Short-term memory in the rhesus monkey: disruption from the anti-cholinergic scopolamine

Two separate experiments were conducted to evaluate the effects of the anti-cholinergic scopolamine on primate visual discrimination and short-term memory (STM). In the first experiment it was shown that relatively mild doses of scopolamine severly impaired visual discrimination performance, even though the test procedure provided strong stimulus control. This deficit in visual discrimination suggested that previous research which used the delayed matching to sample procedure (DMS) to evaluate the role of cholinergics in primate STM may have confounded an accurate measure of specific STM effects because the DMS is inherently dependent on accurate visual discrimination. Therefore, the second experiment evaluated the effects of scopolamine on STM, using an automated apparatus and test procedure designed to minimize the discrimination component and other confounding variables present in the earlier research. In this second experiment, an indirect delayed response (DR) procedure was used, measuring the monkeys' ability to recall simple stimulus events over retention intervals of 0, 2.5, 5, and 10 sec. The monkeys were tested under 2 doses of the anti-cholinergic scopolamine and their performance was compared to that obtained on several nondrug control days. Contrary to earlier reports using the DMS, a clear interaction of drug and retention interval occurred in this situation. Under scopolamine, greatest impairments occurred on the longest delays, with little or no effect with zero second retention. Furthermore, the impairments observed on the longer delays were even greater with the highest dose of scopolamine. These data, therefore,support the notion that cholinergic mechanisms play an important role in the expression of STM in primates.