A team of researchers at UC San Francisco has uncovered the neurological basis of speech motor control, the complex coordinated activity of tiny brain regions that controls our lips, jaw, tongue and larynx as we speak.
Described this week in the journal Nature, the work has potential implications for developing computer-brain interfaces for artificial speech communication and for the treatment of speech disorders. It also sheds light on an ability that is unique to humans among living creatures but poorly understood.
"Speaking is so fundamental to who we are as humans - nearly all of us learn to speak," said senior author Edward Chang, MD, a neurosurgeon at the UCSF Epilepsy Center and a faculty member in the UCSF Center for Integrative Neuroscience. "But it's probably the most complex motor activity we do."
The complexity comes from the fact that spoken words require the coordinated efforts of numerous "articulators" in the vocal tract - the lips, tongue, jaw and larynx - but scientists have not understood how the movements of these distinct articulators are precisely coordinated in the brain.
To understand how speech articulation works, Chang and his colleagues recorded electrical activity directly from the brains of three people undergoing brain surgery at UCSF, and used this information to determine the spatial organization of the "speech sensorimotor cortex," which controls the lips, tongue, jaw, larynx as a person speaks. This gave them a map of which parts of the brain control which parts of the vocal tract.
They then applied a sophisticated new method called "state-space" analysis to observe the complex spatial and temporal patterns of neural activity in the speech sensorimotor cortex that play out as someone speaks. This revealed a surprising sophistication in how the brain's speech sensorimotor cortex works.
They found that this cortical area has a hierarchical and cyclical structure that exerts a split-second, symphony-like control over the tongue, jaw, larynx and lips.
"These properties may reflect cortical strategies to greatly simplify the complex coordination of articulators in fluent speech," said Kristofer Bouchard, PhD, a postdoctoral fellow in the Chang lab who was the first author on the paper.
In the same way that a symphony relies upon all the players to coordinate their plucks, beats or blows to make music, speaking demands well-timed action of several various brain regions within the speech sensorimotor cortex.
Brain Mapping in Epilepsy Surgery
The patients involved in the study were all at UCSF undergoing surgery for severe, untreatable epilepsy. Brain surgery is a powerful way to halt epilepsy in its tracks, potentially completely stopping seizures overnight, and its success is directly related to the accuracy with which a medical team can map the brain, identifying the exact pieces of tissue responsible for an individual's seizures and removing them.