Faculty Bibliography

PURPOSE. To examine the sites of cone sensitivity loss in patients with retinitis pigmentosa by comparing focal electroretinographic and psychophysical modulation thresholds. METHODS. Both psychophysical and electrophysiologic increment threshold curves were obtained in retinitis pigmentosa patients and a group of age-matched, normally-sighted adults. RESULTS. The majority of the retinitis pigmentosa data could be accounted for by a vertical displacement of the normal curve. The retinitis pigmentosa patients showed similar patterns of cone sensitivity losses using both techniques. CONCLUSIONS. The combined electrophysiologic and psychophysical results provide support for an outer retina locus for these cone sensitivity losses. The data suggest that these deficits may be caused by a spatially independent loss of cone photoreceptors with normal adaptation properties in the remaining photoreceptors

PURPOSE. To study the effects of acetazolamide on central and peripheral visual function in patients with retinitis pigmentosa (RP) who showed no evidence of macular edema. METHODS. Thirteen patients with retinitis pigmentosa participated in a preliminary study. Measures of central and peripheral visual function were obtained before and after an 8 wk period on acetazolamide. An additional 10 patients participated in a cross-over study. They were placed on a placebo for an 8 wk period, then on acetazolamide for a second 8 wk period. RESULTS. None of the patients in the preliminary study showed significant changes in visual acuity, color vision, foveal cone pathway sensitivities, focal electroretinogram (ERG) amplitudes, or in any ERG parameter. Three patients, however, showed significant changes in visual field area and in dark-adapted thresholds. None of the patients in the cross-over study showed significant increases in visual field area. CONCLUSIONS. Given the results and the reports of side-effects, it is difficult to justify using acetazolamide to improve retinal function in RP patients who show no evidence of cystoid macular edema

Recent evidence suggests that retinal hypoxia and ischemia affect the standing potential of the eye and the activity of the photoreceptors. To test whether chronically elevated intraocular pressure would produce similar effects, we measured electro-oculograms in two groups of patients: ocular hypertensive patients and patients with primary open-angle glaucoma. There were significant differences among the average electro-oculogram ratios of these groups compared to age-similar controls. The control observers had an average light-peak/dark-trough ratio of 2.86, the ocular hypertensive patients had an average ratio of 2.44, and the patients with primary open-angle glaucoma had an average ratio of 2.07. This indicates that long-term elevations in intraocular pressure can decrease the light peak of the electro-oculogram, even in patients with no other evidence of glaucomatous damage. This deficit may have its origins in the sensitivity of the outer retina to choroidal ischemia

The P300 cortical evoked potential was compared to the P100 wave of the visual evoked potential by means of appearance/disappearance gratings. The spatial frequency of the novel stimulus was varied to compare the effect of task sensory difficulty on both P100 and P300 potentials. The P100 showed a steady increase in latency with the spatial frequency of the uncommon stimulus, and a degree of amplitude tuning consistent with the contrast sensitivity function. The P300 showed marked changes in both amplitude and latency dependent almost wholly on the proximity of the uncommon stimulus spatial frequency to that of the common stimulus. Motor reaction time showed elements of both the P100 and P300 response patterns. The results are consistent with a model in which, after visual information arrives at the visual cortex, processing is parallel and interdependent. In this model, the amplitude and latency of the P100 cortical evoked potential are governed solely by properties intrinsic to the stimulus, whereas the amplitude and latency of the P300 are functions of the degree of stimulus mismatch

The focal electroretinogram (FERG) was used to examine temporal frequency tuning at the outer retinal level in humans by measuring temporal modulation thresholds. Changes in FERG thresholds as a function of ambient light level were compared to temporal modulation thresholds obtained psychophysically using the same stimuli. At lower temporal frequencies, both FERG and psychophysical thresholds changed sensitivity proportional to the mean illuminance level. At higher illuminance levels, both threshold measures were relatively independent of illuminance. The comparison of the FERG to the behavioral data suggest that most of the adaptation-dependent changes in temporal sensitivity in humans occur at the level of the photoreceptor complex