Journal of Neurophysiology, Vol 53, Issue 5 1287-1302, Copyright © 1985 by APS
Orientation sensitivity of the cat assessed from evoked potentials: central and peripheral contributions
J. S. Tootle and M. A. Berkley
To estimate contour-orientation sensitivity of the cat and the degree to
which precortical processing contributes to such estimates, the amplitude
of visually evoked potentials (VEP) recorded from the visual cortex of cats
in response to a visual stimulus (S2) presented at various intervals after
presentation of another visual stimulus (S1) was measured under several
conditions. Recordings were made when both stimuli were presented through
one eye (monoptic condition) or when S1 was presented to one eye and S2 to
the other (dichoptic condition). In some experiments, simultaneous
recordings were made from the optic tract and visual cortex. The stimuli
were pairs of sinusoidal gratings with a spatial frequency of 0.5
cycles/deg and of various orientations. Each stimulus was presented by
stepping the grating contrast from 0.0 (adapting field) to 0.5 for 50 ms.
The intervals between the presentation of the two test stimuli (S1 and S2)
was varied from 0 to 1,550 ms, and on different trials the orientation of
the S2 grating relative to that of S1 was varied from 0 to 90 degrees.
Results showed that under monoptic conditions, the VEP to the second
stimulus (S2) was reduced by presentation of the first stimulus (S1) when
the interstimulus interval was less than 200 ms, whereas under dichoptic
conditions, the response to S2 was reduced with interstimulus intervals
less than 75 ms. The response reduction was always in a forward direction
(e.g., reduced S2 response), increased in magnitude with decreases in the
interstimulus interval, and was larger under monoptic conditions than under
dichoptic conditions. The response reduction produced monoptically was
orientation selective in that it was greatest when the orientation of S1
and S2 was the same, and it recovered by half when the orientation differed
by 6 to 15 degrees (orientation half-band pass). In some cortical
recordings, the orientation-selective response reduction was superimposed
on a response reduction that was not selective for S2 orientation.
Stimultaneous recording in the optic tract also showed a response reduction
of S2 response that was not orientation selective, suggesting that
precortical neural elements contribute to the cortical VEP. With dichoptic
stimulus presentation an orientation-nonspecific response reduction was
obtained. We hypothesized that binocular inhibitory effects, resulting from
disparate retinal input, produced this surprising finding. The results
demonstrate that the VEP recorded at the cortex can be used to estimate
orientation sensitivity, but that response interactions in peripheral
(precortical) neural elements can contribute to such estimates.
