Receptive field organization of complex cells in the cat's striate cortex.

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Abstract
All complex cells in the cat''s striate cortex exhibit gross non-linearities of spatial summation when tested with sinusoidal grating stimuli. Their responses to moving gratings of all but the lowest spatial frequencies are usually dominated by a component that is not modulated by the passage of the bars of the grating across the receptive field. They give responses to temporally modulated stationary gratings that consist mostly of even harmonics of the stimulus frequency and that vary little in amplitude or wave form as the spatial phase of the grating is varied. Complex cells'' receptive fields were compared with their sensitivity to sinusoidal gratings of different spatial frequencies. The receptive field organization of these cells was examined using pairs of stationary lines flashed synchronously on their receptive fields. If both lines are of the same polarity (bright or dark), complex cells respond to the paired stimulus much less well than they do to either of its component bars, unless the bars are separated by less than about 1/4 of the width of the receptive field. The results are independent in general character of the absolute positions of the stimuli within the receptive field. The 2-line interaction profile that plots the change in a complex cell''s response to 1 bar as a function of the position of a second added bar corresponds closely to the receptive field profile predicted from Fourier synthesis of the cell''s spatial frequency tuning curve. These profiles may thus reveal the spatial characteristics of subunits within complex cell-receptive fields. The nature of the interaction was examined between these subunits by performing several 2-line interaction experiments in which the onset of the second bar was delayed some time after the onset of the first. Neighboring subunits may interact in a facilitatory fashion: for an interval after the presentation of 1 bar, responses to neighboring bars are enhanced. The subunits of a complex receptive field may, by their spatial properties, determine the spatial selectivities of complex cells, while the nature of the interaction among the subunits may determine these cells'' sensitivity and selectivity for moving visual stimuli. A model based on the properties of, and interactions among, spatially distributed subunits within complex cell receptive fields appears capable of explaining the visual response properties of these neurons.