A study published in Proceedings of the National Academy of Sciences suggests multiple areas in the brain's frontal lobe control muscles of the voice, a function that was previously presumed to belong to only one brain region, the primary motor cortex.
Researchers in this study mapped out neural networks in marmoset and macaque monkeys to link muscles used in vocalizations to brain regions. Marmosets vocalize similarly to humans in terms of turn taking and altered volume, timing, and pitch. Macaques, on the other hand, produce relatively simpler vocalizations.
Neural networks were mapped out by injecting the cricothyroid muscle of the monkeys' larynxes with a tracer made from the rabies virus. Like the rabies virus, the tracer traveled from the muscle to nerve cells to the corresponding cerebral cortex regions that control the muscle.
Along with the primary motor cortex, multiple premotor areas in the frontal lobe were associated with control of the cricothyroid muscle of the larynx, in both types of monkeys. However, in marmosets, the monkeys with relatively more complex vocalizations, a larger portion of the premotor areas of the brain was associated with vocal productions. This challenges the long-held assumption that evolution in the primary motor cortex alone is responsible for more complex speech output.
Lead investigator Peter L. Strick, Ph.D., commented, "It appears there is no single control center, but rather parallel processing sites that enable complex vocalization and, ultimately, speech."
Uncovering new brain areas associated with speech production may help researchers in investigating voice and speech disorders and their treatments. The NIH estimates voice disorders affect 7.5 million people in the US.