Many characteristics of visual temporal integration, found in both psychophysics and physiology, have been thought of as arising from a number of cascaded integrations, or exponential delays. As an alternative, a single process is proposed which has a temporal response identical with that of the many-stage model. This process is readily visualized as taking place within a single receptor cell. It consists of a number of subprocesses, the number being proportional to the number of photons absorbed. Each subprocess consists of three steps: (1) initiation by photon absorption, (2) counting of a series of random events, and (3) emission of a signal when the count reaches a specific number. Response is taken to be the summation of all the signals produced by the subprocesses. Temporal delay and spread of the response is entirely attributed to the time required to accumulate the independent random events of step (2). Possible physiological correlates of the three steps are touched on, but only speculatively.
JOHN LEVINSON, "One-Stage Model for Visual Temporal Integration," J. Opt. Soc. Am. 56, 95-97 (1966)