Minimum duration of light signals capable of producing the Aschoff effect

To further explore the validity of the non-parametric model of entrainment for predicting the phase-shifting effects of light pulses, we exposed rats to several intensities of continuous light (LL) and feedback lighting (LDFB). LDFB is a lighting condition that exposes the animal to light only during the interval of active locomotion; this interval is coincident with the photosensitive portion of the circadian cycle as defined by the phase-response-curve. This is achieved by linking lights on with locomotor activity. In addition to the comparison of LL with LDFB, the duration of the LDFB pulse was also varied in four rats. Whether rats were exposed to LL or LDFB, as light intensity increased, the free-running period (tau) of the locomotor activity rhythm also increased. This intensity-related increase in tau, which is known as the Aschoff effect, was similar for LL and LDFB 2 min at each light intensity (0.1, 1, and 100 lux). However, when the LDFB pulses were shortened from a duration of 2 min to a duration of approximately 1 sec, tau shortened significantly. These results demonstrate that the non-parametric model of entrainment adequately explains the major period-lengthening effects of LL. However, there are temporal limits to the light pulses that can be used to simulate the effects of LL (i.e., one second light pulses fail to produce the effects brought about by longer pulses).