On the Nature of Directed Behavior to Drug-Associated Light Cues in Rhesus Monkeys (Macaca mulatta)

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The present study investigated the role of drug-paired stimuli in controlling the behavior of rhesus monkeys. Systematic observations were made with 9 monkeys who had a history of drug self-administration; they had been lever pressing to produce intravenous infusions of various drugs. These observations revealed that the stimulus light co-occurring with drug infusion produced robust and cue-directed behavior such as orienting, touching, and biting. Experiment 1 showed that this light-directed behavior would occur in naïve monkeys exposed to a Pavlovian pairing procedure. Four monkeys were given response-independent injections of cocaine. In 2 monkeys, a red light preceded cocaine injections by 5 s, and a green light co-occurred with the 5-s cocaine injections. In the other 2 monkeys, the light presentations and cocaine injections occurred independently. Light-directed behavior occurred in all 4 monkeys within the first couple of trials and at high levels but decreased across sessions. The cocaine-paired stimulus maintained behavior longer and at higher levels than the uncorrelated stimuli. Furthermore, light-directed behavior was not maintained when cocaine was replaced with saline. Light-directed behavior did not occur in the absence of the lights. When these monkeys were subsequently trained to lever press for cocaine, light-directed behavior increased to levels higher than previously observed. Behavior directed toward drug-paired stimuli is robust, reliable, and multiply determined; the mechanisms underlying this activity likely include Pavlovian conditioning, stimulus novelty, habituation, and operant conditioning.

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