Effects of tegmental lesions on the isolation call of squirrel monkeys.

The present study is concerned with identifying brain mechanisms underlying a basic mammalian vocalization known as the isolation call. The call serves to reestablish contact of separated individuals. Adult squirrel monkeys were used as experimental subjects because the isolation call in these animals has been shown to be stable, well-defined, and readily elicited under experimental conditions. Bilateral, symmetrical electrocoagulations in certain parts of the tegmentum and core gray matter of the thalamus and midbrain variously altered the character and production of isolation calls, but had no apparent effect on other vocalizations. In respective cases the changes were characterized by: (1) reduction in number of calls; (2) calls with abnormal structure; and (3) calls of infantile character. As opposed to earlier investigations on mammals, the present study has shown that damage to certain brain structures may not only affect the production of a vocalization but also its physical characteristics.