Cortical field potentials preceding vocalization in monkeys.

Field potentials before vocalization were studied with electrodes chronically implanted on the surface and at a 2.0-3.0 mm depth in various cortical areas in three monkeys. Surface-negative, depth-positive (s-N, d-P) slow potentials prior to vocalization were recorded in the premotor (including the Broca's homolog), motor and somatosensory cortices, and the supplementary motor area (SMA). Such premovement potentials were recorded also in the cingulate and prefrontal cortices when a monkey uttered a sound with intense motivation for reward. Cerebellar hemispherectomy on the right side eliminated the s-N, d-P premovement potentials in the motor cortex and in the posterior bank of inferior limb of the arcuate sulcus (homolog of Broca's area) in the left hemisphere. After the operation, the tone came to have less components of different frequencies, and its duration varied much more than before. These facts suggest that the neocortical area homologous to the human speech area takes part in the generation and control of monkey vocalization together with the cerebellum possibly through cerebro-cerebellar interactions. This is against ideas so far proposed on nonhuman primate vocalization, i.e., it is generally considered that animal vocalization differs fundamentally from human speech.