Light adaptation has similar effects on response times generated by the L-, M-, and S-cones, for both increments and decrements.

Visual system response kinetics change based on the adapting ambient light level, test stimulus properties (e.g., test polarity), and how a test stimulus is being processed by post-receptoral mechanisms (e.g., summation or differencing of the L-, M-, and/or S-cones). Light adaptation shortens the time constants of the photoreceptor response, reducing integration time, regulating sensitivity of the visual system: the visual system speeds up as a function of light level. Beginning in the retina at the cone-bipolar synapse, visual information is organized into parallel systems, signaling either a light increment (ON) or light decrement (OFF); the OFF pathway has been shown to be faster. We studied the effects of light adaptation as a function of test polarity and cone class by measuring response time (RT) distributions at low (0.31 or 1.21 Log Td) and high (4.13 Log Td) adapting background luminances. Stimuli were 2X threshold two-dimensional blurred rectangles calculated to be cone-isolating increments or decrements, to selectively modulate individual cone classes and activate either ON or OFF pathways. We found that: (a) The visual system speeds up similarly for all cone classes; average median RT decreased by ~90 msec as light level increased by ~3.4 log units. (b) Decrement RTs were faster than increment RTs by ~18 msec at both light levels. (c) At both light levels, latency is ~3 times longer for S-cones compared to the L- and M-cones. The simplest conclusion is that the speeding up of the visual system as luminance increases is predominantly due to changes at the photoreceptor level, such that increments and decrements see a similar decrease in RT. Despite the dramatically different post-receptoral pathways carrying S-cone signals, the temporal advantage of decrement perception occurs for S- as well as L- and M-cones, likely stemming from the faster kinetics of the OFF bipolar cells.