In a study published today in theProceedings of the National Academy of Sciences, neurobiologists at the University of Pittsburgh School of Medicine discovered that multiple areas in the brains frontal lobe were capable of controlling vocalization and therefore, they could provide a foundation for complex speech.
According to the study, only the primary motor cortex, nicknamed M1, directly influences the larynx or voice box, according to the principal investigatorPeter L. Strick, Ph.D., and Thomas Detre, the chair ofneurobiology at Pitt. Instead, several cortical regions send signals to laryngeal muscles to increase vocal finesse in some nonhuman primates.
This type of parallel processing in our neural wiring might explain why humans are capable of very useful language, which allows us to share information, express and perceive emotion, and tell memorable stories, according to Strick, who is also a scientific director ofPitts Brain Institute. Our exceptional speech abilities are primarily due to increased brains, not better muscles.
Investigators compared marmosets and macaques neural networks that are the origin of ascending command signals to control monkey vocalizations, led by Christina M. Cerkevich, a research assistant professor of neurobiology.
Cerkevich explained that these two monkey species were chosen because of the profound differences in their vocal behavior. Marmosets express their connection in ways that are similar to humans by taking turns to speak and altering the volume, timing, and pitch of their calls to each other. Macaques, on the other hand, make mostly simple, spontaneous calls.
The researchers then transplanted a transneuronal tracer from the rabies virus into the larynxes'' cricothyroid gland. It has the unique ability of moving from one neuron to another only at synapses, which are the special areas where neurons interact. This makes it possible to track neuronal circuits from the muscle back to the areas of the cerebral cortex that control it.
Both monkeys had multiple premotor areas in the frontal lobe that sent descending command signals to the cricothyroid gland. However, two of the premotor areas provided a substantially greater source of descending output in marmosets, implying that the increased vocal motor skills of these monkeys are partly due to the expansion of neural signaling from these premotor areas.
This result dispels the long-held view that vocalization improvements are due mainly to M1''s primary motor cortex. It appears there is no single control center, but parallel processing sites that enable complex vocalization and, ultimately, speech.
Future tasks include analyzing other members of the vocal motor network and developing how these changes may impact or result in vocal aches, including stuttering and speech apraxia.
Jean-Alban Rathelot, Ph.D., ofAix-Marseille Universitein Marseille, France, was also a member of the research team. R01NS24328 352, R01AT010414, P40OD010996, and T32NS086749 were among the students receiving research funding. TheDSF Charitable Foundation has provided some information.