Delta-9-THC and nonverbal communication in monkeys.

Male rhesus monkeys were trained to perform an instrumental avoidance conditioned response. They were then paired in the "cooperative conditioning" paradigm, a situation designed to measure the quality of nonverbal communication by changes in facial expression. A "responder monkey" was able to perform discriminated instrumental avoidance responses by processing facial expressive cues provided through a closed circuit television picture of a "stimulus monkey" which was receiving presentations of the conditioned stimulus. Oral administration of 1.0 mg/kg of delta-9-tetrahydrocannabinol (delta-9-THC) to the stimulus animal, the responder, both animals, or neither animal was given in all possible combinations of pairings to determine if delta-9-THC affects the expressiveness of the stimulus monkey or the sensitivity of the responder to the expressions of others. The drug impaired the display of facial expression in "stimulus" subjects but enhanced the ability of "responders" to discriminate the expressions of their partners. Measures of heart rate during cooperative conditioning revealed that delta-9-THC lowered cardiac rates of responder monkeys.