An analysis of the variability of unit activity in the cat's visual cortex

The spontaneous firing of single units in the cat's visual cortex, and their responses to repeated presentations of a constant visual stimulus, have been analysed quantitatively. Although the responses of most cells showed some habituation to the stimulus, this was less extensive than the fluctuations in responsiveness which occurred from one set of presentations to the next, over the course of several minutes. Fluctuations occurred largely independently in both the spontaneous and the stimulus-evoked firing of the cell. This suggests that separate mechanisms determine spontaneous and evoked firing. Quantitative estimates of the reliability (the reciprocal of the variability) of responses were higher if the peak firing rate of the cell was measured, rather than the total number of spikes fired by the cell. Thus, peak firing rate is more likely than total spike count to be the signal of biological significance.