Alterations in aversive and aggressive behaviors following orbital frontal lesions in rhesus monkeys.

We utilized the methods of comparative psychology and of ethology to assess the effects of frontal lesions on species-specific aversive and aggressive behaviors in rhesus monkeys. Removal of orbital frontal (OF) cortex enhanced aversive reactions and reduced aggressive reactions in several threatening situations, including a social colony. The deficit in aggresion was not due to enhanced aversion or to locomotor hyperactivity. Nor did the OF monkeys lose the capacity to execute aggressive reactions. Rather, their degree of deficit seemed to vary with the degree of threat in the test situation. Moreover, these emotional alterations are produced by lesions restricted to the posteromedial portion of OF cortex, which is closely associated with limbic structures, as well as by lesions of the dorsomedial nucleus, with which this portion of OF cortex is interconnected. While lesioning either of these structures reduced aggression and enhanced aversion, amygdalectomy reduced both aggression and aversion. The view that monkeys with OF lesions are overaroused by threatening situations was not supported by analysis of autonomic responses following OF lesions. The emotional alterations produced by OF lesions may be due to the disturbance of stress mechanisms which prepare the organism for reacting appropriately to highly threatening situations.