Coding of disparity information in extrastriate cortex of the cat

We have used information theory to analyse the responses of neurons in area 21a of the cat to disparity stimuli. Visual stimuli consisted of drifting sinusoidal gratings presented simultaneously to each eye. The relative spatial phase of the gratings varied between stimulus periods in a pseudo-random sequence of 45 degrees increments that covered the full 360 degrees. The mean information content of the responses of all neurons across all phases was 0.72 bits (+/-0.10, SE, n=29). The information conveyed by each neuron was well correlated with the extent to which the interocular phase difference modulated the response of the cell. However, information content was not simply related to firing rate, as there was usually significant information content in the neuronal responses to phase differences that elicited the minimum firing rate. In general, burst responses (impulse intervals <4 ms) did not convey more information than that conveyed by the total response. The contribution to the cumulative information of the response in successive 100-ms segments decreased over the course of the 1-s stimulus. The ratio of information transmitted at 200 ms to that transmitted over the full second had a median of 0.30 while the ratio of 500 ms to 1 s was 0.68.