In previous work we reported that fixation stability did not deteriorate in older adults over relatively long viewing durations. In the present study we reanalyzed the data to examine potential aging effects on fixational control for viewing durations typically used in psychological experimentation. Monocular eye movements were recorded in 12 older and 12 younger observers using a dual Purkinje image technique, while observers fixated a stationary target. The two-dimensional scatter of eye positions was measured during nine viewing durations ranging from 100 ms to 12.8 s. Fixational control of the two groups was comparable at all of the viewing durations. Both younger and older observers were able to maintain fixation within an area several times smaller than the size of the fovea. Implications for aging studies that use briefly presented visual stimuli are discussed.
Practice improves observers' ability to discriminate between highly similar directions of motion. In an effort to clarify the basis for this improvement, we recorded an observer's eye movements while he made direction discriminations. We found that the observer did not need to track the moving target in order to learn the discrimination. Both at the beginning and at the end of training the observer's eye movements more closely resembled movements made while fixat-ing a stationary target, and did not at all resemble movements made while intentionally tracking the stimulus. These results suggest that the learned discrimination of the direction of moving targets is perceptual in nature and does not depend on the learning of a sensorimotor response.
Adaptation ability in the center of vision was tested in diabetic patients treated with pan-retinal photocoagulation for diabetic retinopathy and in normal control subjects. This was accomplished with a specially designed contrast sensitivity testing regimen. Patients with retinopathy showed "hidden" deficits, losses in adaptation ability that could not be measured by standard clinical tests, such as visual acuity. This new test thus quantifies a previously unmeasured visual problem that affects the everyday visual function of these patients.
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