Abstract
The subject, a young female chimpanzee, was reared in a room where the room light was controlled by an automatic timer, turning on at 6 a. m. and off at 6 p. m. for a few years. During experimental sessions, switching of the light was left to the care of the subject through a manual switch in the room.
The time pattern of illumination of each day, generated by the subject, showed a continuous 12-hr rest period while the subject kept the light off and stayed in the nest and a 12-hr active period while the subject turned the light on and off repeatedly. Further, the starting time of the active period became earlier and earlier with daily sessions until the phase shift reached a 3-hour difference from the original state. The termination of the shift was probably caused by a fixed feeding-time occurring around 11 : 45 a. m. on each day.
According to ASCHOFF'S rule, it can be concluded that periodicity of light switching generated by the subject is analogous to continuous illumination with respect to circadian rhythm. Based upon this observation and analysis of effects of light upon operant behavior, it is proposed that the light-effect as a “Zeitgeber” may be weak at least in higher primates.
