The Journal of Neuroscience, April 15, 2000, 20(8):3017-3032
Contrast Gain Control in the Visual Cortex: Monocular Versus
Binocular Mechanisms
Anthony M.
Truchard,
Izumi
Ohzawa, and
Ralph D.
Freeman
Group in Vision Science, School of Optometry, University of
California, Berkeley, California 94720-2020
In this study, we compare binocular and monocular mechanisms
underlying contrast encoding by binocular simple cells in primary visual cortex. At mid to high levels of stimulus contrast, contrast gain of cortical neurons typically decreases as stimulus contrast is
increased (Albrecht and Hamilton, 1982). We have devised a technique by
which it is possible to determine the relative contributions of
monocular and binocular processes to such reductions in contrast gain.
First, we model the simple cell as an adjustable linear mechanism with
a static output nonlinearity. For binocular cells, the linear mechanism
is sensitive to inputs from both eyes. To constrain the parameters of
the model, we record from binocular simple cells in striate cortex. To
activate each cell, drifting sinusoidal gratings are presented
dichoptically at various relative interocular phases. Stimulus contrast
for one eye is varied over a large range whereas that for the other eye
is fixed. We then determine the best-fitting parameters of the model
for each cell for all of the interocular contrast ratios. This allows
us to determine the effect of contrast on the contrast gain of the
system. Finally, we decompose the contrast gain into monocular and
binocular components. Using the data to constrain the model for a fixed contrast in one eye and increased contrasts in the other eye, we find
steep reductions in monocular gain, whereas binocular gain exhibits
modest and variable changes. These findings demonstrate that contrast
gain reductions occur primarily at a monocular site, before convergence
of information from the two eyes.
Key words:
contrast gain control; simple cells; striate cortex; binocular vision; cat; nonlinearity
Copyright © 2000 Society for Neuroscience 0270-6474/00/2083017-16$05.00/0
Previous Article | Next Article
