Rod flicker perception: Scotopic duality, phase lags and destructive interference

Abstract

Rod vision has a duality of organization: at mesopic luminances rod signals have access to a slow, sensitive pathway (which we refer to, following Stiles, as π 0) and a fast, insensitive pathway ( π' 0). The phase lag between the two rod signals increases with frequency until at 15-Hz the rod signals transmitted through the two pathways emerge out-of-phase, so that destructive interference produces a nulling of the apparent flicker. Relative to the cones, the phase lag of π' 0 is roughly half that of π 0. Thus at 15-Hz π' 0 signals can be out-of-phase with cone signals, so that the signals from the slower pathway, π 0, are actually in phase with cone signals. We have investigated the frequency response, adaptation behavior and phase characteristics of the two rod processes. The slower process, π 0 is more sensitive than π' 0, and dominates from absolute threshold up to low mesopic levels. The adaptation of π 0 seems not to be associated with a change in time constant, but rather with simple response compression or sensitivity scaling. The time constant of π' 0, however, does change with adaptation. There are large differences in the way that light adaptation changes the sensitivity of the two processes: signals from π' 0 may evade part of the postreceptoral sensitivity regulating mechanism normally associated with rod vision. The ability of signals from π 0 and π' 0 to reinforce or cancel each other, however, suggests that they are later reunited in a common pathway.