Abstract
There is a popular belief that the sensory response of single cells throughout visual cortex is highly variable from trial to trial, and that this variability places a fundamental limit on the information processing capacity of individual neurons. Often this variability is characterized by recording the activity of a cell during multiple repetitions of a particular stimulus condition, and comparing the mean number of action potentials occurring in each repetition to the variance of these counts. An obvious question to be addressed before conducting such an analysis is asking what actually constitutes a “trial”? How critically does the trial to trial reliability of a single cell depend on the time window within which spikes are counted? Recent data from primate V1 (Müller et al., 2001) suggest that a relatively short epoch surrounding the cell's onset transient is both selective and reliable. Here we address this question by examining the activity of visually driven cells located in the temporal cortex of the monkey during both fixation and object identification. We find that for this population of cells, the choice of time windows is critical. Cells that appear to respond predictably are indeed quite reliable — with mean to variance ratios significantly in excess of one (as would be predicted by a Poisson process). However, this period of reliability is limited to short intervals — on the order of 75 to 100 milliseconds. Furthermore, for most cells, reliable responses are evoked for only a small subset of test patterns. We suggest that this period of maximal reliability, which generally occurs within 200 milliseconds of stimulus onset, is also time period over which these cells are most critical for signaling the presence of well known visual patterns.