Effect of destruction of the 5-hydroxytryptaminergic pathways on temporal memory: Quantitative analysis with a delayed interval bisection task

This experiment examined the effect of destruction of the ascending 5-hydroxytryptaminergic (5HTergic) pathways on memory for duration, using a delayed interval bisection task. Rats that had received injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei, and sham-lesioned control rats, were trained in a series of discrete trials to press lever A following a 2-s presentation of a light stimulus, and lever B following an 8-s presentation of the same stimulus. Following stimulus offset a response on a panel placed midway between the two levers was required in order to initiate lever presentation; a single response on either lever resulted in withdrawal of both levers and, in the case of a 'correct' response, reinforcer delivery. When > 90% correct choices had been attained, an 8-s (phase I) or a 12-s (phase II) delay was interposed between stimulus offset and lever presentation in 50% of the trials, and probe trials (10% of both non-delay and delay trials) were introduced in which the light was presented for intermediate durations. Logistic functions were derived relating percent choice of lever B to stimulus duration. In both groups, the imposition of post-stimulus delays displaced the bisection point (duration yielding 50% choice of lever B) towards longer durations; this effect was significantly greater in the lesioned group than in the control group. Imposition of post-stimulus delays resulted in increases in the Weber fraction, which did not differ significantly between the two groups. 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.