Figure 3.
Variability and cortical state. A, The stimulus sequence expressed as a temporal sequence of grating orientations. Only 1 s of stimulus is shown (stimuli typically lasted 6 s). B, The population response measured in one trial, as a function of time and preferred orientation. C, The population response, averaged across 10 trials in this session (Experiment 79-12-3). D, A single example of ongoing activity of the same population, recorded during the presentation of a blank screen during the same session. E, Total population rate measured during the ongoing activity. Continuous red lines indicate average over time. Dotted red lines indicate 1 SD away from this average. Right, Histograms, Log probability of each firing rate. F–J, The same stimulus sequence was used in another experimental session (Session b) recording from the same population of neurons, 24.5 h earlier (Experiment 79-9-11). K, Mean activity against the variance in activity for all experiments. Mean and variance were computed over 8 ms time bins and averaged across sites. Dots represent 23 experimental sessions in 9 animals. A–J, The two example experiments (recorded from the same neurons) are colored in red (Session a) and blue (Session b). L, Relationship between “global fluctuation index” (measured during ongoing activity) and “multiunit variability index” (measured from stimulus-driven activity) across experiments. Solid line indicates the result of linear regression. Dotted line indicates an arbitrary distinction between “synchronized” and “desynchronized” experiments. Red and blue symbols represent the two example sessions.