Quantitative determination of orientational and directional components in the response of visual cortical cells to moving stimuli.

The response characteristic of visual cortical cells to moving oriented stimuli consists mainly of directional (D) and orientational (O) components superimposed to a spontaneous activity (S). Commonly used polar plot diagrams reflect the maximal responses for different orientations and directions of stimulus movement with a periodicity of 360 degrees in the visual field. Fast Fourier analysis (FFT) is applied to polar plot data in order to determine the intermingled S, D, and O components. The zero order gain component of the spectrum corresponds to a (virtual) spontaneous activity. The first order component is interpreted as the strength of the direction selectivity and the second order component as the strength of the orientation specificity. The axes of the preferred direction and optimal orientation are represented by the respective phase values. Experimental data are well described with these parameters and relative changes of the shape of a polar plot can be detected with an accuracy better than 1%. The results are compatible with a model of converging excitatory and inhibitory inputs weighted according to the zero to second order components of the Fourier analysis. The easily performed quantitative determination of the S, D, and O components allows the study of pharmacologically induced changes in the dynamic response characteristics of single visual cortical cells.