Fig. 3: Gain changes during light adaptation occur on a slower timescale in foveal compared to peripheral cones.

A Light stimulus used to assess the time course of rapid gain changes during light adaptation. Five brief light flashes (black trace) were superimposed on an adapting light step (gray trace); the first, third, and fifth flashes were fixed in time (black), and the second and fourth were presented at variable time delay from step onset and offset. In this exemplar trace, time delay = 160 ms. The middle panel shows the mean response of a peripheral cone to the adapting step in combination with the five flashes (black trace) for time delay = 160 ms, as well as to the light step alone (gray trace). The bottom panel shows flash responses isolated by subtracting the response to the light step alone. The first and fifth light flashes are the unadapted responses, whereas the smaller and faster response to the third flash (near the end of the step) is the adapted response. The light flashes following step onset and offset evoke cone responses that adapt from a lower to higher mean luminance, as well as responses that adapt from a higher to lower mean luminance. B The top panel shows rapid gain changes of the peripheral (left) and foveal (right) cone in (A) at light onset. Response gains for each of the flashes were obtained by dividing the response by the flash strength and normalizing to the response gain of the flash at lower background luminance (far left black trace; unadapted flash); the steady-state adapted gain is denoted by the far right purple trace and colored traces in between correspond to flashes with a variable delay from the step onset. The kinetics of the gain changes were tracked by identifying the peaks and fitting their time course with a single exponential function. The time constant of the best-fit exponential was tau_On = 13.5 ms (red smooth line) for the peripheral cone and 30.92 ms for the foveal cone. The bottom panel shows the time course of gain changes at light offset (gains normalized to response gain of the flash delivered well after light offset). Black trace corresponds to the initial steady-state adapted gain (far left) and the purple trace corresponds to the final steady-state gain after step offset (far right). Colored traces correspond to flashes with a variable delay from the step offset (same delays as in the top panel). The time constant of the best-fit exponential for this exemplar peripheral cone was tau_Off = 127.3 ms (red smooth line) for the peripheral cone and 250.63 ms for the foveal cone. C–E Time course of gain changes at step onset of foveal and peripheral cones for light steps of two distinct magnitudes 1000–10,000 R^*/s (n_f = 6; n_p = 15) and 5000–50,000 R^*/s (n_f = 12; n_p = 23). The foveal cones show a significantly slower time course of adaptation both at light onset and offset. E Gain changes several folds more rapidly during step onset than at step offset. The ratio of tau_onset over tau_offset was above 1 for both foveal and peripheral cones and significantly different between foveal and peripheral cones for only the 1000–10,000 R^*/s light step. Lighter open circles represent peak response gains from each cell. Points with error bars represent mean ± sem. Source data are provided as a Source Data file. We used the unpaired t-test for all the statistical analyses in this figure. The significance threshold was placed at α = 0.05 (n.s., p > 0.05; *p < 0.05; **p < 0.01; ***p < 0.001).