Alterations in the pattern of mastication after ablations of the lateral precentral cortex in rhesus macaques

Abstract

Patterns of mandibular movement and jaw muscle activity in two monkeys were recorded during mastication of natural foods before and after ablations of the lateral precentral cortex. Bilateral, but not unilateral, lesions that included the “face” area of the precentral cortex produced permanent changes in patterns of mandibular movement during mastication, including decreased jaw opening and lateral deviation. The basic chewing rate, however, was not greatly altered. Paresis of the tongue and facial muscles, possibly combined with loss of the ability to produce complex jaw movements, hindered manipulation and positioning of food required for mastication. Bilateral lesions of the far-lateral precentral cortex, a region that evokes rhythmic jaw movement when electrically stimulated, initially produced symptoms like those described above, but after a period of recovery, patterns of mastication became normal. The results indicate that the precentral cortex, particularly the face area, is involved in coordination of the tongue, jaw, and facial muscles necessary for normal, efficient mastication. At the same time, the results confirm the existence, in the nonhuman primate, of a subcortical pattern generator capable of producing a pattern of rhythmic jaw movement similar in some respects to masticatory jaw movements of normal monkeys.