Rats press a lever to halve the frequency of a prolonged rewarding brain stimulus. The rats are given a choice between a lever which halves the frequency and a second lever on which the overall frequency of pulses remains the same but each even pulse follows each odd pulse after a short interval. When the interval between odd and even pulses on this second lever is less than 0.5 msec, this lever is pressed as frequently as the first, as if the even pulses had no effect. However, if the even pulses are placed at an interval which exceeds 0.5 msec, preference shifts to the first lever. The opposite effect occurs at the same parameters if there is no prolonged brain stimulus and the rat works simply to turn on the stimulus, pointing to the same neural substrate for the turning on and turning off of the brain stimulus. This supports the notion that turning off of the stimulus is due to an adaptation of the neural pathway subserving reward.
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