LINEARITY AND NORMALIZATION IN SIMPLE CELLS OF THE MACAQUE PRIMARY VISUAL-CORTEX

Simple cells in the primary visual cortex often appear to compute a we ighted sum of the light intensity distribution of the visual stimuli t hat fall on their receptive fields. A linear model of these cells has the advantage of simplicity and captures a number of basic aspects of cell function. It, however, fails to account for important response no nlinearities, such as the decrease in response gain and latency observ ed at high contrasts and the effects of masking by stimuli that fail t o elicit responses when presented alone. To account for these nonlinea rities we have proposed a normalization model, which extends the linea r model to include mutual shunting inhibition among a large number of cortical cells. Shunting inhibition is divisive, and its effect in the model is to normalize the linear responses by a measure of stimulus e nergy. To test this model we performed extracellular recordings of sim ple cells in the primary visual cortex of anesthetized macaques. We pr esented large stimulus sets consisting of (1) drifting gratings of var ious orientations and spatiotemporal frequencies; (2) plaids composed of two drifting gratings; and (3) gratings masked by full-screen spati otemporal white noise. We derived expressions for the model prediction s and fitted them to the physiological data. Our results support the n ormalization model, which accounts for both the linear and the nonline ar properties of the cells. An alternative model, in which the linear responses are subject to a compressive nonlinearity, did not perform n early as well.