Spatial summation in the receptive fields of simple cells in the cat's striate cortex.

Abstract

The responses of simple cells in the cat''s striate cortex to visual patterns designed to reveal the extent to which these cells may be considered to sum light-evoked influences linearly across their receptive fields are examined. One-dimensional luminance-modulated bars used with gratings as stimuli; their orientation is always the same as the preferred orientation of the neuron under study. The stimuli are presented on an oscilloscope screen by a digital computer, which also accumulates neuronal responses and controlls a randomized sequence of stimulus presentations. The majority of simple cells respond to sinusoidal gratings that are moving or whose contrast is modulated in time in a manner consistent with the hypothesis that they have linear spatial summation. Their responses to moving gratings of all spatial frequencies are modulated in synchrony with the passage of the gratings'' bars across their receptive fields, and they do not produce unmodulated responses even at the highest spatial frequencies. Many of these cells respond to temporally modulated stationary gratings simply by changing their response amplitude sinusoidally as the spatial phase of the grating is changed. Their behavior appears to indicate linear spatial summation, since the absence of a null phase in a visual neuron need not indicate non-linear spatial summation, and that a linear neuron lacking a null phase should give responses of the form that are observed in this type of simple cell. A minority of simple cells appears to have significant non-linearities of spatial summation. These neurones respond to moving gratings of high spatial frequency with a partially or totally unmodulated elevation of firing rate. They have no null phases when tested with stationary gratings, and reveal their non-linearity by giving responses to gratings of some spatial phases that are composed partly or wholly of even harmonics of the stimulus frequency (on-off responses). Simple receptive fields were compared with their sensitivity to sinusoidal gratings of different spatial frequencies. Qualitatively, the most sensitive subregions of simple cell receptive fields are roughly the same width as the individual bars of the gratings to which they are most sensitive. Quantitatively, their receptive field profiles measured with thin stationary lines, agree well with predicted profiles derived by Fourier synthesis of their spatial frequency tuning curves.