Functional muscle partitioning during voluntary movement: facial muscle activity for speech.

Electromyographic activity was recorded from multiple intramuscular sites in the orbicularis oris muscle for a set of speech and other motor tasks. Because of its geometric heterogeneity and multiple functions, it was hypothesized that this muscle might be partitioned in a task-dependent manner. Comparison of the timing and amplitude of the EMG activity among the multiple recording sites supported this hypothesis. Because some facial motoneurons in primates, including humans, are monosynaptically activated from area 4 of the motor cortex, it is suggested that this task-dependent muscle partitioning may reflect an aspect of cortical organization.