Impulse responses of the simple fields cat visual cortex were found to be modulated by gratings passing the field. The complex fields proved to be of three types: with modulated responses, unmodulated responses, and with modulated responses against unmodulated background. Amplitude-phase characteristic (APC) measured were inverse Fourier transformed to obtain the field's weighting function. Simultaneously the APC was reconstructed from the responses to edges and bars, with the use of the Fourier transform. Cross-comparison of the reconstructed APC and the WF showed that a RF has some linear properties but, strictly considered, is a non-linear system. Simple fields display the largest degree of linearity. The more complex field is the greater departures from linearity. As linear methods are inadequate for dealing with cortical RFs, their identification was performed in model experiments on a computer. The evidence obtained suggest that the RFs form a system of operators which perform the expansion of the image in non-classical pattern. Such an expansion can be termed quasi-Fourier-description.
