Stimulus parameters and visual evoked potential diagnosis.
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Evoked Potentials, Visual↗
Biomedical subjects
J Camisa
Publications and source records attributed to J Camisa.
The effect of stimulus orientation on the visual evoked potential in multiple sclerosis.
Visual evoked potential (VEP) latency was measured in 89 patients with multiple sclerosis (MS) using both a checkerboard stimulus and an alternating vertical grating. Thirty-seven patients had abnormal VEP latencies to the check stimulus, but 63 were classified as abnormal when the grating stimulus was employed. In an additional 22 MS patients, each eye was tested with a checkerboard stimulus, a vertical grating, and a horizontal grating to determine if edge orientation of the stimuli was responsible for the difference between grating and check diagnostic yields. The resulting diagnostic differences could be explained by taking into account the sinusoidal components of these stimuli. After equating gratings and checks for the fundamental Fourier component, each eye of 15 of the MS patients was retested with gratings in three orientations: vertical, horizontal, and oblique. Nine of the 15 patients had an orientation-dependent abnormality of VEP latency. The results suggest that MS causes an orientational imbalance in the human visual system and that VEP testing with stimuli in more than one orientation can increase the diagnostic yield in MS patients.
Adolescent↗
On the inhibitory nature of binocular rivalry suppression.
During binocular rivalry the average duration of a suppression phase depends on the stimulus strength (e.g., contrast) of the input to the suppressed eye. To determine if a similar relationship exists between stimulus strength and the inhibitory effect of suppression on test probe detectability, a series of experiments was performed. Using two-alternative forced-choice procedures, increment detection thresholds were measured during phases of dominance and suppression. Results from three trained observers show that detection performance is significantly impaired during suppression by an amount that is independent of any difference in contrast between the rivalrous stimuli. These data indicate that the magnitude of the inhibitory effect of suppression is governed by a mechanism other than that which determines suppression duration.
Adult↗
Is binocular vision always monocular?
Visual sensitivity of one eye was determined under binocular stimulus conditions yielding apparent fusion, stereopsis, monocular dominance, and monocular suppression. Marked losses in sensitivity accompanied monocular suppression but were not evident during stable singel vision. The results are inconsistent with the hypothesis that supression alone mediates binocular single vision.
Functional Laterality↗
Visual movement perception in the cat is directionally selective.
Behavioral experiments show that the visual system of cat contains mechanisms which are selective for direction of stimulus movement. The cat's contrast detection threshold for a drifting grating is unaffected by the addition of a grating moving in the opposite direction; this same pattern of results is found for human observers. The convergence of cat and human psychophysical data suggests that man's brain may hold direction-specific neurons, similar to those known to exist in the cat brain.
Animals↗
Stimulus orientation and contrast constancy.
The human visual system requires less contrast to detect patterns oriented vertically or horizontally than those oriented obliquely. We investigated whether this orientational anisotropy persists at suprathreshold contrasts. Using a contrast-matching technique, we found that it disappears at contrasts just above threshold, given the stimulus conditions employed. These results suggest that a neural mechanism, at suprathreshold, adjusts the gain of orientational subsystems to compensate for the lower sensitivity of the visual system to oblique patterns. A similar suprathreshold effect, referred to as contrast constancy, has been observed in studies employing grating patterns varying in spatial and temporal frequency. We argue, based on previous electrophysiological findings, that this neural compensation results from antagonism between excitatory and inhibitory processes in the visual cortex.
Anisotropy↗