Contrast gain control in the visual cortex: Monocular versus binocular mechanisms

In this study, we compare binocular and monocular mechanisms underlying con trast encoding by binocular simple cells in primary visual cortex. At mid t o high levels of stimulus contrast, contrast gain of cortical neurons typic ally decreases as stimulus contrast is increased (Albrecht and Hamilton, 19 82). We have devised a technique by which it is possible to determine the r elative contributions of monocular and binocular processes to such reductio ns in contrast gain. First, we model the simple cell as an adjustable linea r mechanism with a static output nonlinearity. For binocular cells, the lin ear mechanism is sensitive to inputs from both eyes. To constrain the param eters of the model, we record from binocular simple cells in striate cortex . To activate each cell, drifting sinusoidal gratings are presented dichopt ically at various relative interocular phases. Stimulus contrast for one ey e 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 a ll of the interocular contrast ratios. This allows us to determine the effe ct of contrast on the contrast gain of the system. Finally, we decompose th e 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 bino cular 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.