The effects of scopolamine on proactive interference and spatial delayed matching-to-sample performance in rats

Abstract

Rats were trained in a three-alternative spatial delayed matching-to-sample task in which the samples were rewarded forced choices of one arm of a three-arm starburst maze and retention was indicated by returning to that arm following a delay or retention interval. If the rat made an error on its first free choice of a trial, the chosen arm was blocked off and the rat was allowed a second choice between the remaining two arms. In two experiments, we investigated rats’ performance following presession treatment with scopolamine hydrobromide or following one of several control treatments presented in a repeated-measures design. In the first experiment, rats were given 0.1-mg/kg scopolamine hydrobromide, 0.1-mg/kg scopolamine methylbromide, or an equal volume of saline vehicle. In the second experiment, we tested new groups of rats under the same drug conditions as in Experiment 1 and in an additional condition without drugs, but with a longer retention interval. Experimental sessions were separated from one another by at least 1 rest day and 1 drug-free short-delay reminder session. Choice accuracy was lower following injections of scopolamine hydrobromide relative to the other conditions. Proactive interference, in the form of intrusions by previous samples or intrusions of the previous choices, however, was unaffected by drug treatment. These results suggest that scopolamine interferes with choice accuracy by decreasing the retention or encoding of information about the location of the sample, rather than by decreasing the animal’s ability to differentiate the responses made on one trial from those made on the previous trial.