Studies on phase-shifts in endogenous rhythms

Abstract

Cultures of Drosophila pseudoobscura pupae raised in 12:12 hours L/D cycles were subjected to brief light pulses and light steps during early and late subjective night phases.1. A light pulse and a light step during early subjective night evoke dissimilar responses, the pulse effecting a delay phase-shift and the step an advance phaseshift. But a pulse and a step in the latter part of the subjective night evoke a similar response from the system and advance the phase. 2. The results are explained by assuming a differential light sensitivity of the underlying system during the subjective night phase itself, with a phase-point of maximum light sensitivity. 3. It is postulated that the light "off" fraction of a pulse acts as a new "dusk" in the early subjective night and that the "on" fraction acts as a new-"dawn" in the late subjective night. 4. The results of an experiment where a light pulse and a light step combined to form the light treatment bear out the assumptions made above and indicate that the photoinducible phase itself is not phase-locked to environmental time.