Electrical stimulation of the rat diencephalon: differential effects of interrupted stimulation on on- and off-responding.

Rats were trained to turn on and to turn off electrical stimulation of the hypothalamus. Breaking trains of stimulation into bursts of pulses separated by intervals of no-stimulation attenuated off-responding more than on-responding. Current intensity was raised in an attempt to maintain a constant level of performance when either burst duration was decreased or interburst interval was increased. Current increases necessary to maintain on-responding were consistently smaller than the increments required to maintain off-responding. At shorter burst durations, off-responding usually ceased while on-responding continued. Four interpretations of the results are discussed: (1) temporal integration characteristics of the underlying neural systems, (2) reward adaptation, (3) electrode location, and (4) procedural artifacts. Only the first explanation which holds that the buildup of activity in the rewarding system is more rapid than in the aversive system is consistent with all the results. If correct, this conclusion indicates that multiple effects of electrical stimulation at a single brain site can be differentiated by manipulating the temporal pattern of the stimulation.