Faculty Bibliography

The aim of this study was to determine the relative effects of age and compromised vision on driving-related skills and on-road accidents. A total of 107 subjects were tested. They represented four groups that varied in age and visual status, as follows: (1) a younger, normally sighted group; (2) an older, normally sighted group; (3) a younger, visually compromised group; and (4) an older, visually compromised group. Driving performance was assessed by self-reported and state-recorded accident frequency and by an evaluation of performance on an interactive driving simulator. The older groups had poorer driving-related skills, as measured with our interactive driving simulator, than had the younger groups, but they did not have significantly higher on-road accident rates than the younger groups. The older subjects and those with compromised vision had reduced risk-taking scores, as measured with a self-report questionnaire. In addition, all older drivers had increased eye movements and had slower simulator driving speeds, which suggests that behavioral compensation is made for visuocognitive/motor deficits. Regression analyses showed that compromised vision and visual field loss predicted real-world accidents in our study population

To investigate the relative sensory and perceptual contributions to central visual function of patients with retinitis pigmentosa (RP), we tested symmetry discrimination using block patterns with varying types of symmetric organization. Eleven control subjects with normal vision and 11 patients with RP with 20/30 visual acuity or better, viewed patterns presented for 255 msec. The patterns differed in the type of symmetric organization and the subjects were required to identify the type. The control subjects performed significantly better (89.2%) than the patients (74.5%). Four hypotheses to account for these findings were tested and the results were as follows, (1) A reduction in pattern luminance did not change symmetry discrimination performance in the control subjects. (2) Large reductions in pattern contrast did not alter symmetry discrimination in the control subjects. (3) Reductions in stimulus duration, likewise, did not produce similar error patterns in the control subjects as those observed in the patients with RP. (4) Alterations in spatial sampling density did not completely account for the patients' deficits. None of the retinally based explanations alone was sufficient to account for our findings. Additionally, we suggest that alterations of sensory input may affect the perceptual encoding of the relationship among pattern elements.

We measured contrast processing as a function of spatial and temporal frequency using three measurement techniques: psychophysical thresholds, visual evoked potential (VEP) thresholds and VEP amplitudes. Measurements were made using the same stimulus parameters and on the same group of subjects. The agreement among these functions depended upon the particular spatio-temporal stimulus employed and in general, there were more differences than similarities among these measures. For example, VEP derived functions (both amplitude and threshold) peaked at 4 c/deg, whereas, the psychophysical functions peaked at 1 c/deg. VEP amplitude functions were maximum at > or = 3.5 Hz, whereas both psychophysical and VEP sensitivity were maximum at < or = 2.5 Hz. VEP thresholds were broad and low pass in shape and in contrast, the psychophysical and VEP amplitude functions were band pass. In summary, comparisons among these measures of contrast processing must be made with caution

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