The effects of morphine and scopolamine on auditory discrimination in squirrel monkeys.

In squirrel monkeys trained to discriminate between two tones, morphine consistently increased the percent of response failures during tone periods; however, it had no consistent effect on the percent correct responses. In contrast, scopolamine decreased percent correct responses as well as increasing percent response failures in two monkeys not required to respond to produce the tone periods. In the two monkeys required to initiate tone periods by responding, high doses of scopolamine reduced the number of tones presented.