Contrast Gain Control in the Visual Cortex: Monocular vs. Binocular Mechanisms
Anthony M. Truchard, Izumi Ohzawa, and Ralph D. Freeman (2000)
Contrast gain control in the visual cortex: monocular versus binocular mechanisms.
J. Neuroscience 20: 3017-3032.
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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 (). 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.