Dynamic response of a circadian pacemaker

The Culex circadian pacemaker's response to phase-resetting light signals was studied in the first 3 cycles of darkness following a 12h light exposure. (1) In both cycles 1 and 2 there is a clear change from type 1 to type 0 phase-resetting as the resetting signal is prologed (Fig. 2). (2) Mosquitoes in cycle 1 are about half as sensitive to phase-resetting as those in cycles 2 or 3 (the criterion being the minimum pulse duration required to produce type 0 phase-resetting) (Fig. 2). (3) Each cycle appears to have a corkscrew-shaped phaseresetting surface and a phase singularity (Figs. 4, 5, and 7). The hypothesis that the Culex pacemaker reaches a stable limit cycle within the first cycle leads to an economical explanation of the results.