Recent extra- and intracellular recordings from simple cells in the striate cortex have led to the development of detailed models of how novel receptive field properties are generated by the neuronal circuitry of the cortex. The first stage in assembling simple receptive fields appears to be a linear combination of visual inputs. The output of the linear stage is then normalized by a contrast gain control mechanism. Finally, a threshold and expansive nonlinearity in the spike-generating mechanism enhances selectivity for stimulus features such as orientation, direction, and spatial frequency.