EFFECTS OF ASPIRATION VERSUS NEUROTOXIC LESIONS OF THE AMYGDALA ON EMOTIONAL RESPONSES IN MONKEYS

Abstract

All previous reports describing alterations in emotional reactivity after amygdala damage in monkeys were based on aspiration or radiofrequency lesions which likely disrupted fibers of passage coursing to and from adjacent ventral and medial temporal cortical areas. To determine whether this associated indirect damage was responsible for some or all of the changes described earlier, we compared the changes induced by aspiration of the amygdala to those induced by fiber-sparing neurotoxic lesions. Four different stimuli, two with and two without a social component, were used to evaluate the expression of Defense, Aggression, Submission, and Approach responses. In unoperated controls, Defense and Approach behaviors were elicited by all four stimuli, "social" and inanimate alike, whereas Aggression and Submission responses occurred only in the presence of the two "social" stimuli. Furthermore, all Defense reactions were reduced with an attractive inanimate item, while Freezing was selectively increased with an aversive one. Relative to controls, monkeys with neurotoxic amygdala lesions showed the same array of behavioral changes as those with aspiration lesions, namely reduced fear and aggression, increased submission, and excessive manual and oral exploration. Even partial neurotoxic lesions involving less than two-thirds of the amygdala significantly altered fear and manual exploration. These findings convincingly demonstrate that the amygdala is crucial for the normal regulation of emotions in monkeys. Nevertheless, since some of the symptoms observed after neurotoxic lesions were less marked than those seen after aspiration lesions, the emotional disorders described earlier after amygdalectomy in monkeys were likely exacerbated by the attendant fiber damage.