Figure 5.

Population effect of stimulus contrast on area summation during the control condition. (A–D) Summation curve features: difference in summation field (SF), peak response rate (R_peak), asymptote response rate (R_asym), and surround suppression index (SSI) between high and low contrast stimuli. Black triangle indicates population mean (with two-tailed t-test p-value). (E–H) Parameter estimates (k_D, k_N, w_D, w_N) for all units plotted for high contrast vs. low contrast conditions. p-values correspond to a two-tailed Mann-Whitney U-test for k_D and k_N, and a t-test for w_D and w_N. Triangles and circles indicate single- and multi-units, respectively. Differences were not detected in any feature (A–D) or parameter (E–H) when comparing across single- and multi-unit subpopulations (each p > 0.05). In all panels, the magenta and orange symbols indicate features for example units in Figure 1. Reducing contrast increased the summation field, decreased the peak response rate, and yielded a small decrement in the asymptotic rate and suppression index. Both gain parameters increased and w_D decreased with high contrast.