Figure 3.

LIP neuronal Fano factor increases with mean spike rate in the surround. A, Population average PSTF of the Fano factor indicates a larger Fano factor in the epoch between target and distractor appearance for larger mean spike counts. Target locations >25° from the RF center were rank ordered by mean spike rate and then split into three equal classes for each neuron before averaging. The lowest one-third of spike rates is shown in blue, the middle one-third in red, and the highest one-third in magenta PSTF. B, Population average PSTHs of the corresponding mean spike rate for the PSTFs plotted in A confirm differences in spike rate, as expected from the sorting procedure. C, D, Pairwise comparisons confirm the trends shown in A: scatter plot comparing each neuron's Fano factor (average of five non-overlapping 100 ms bins, −470 to 30 ms relative to distractor onset, gray bar in A) for trials with the middle one-third of spike rates (abscissa) to those with the lowest one-third of spike rates (ordinate in C; signed-rank test, p < 0.0001; n = 72 neurons) and the highest one-third of spike rates (ordinate in D; signed-rank test, p < 0.0001; n = 72 neurons). Two points in D (x-axis: 1.63, 2.39; y-axis: 4.98, 4.09) were omitted from the display for better visibility. Figure format for PSTFs, PSTHs, and scatter plots are as in Figure 1.