Different roles of the primary motor and ventral premotor cortex in jaw movements

To determine whether the primary motor cortex (MI) and ventral premotor cortex (PMv) play different roles in the control of jaw movements, single neuronal activities were recorded from these areas while a monkey closed or opened its mouth in response to a visual stimulus. There were a total of 134 neurons exhibiting task-related activities, of which 45 MI and 53 PMV neurons showed significant activity when the monkey performed the jaw movements, and thus, were designated as movement-related neurons. The movement-related activities could be classified into phasic, phasictonic, and tonic patterns based on the changes in the discharge rate, and they probably play specific roles during different jaw movements. We also obtained sustained activity during the delay period in four MI and 29 PMv neurons, and considered these neurons as set-related. It is thought that the sit-related neurons contribute to the subsequent jaw movement, since the masticatory muscles showed no clear changes during the delay period. These results suggest that the MI may be involved predominantly in the initiation and control of jaw movements, and that the PMV may be involved in motor preparation and play a role as a higher order motor area related to the initiation and control of jaw movements.