Response variability and orientation discrimination of single cells in striate cortex of cat.

The response of single cells in the striate cortex of cat to a moving light bar of variable orientation was measured by a method providing data on the mean response as well as the standard deviation (SD) at the different stimulus orientations. At the optimal stimulus orientation the SD was about 1/3 of the mean response. Marked differences in this respect were found between simple and complex cells, i.e., the SD for the simple cells was about 1/2 of the mean response and about 1/4 for the complex cells. The variation coefficient (Vc = SD/mean) was minimal at the optimal orientation and increased relatively in the same manner for simple and complex cells as the stimulus orientation was varied away from optimal orientation. The Vc varied with the mean response at optimal orientation in a nonlinear manner. A function is proposed which fits this relationship and which is equally applicable for both simple and complex cells. The mean orientation discrimination (MOD) was defined as that change in orientation angle away from the optimal which produced a response statistically different--on the 1% level--from the response to the optimal orientation. There were differences in MOD between the two sides of the orientation tuning curve: the mean of the smaller of the two values was 13.5 deg and of the larger 19.7 deg. No significant difference in MOD was found between simple and complex cells despite the fact that the halfwidth of the tuning curves for the two cell types was 19.5 deg and 31.6 deg, respectively. The preciseness in localization of the most sensitive part within the receptive field of single cells was calculated from the variability in time of occurrence of the smallest interspike interval. The degree of preciseness was found to be of the order of 1/4 of the receptive field diameter in both simple and complex cells. When nonoptimal stimulus orientations were presented, the preciseness significantly decreased in complex cells whereas it remained unchanged in simple cells. It is suggested that the same type of intracortical wiring produces orientation selectivity in simple and complex cells, and that the differences in tuning width are mainly due to a larger extension of inhibitory fields in the simple cells. Considering the cortical visual cells as elementary units in a network built for orientation detection and discrimination, the tuning width seems of minor importance for that function.