Fig. 2.

Schematic illustration of correlogram calculation. A: shuffled auto correlogram (SAC) computation. A1: the intervals between a certain spike (surrounded by an ellipse in this example) and every other forward spike in the other trains (but not the spikes from the same train) are computed and tallied in a histogram (A2). This operation is repeated for every spike of every spike train and the results mirrored (as the auto correlation is symmetrical), yielding the SAC (A3). The half-height width (HHW) is the width of the main lobe where the values are half of the SAC peak. CI, correlation index. B: cross-stimulus autocorrelogram (XAC) computation. Same as in A but in this case, the intervals are computed between individual spikes from the responses to a repeated stimulus at one stimulus level and all spikes from a repeated stimulus at another stimulus level (including the same trial index). Both forward and backward intervals are considered (B2). Repeating this operation for every spike yields the XAC (B3). The lag is defined as the position of the main lobe peak.