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, usin g a delayed interval bisection task. Rats that had received injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei, a nd sham-lesioned control rats, were trained in a series of discrete tr ials 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. Foll owing stimulus offset a response on a panel placed midway between the two levers was required in order to initiate lever presentation; a sin gle 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 presentatio n in 50% of the trials, and probe trials (10% of both non-delay and de lay trials) were introduced in which the light was presented for inter mediate durations. Logistic functions were derived relating percent ch oice of lever B to stimulus duration. In both groups, the imposition o f post-stimulus delays displaced the bisection point (duration yieldin g 50% choice of lever B) towards longer durations; this effect was sig nificantly greater in the lesioned group than in the control group. Im position of post-stimulus delays resulted in increases in the Weber fr action, which did not differ significantly between the two groups. The levels of 5HT and 5-hydroxyindoleacetic acid were reduced in the brai ns of the lesioned rats, but the levels of noradrenaline and dopamine were not altered.