Photic and nonphotic circadian phase resetting in a diurnal primate, the common marmoset

Despite the considerable literature on circadian entrainment, there is litt le information on this subject in diurnal mammals. Contributing to this lac k of understanding is the problem of separating photic from nonphotic (beha vioral) phase-resetting events in diurnal species. In the present study, ph otic phase resetting was obtained in diurnal common marmosets held under co nstant dim light (DimDim; <0.5 1x) by using a 20-s pulse of bright light to minimize time available for behavioral arousal. This stimulus elicited pha se advances at circadian time (CT) 18-22 and phase delays at CT9-12. Daily presentation of these 20-s pulses produced entrainment with a phase angle o f <similar to>11 h (0 h = activity onset). Nonphotic phase resetting was ob tained under DimDim with the use of a 1-h-induced activity pulse, consistin g of intermittent cage agitation and water sprinkling, delivered in total d arkness to minimize photic effects. This stimulus caused phase delays at CT 20-24, and entrainment to a scheduled daily regimen of these pulses occurre d with a phase angle of similar to0 h. These results indicate that photic a nd nonphotic phase-response curves (PRCs) of marmosets are similar to those of nocturnal rodents and that nonphotic PRCs are keyed to the phase of the suprachiasmatic nucleus pacemaker, not to the phase of the activity-rest c ycle.