Functional properties of the primary motor cortex and ventral premotor cortex in the monkey during a visually guided jaw-movement task with a delay period.

This study investigated single neuronal activity in the face area of the primary motor cortex (MI) and ventral part of the premotor cortex (PMv) while a monkey performed a visually guided jaw-movement task with a delay period. When the monkey executed the jaw movements, 48 MI and 53 PMv neurons showed statistically significant activities time-locked to jaw movements and were defined as movement-related neurons. The activities of movement-related neurons could be classified into phasic, phasic-tonic and tonic patterns based on the changes in discharge rate. Most of the neurons exhibiting phasic and phasic-tonic activities probably contributed to the initiation of jaw movements, since they exhibited transient responses immediately after the onset of the go-cue indicating the jaw movement. In contrast, the sustained activity of the movement-related neurons exhibiting phasic-tonic and tonic activities may be involved in controlling and/or maintaining jaw position. Sustained activity was also detected during the delay period in 4 MI and 29 PMv neurons and these neurons were defined as set-related neurons. It is thought that these set-related neurons are involved in the preparation for the subsequent jaw movement, since the masticatory muscles showed no significant changes during the delay period. These findings 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 may play a role as a higher-order motor area related to the initiation and control of jaw movements.