Visual cortical neurons as localized spatial frequency filters

This paper relates to the receptive field properties of neurons in the primary visual cortex, i.e. the striate cortex, to current issues in spatial visual information processing. Particular attention is given to the fact that receptive field profiles of simple cells in the visual cortex often resemble even-symmetric or odd-symmetric Gabor filters; i.e. their receptive field profiles can be described by the product of a Gaussian and either a cosine or sine function. Their spatial frequency tuning is of medium bandwidth (~one octave) which is narrow enough for a cell to distinguish the third harmonic from the fundamental frequency for square-wave gratings of low spatial frequency. The responses of adjacent simple cells, tuned to the same spatial frequency, orientation, and direction, differ in their phase response to drifting sine-wave gratings by approximately either 90° or 180°. This latter result makes it possible to consider two adjacent simple cell pairs as operating like paired Gaussian-attenuated sine and cosine filters of Gabor filters for restricted regions of visual space. The entire set of simple cells provides a complete representation of the visual scene, yet each simple cell is unique in its response properties. At the complex cell stage, the cell´s mean firing rate appears to represent the amplitude of a local Fourier coefficient, but phase information is seldom conveyed with much precision in the action potential code.