obstacle avoidance in adults with low vision.

This study examined how mobility performance in a heterogeneous sample of visually impaired adults relates to measures of visual sensory and perceptual function. We found that the best predictors of mobility performance under photopic and scotopic lighting conditions were models that incorporated visual field extent (VFE) and scanning ability. Together with measures of contrast sensitivity and spatial resolution, these models accounted for approximately 50% of the variance in mobility performance. We also found that VFE and scanning ability were the dominant predictor variables when the sample was broken down by subjects' type of vision loss into an acuity loss, a visual field restriction, and a combination loss group.

Environmental variables and mobility performance in adults with low vision.

BACKGROUND: Surveys and empirical studies of visually impaired (VI) persons suggest that avoidance of objects while walking may depend on type of vision loss and be influenced by light level, object contrast, and object type. METHODS: Mobility performance in 88 VI adults was assessed on an indoor obstacle course under photopic and mesopic lighting conditions. Subjects were divided into three categories: primarily an acuity loss, primarily a peripheral field restriction, and a combination of the two. Performance measures were time to walk the course and total number of contacts with objects in the course. RESULTS: Decreasing light level from photopic to mesopic resulted in a significant increase (roughly double) in the time required to complete the course and in the total number of contacts, regardless of type of vision loss. Under photopic illumination, subjects with acuity loss took less time on average to complete the course and contacted fewer objects than the other two groups. At mesopic levels, acuity loss subjects performed better than those in the other categories. Low contrast obstacles were contacted more frequently than high contrast ones at both light levels. Finally, floor level walk-around objects were contacted significantly less often than either step-over or head level objects, regardless of type of vision loss and light level. CONCLUSIONS: The ability of VI persons to avoid obstacles is significantly impaired under mesopic illumination. Object contrast and location are significant factors in determining the success of VI persons in avoiding obstacles in the travel path.

Suppressive effects of peripheral flicker on foveal bichromatic mixture thresholds.

Increment thresholds were determined for bichromatic mixtures of 660 and 520 nm (red + green), and 440 or 460 and 570 nm (blue + yellow). These measurements were made against a 1000-td white background that was surrounded by a larger, luminance matched annulus that appeared steady or flickered at 10 Hz. Targets were circular spots that subtended 60 and 15 min arc and had durations of 10 and 200 msec. All mixture thresholds with the steady surround were non-additive. The flickered surround gave rise to a periphery effect (PE), in the form of elevated mixture thresholds compared to those using the steady surround, for both red + green and blue + yellow mixtures when the 60 min arc, 10 msec spot was used. A PE was not consistently observed for the other three stimulus conditions. We propose the non-additive results indicate parvocellular (P-) pathway involvement in detection, which occurred under all stimulus conditions tested. Furthermore, that the PE were found when mixture stimuli were large, and brief indicates magnocellular (M-) pathway involvement in detection.

The effects of peripheral flicker on foveal spectral sensitivity.

A flickering surround reduced sensitivity to large, briefly flashed monochromatic stimuli superimposed on a steady white background, compared to sensitivity measured when the surround was steady. The flickering surround had no effect on stimuli that were large and of long duration or small in size. Increasing the diameter of the background reduced the magnitude of the effect, but did not eliminate it.

Efficacy and stability of amblyopia therapy.

To determine the efficacy and stability of therapy, we reviewed the charts of 64 amblyopes with strabismus and/or anisometropia who had been treated by direct occlusion. For patients aged 7 years or less (N = 39), 90% (35/39) showed some acuity gain, with 69% (27/39) achieving at least a doubling of acuity (0.3 log units). Fifty-four percent obtained 20/40 (6/12) or better after an average treatment period of 3.8 months. Some reduction in visual acuity (VA) subsequently occurred for 75% (24/32) of those patients followed. For patients aged 8 years or more (N = 26), 77% (20/26) showed some acuity gain with 31% (8/26) improving at least 0.3 log units. Twenty-seven percent (7/26) obtained 20/40 (6/12) or better after an average treatment period of 4.2 months, although no patients older than 10 years (N = 13) achieved 20/40 (6/12). Loss of some of the acuity gain subsequently occurred for 67% (12/18) of those followed. These findings indicate that VA can be improved by patching therapy in most patients older than 7 years, but the acuity improvement is somewhat less than in younger patients. At least 67% of all amblyopes followed for 1 year lost some of the acuity gain after cessation of therapy, regardless of the age when treated. As a reduction of the acuity gain is likely to occur within the first year after cessation of therapy, it is recommended that amblyopic patients of all ages be followed at regular intervals.

Ganzfeld blankout occurs in bowl perimetry and is eliminated by translucent occlusion.

An intermittent darkening of the visual field (ganzfeld "blankout") is perceived during bowl perimetry. Since this may be a form of rivalry, we hypothesized that occluding the nontested eye with a translucent occluder may eliminate the phenomenon. Ten normal subjects underwent visual field testing with both a translucent and an opaque occluder. Eight of the 10 reported darkening to occur with the opaque patch, while none reported it with the translucent occluder. The darkening occurred with a mean latency of 10.9 seconds, occupied 34.4% of the time, and on average occurred 3.25 times per minute. With the translucent occluder, retest threshold variability was 18.8% less and sensitivity was increased by 0.7 dB. The elimination of this darkening using translucent occluders in bowl perimetry will allow reduced variability and increased sensitivity and comfort.