Linearity and non-linearity in cortical receptive fields

Visual neurons in striate (V1) cortex have been studied as feature detectors or as spatiotemporal filters. A useful way to distinguish between these two conceptual approaches is by studying the way in which visual signals are pooled across space and time. Many neurons in layer IV of striate cortex exhibit linear spatial summation and their response time course is consistent with linear temporal summation. Neurons in supragranular and infragranular layers sum signals in a non-linear manner. A particularly important non-linearity seen in many cortical complex cells is non-linear summation along an axis parallel to their preferred orientation. This leads to responsiveness to 'illusory contours', borders defined by texture differences only. These and other results on non-linear summation of chromatic and achromatic signals imply that V1 cortex performs sophisticated and complex image processing and is not simply an array of spatiotemporal filters.