Figure 4.

Synchronous discharge carries directional information beyond that expected by firing rate alone. (A) The temporal evolution of mutual information between synchrony defined at 1 ms precision and movement direction (blue crosses and line) and trial-shuffled estimates of mutual information (red dots) for a single pair of neurons. The mutual information is significantly (P < 0.03) larger than chance at around movement onset (green dots). (B) In another cell pair, a sustained level of significant mutual information is observed after movement onset by using a 5 ms temporal precision. The means and SDs of the trial-shuffled estimates are shown in red. (Inset) The difference between the mutual information and the mean of the shuffled estimates. (C and D) The temporal evolution of mutual information for the same cell pair used in A by using a 5 ms bin width excluding coincident spikes at 1 ms precision and by using a 15 ms bin width excluding coincident spikes at 5 ms precision, respectively. (E) Z-scores of mutual information derived from the pair of neurons used in A, C, and D for all three levels of temporal precision: 1 ms (red), 5 ms exclusive of 1 ms synchrony (blue), and 15 ms exclusive of 5 ms synchrony (green), respectively. Scale bar represents a z-score of 4. Z-scores were computed by subtracting the mean from the estimates of mutual information and normalizing for variance. This measure reveals the additional information about direction that occurs at movement onset that exceeds chance. (F) For the same cell pair used in A, C, and D, all three levels of temporal precision together were used to estimate the mutual information between synchronous spikes and direction. Scale bar for all graphs except for E represents 0.05 bits/50 ms interval. All figures except for B are based on a two-direction task (i.e., 1-bit task). Figure B is based on an eight-direction task (i.e., 3-bit task).