Faculty Bibliography

The timing of visual evoked potential (VEP) amplitude and phase changes following abrupt increases or decreases in contrast was examined. Gratings (1 c/deg) were presented at a low contrast for 8 sec, increased to a higher contrast for 8 sec, and then decreased to the initial lower contrast for another 8 sec. Second harmonic VEP amplitude and phase were recorded continuously and averaged in 1 sec epochs. Both amplitude and phase exhibited delays in reaching a stable level following the contrast change. For amplitude, the length of the delay was dependent on the magnitude and direction of the contrast step and on the spatial frequency of the stimulus. Time constants for the change in amplitude following step increases in contrast ranged from 0.2 sec for a 12% contrast step to 1.34 sec for a 37% contrast step. The timing of phase changes, however, was independent of the size of the contrast increases (tau = 0.7 sec). For step decreases in contrast, both amplitude and phase were relatively independent of the size of the change (tau = approx. 0.9 sec for amplitude and tau = 0.15 sec for phase). Amplitude time constants also increased with increasing spatial frequency (tau = 1.2 sec for 1 c/deg, tau = 1.6 sec for 4 c/deg and tau = 2.3 sec for 8 c/deg); phase time constants, however, did not change as a function of spatial frequency (tau = 0.7 for all spatial frequencies). These findings demonstrate that a unitary process may not always be tapped by signal averaging techniques. Additionally, swept stimulus VEP techniques may produce considerable errors in threshold estimation depending on the stimulus spatial frequency and on the slope and direction of the contrast change

It has been assumed that the 'ON' and 'OFF' responses of the human electroretinogram (ERG) interact to produce a single waveform when brief flashes are used. To test this assumption, we examined the separate effects of stimulus intensity, the level of retinal illuminance and stimulus duration on the ERG. Both 'ON' and 'OFF' response amplitude decreased as stimulus intensity was decreased and as the level of retinal illumination was reduced. When stimulus duration was reduced, the amplitude of the 'OFF' response decreased; however, the amplitude of the 'ON' response increased. Summing of the 'ON' and 'OFF' response waveforms could not account for the increased amplitude in response to brief stimuli or for the changes in ERG wave shape. These results indicate that there is not a significant corneally recordable 'OFF' response elicited by brief stimuli, such as those commonly used to record the flash ERG

Single-cell electrophysiologic studies have shown that dopamine modulates retinal activity, but its role in human retinal processing is unclear. We investigated the effects of short-term oral administration of dopaminergic receptor blocking agents on the flash electroretinogram in humans. Both chlorpromazine (25 and 50 mg) and fluphenazine (1 and 2 mg) significantly reduced electroretinogram b-wave amplitudes and also selectively reduced the amplitude of the first oscillatory potential. Implicit times were not altered. Metoclopramide (10 and 20 mg) had no effect on any electroretinographic variable. Our study indicates that dopamine receptor blocking agents with both D-1 and D-2 receptor affinities reduce the amplitude of the electroretinogram in humans

PURPOSE/OBJECTIVE:With the use of an interactive driving simulator, we examined the driving performance of older patients with either homonymous or quadrantic hemianopsia with primarily occipital lobe damage resulting from cerebrovascular accidents (CVA's). METHODS:We compared the performance of these patients with that of a normally sighted, age-similar control group and that of a normally sighted younger group. RESULTS:The driving performance of the patients was either worse than, or similar to, that of the older control group; all of the older individuals (both patients and normally sighted subjects) had worse performance than the younger group. CONCLUSIONS:Age-related effects combined with the effects of visual field losses in older patients with cerebrovascular accidents had a negative impact on driving skills.

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