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Steady-state stimuli are repeated continuously at a frequency that evokes averaged EEG waveforms with components at a frequency which is the same or some multiple of the stimulus rate. A variety of stimulus parameters are readily manipulated and controlled. We wished to assess the suitability of this approach for animal experimentation, and chose to evaluate steady-state visual evoked potential (VEP) responses to counterphased checkerboard stimuli in anesthetized adult male Rhesus monkeys. Large VEPs were reliably produced over the range of 3-6 Hz reversal rate; at 10 Hz, the VEP spectral power was clearly diminished in all monkeys. At each given reversal rate, monkeys differed substantially in variability and amount (up to 3-fold) of spectral power. These large inter-subject differences paralleled what we have seen in similar studies of unanesthetized humans. All monkeys developed large VEPs at the midline, but the topographic distribution of maximum VEP was not uniform; in 3 monkeys the power was much greater over one hemisphere than the other. Fatigue or habituation was not evident; in each monkey, there were usually no significant differences in VEP power over the 3 consecutive 30 sec segments of a given stimulation. We conclude that the anesthetized monkey can be a suitable model for study of steady-state phenomena, as long as the proper stimulus parameters are employed and as long as the level of variance is taken into account.

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The reliability of steady-state visual evoked responses (VER's) was determined for nine normal subjects using Fourier analyses with 1.0 and 0.25 Hz bin resolutions. No correlations were found between VER amplitudes and subjects' reports of attention, accommodation, fixation, or perceived organization of the stimulus. Across subjects, there was also no sustained amplitude modulation of the VER by any frequency (including alpha), and frequency drift of the VER did not contribute significantly to its amplitude variability. Modeling, using mixed sine waves to simulate different signal/noise (S/N) ratios, established that a significant portion of VER amplitude variability can be accounted for by noise which occurs at the same frequency as the VER and which is not removed by ensemble averaging.

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In this study we evaluated in humans the question of whether contrast effects with patterned color stimuli varied in the same way as is known to occur with black-and-white stimuli. Using a counterphasing checkerboard pattern, we evaluated the steady-state visual evoked potential (VEP) in 10 subjects for the response to different contrast levels in each of the 3 primary colors. Overall mean luminance of each color was photometrically equated and kept constant during all trials. The VEP was computer-averaged for 90 consecutive 1 sec epochs of stimulation, and the power at the appropriate frequency was calculated. For each color, the contrast-response curves revealed small power values at low contrast (0.1) and larger power at the 3 high-contrast settings (0.3-0.5). Power varied markedly by color and by subject. The shape of the curves, depending on color and subject, often indicated a saturation of response. A given subject commonly had a physiologically 'preferred' color in that the power with that color was consistently larger. Most subjects had definite subjective color 'preferences,' believing that they perceived the contrast better for one or two colors. However, these impressions were often not validated by the VEP responses to the various colors. These results indicate that white-light VEP responses may not necessarily reflect the response characteristics of specific colors, nor do they necessarily reflect the large inter- and intra-subject differences in color responses noted in this study.

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