In Praise of Masking: Behavioural Responses of Retinally Degenerate Mice to Dim Light

There are two ways in which animals compartmentalize their activities into day or night: they can have an endogenous clock which is synchronized each day by a light-dark cycle or there can be a direct response to light such as the decrease in activity by a nocturnal rodent. In the first case activity is said to be entrained by light. In the second case activity is said to be masked by light. Any demonstration of entrainment by periodic presentation of a stimulus must show that activity occurring in phase with that stimulus is not simply a direct response to the stimulus but represents control of phase by an endogenous clock. Thus masking has come to be something to be avoided and excluded in experiments on circadian rhythms. This has led chronobiologists into displaying a lack of interest or a negative attitude toward masking, though there are some exceptions (e.g., refs. 1-5).