Figure 5. The roles of refractory period to light adaptation.

A, the event rate (illustrated by unitary QBs) in a microvillus depends on the incoming photon rate. In dim conditions, photon arrivals to an individual microvillus are rare and virtually every photon hit (absorption) evokes a QB. With increasing photon rate (medium light), the quantum efficiency (photon to bump conversion probability) decreases gradually. In very bright daylight, photon arrivals can be so frequent that the refractory period effectively sets the maximum QB production rate (sample rate). B, quantum efficiency changes automatically with brightening from 100% to 1%, acting as a powerful gain control mechanism. C, a fly photoreceptor's LIC response to a dim light pulse. D, a fly photoreceptor's LIC response to a bright light pulse. E, during a dim light stimulus, the QB count (samples) from the activated microvilli does not show a fast adapting peak. F, in response to a bright pulse, the QB count (samples) from the activated microvilli first peaks, then rapidly falls, before settling to a steady‐state as the photon arrivals and refractory periods balance. G and H, QB size must reduce over time to account for the slow exponential response trend. The receptor current displayed in C and D can be simulated by taking account of both factors: the reduction of activated microvilli number (E and F), and QB size reduction (G and H) (Song et al. 2012).