This experiment examined the effect of destroying the 5-hydroxytryptaminergic (5HTergic) pathways on rats' ability to discriminate between two durations. Rats received injections of 5,7-dihydroxytryptamine into the median and dorsal raphe nuclei or sham lesions. They were trained to press lever A following a 2-s presentation of a light and lever B following an 8-s presentation of the light. For some rats, the levers were inserted into the chamber immediately after stimulus presentation (''no-poke-requirement''); for others, the levers were not inserted until a flap covering the food tray positioned midway between the levers had been depressed (''poke-requirement''). When stable performance was attained, ''probe'' trials were introduced in which the light was presented for intermediate durations. Logistic functions were derived relating percent choice of lever B to log stimulus duration. Under the ''no-poke-requirement'' condition, the bisection point (duration yielding 50% choice of lever B) was shorter in the lesioned rats than in the control rats. Under the ''poke-requirement'' condition, this effect of the lesion was attenuated. There was no effect of the lesion on the Weber fraction under either condition. The levels of 5HT and 5-hydroxyindoleacetic acid were reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not altered. It is proposed that rats may attain accurate timing under the interval bisection task by moving from one lever to the other during stimulus presentation; this movement may be facilitated by destruction of the 5HTergic pathways. Accurate timing is still possible when this movement is suppressed by the introduction
