The study, conducted by researchers at McGill University and the Yale School of Medicine, could reshape scientific understanding of how speech is learned and influence the design of future speech recognition and brain-based communication technologies.

Sensory Brain Regions Take Center Stage

For years, researchers have generally assumed that learning and remembering the complex movements required for speech depended primarily on motor areas of the brain. Those regions control the movements of the face, mouth, and vocal tract that make speaking possible.

The new findings point in a different direction. Instead of highlighting motor regions as the main driver of speech learning, the research suggests that auditory and somatosensory systems are critical for acquiring and retaining new speech patterns.

"Sensorimotor neuroscience has traditionally focused on frontal motor areas as the principal drivers of movement. This study changes that understanding by showing that human speech learning is extensively sensory in nature," said David Ostry, Professor of Psychology at McGill University.

The results may also help guide the development of emerging brain-speech technologies. Such systems could one day help restore communication abilities after stroke by incorporating sensory processes to improve performance and usability.

Testing Speech Learning With Brain Stimulation

To investigate how different brain regions contribute to speech learning, the researchers first altered participants' speech in real time and played the modified speech back through headphones. This approach encouraged participants to adapt their speech patterns, creating a form of speech motor learning.

The team then used transcranial magnetic stimulation (TMS), a non-invasive method of brain stimulation, to temporarily disrupt activity in three key brain regions involved in speech: the auditory cortex, the somatosensory cortex, and the motor cortex.

Researchers evaluated retention of the newly learned speech patterns 24 hours later.

Their prediction was straightforward. If a particular brain region was essential for learning and storing speech-related memories, disrupting that area should reduce retention. If the region was not critical, retention should remain unchanged.

The results strongly supported the importance of sensory processing. When activity in either the auditory cortex or somatosensory cortex was disrupted, participants showed significantly poorer retention of the speech movements they had learned. In contrast, disrupting the motor cortex had little effect on retention.

"Our study challenges the assumption that new speech memories are solely reliant on changes in motor areas of the brain. Instead, it underscores the importance of changes in auditory and somatosensory brain areas in shaping how we learn to speak," said study co-author Nishant Rao, Associate Research Scientist at Yale University.

Brain Plasticity and Future Stroke Therapies

The research is part of a larger effort to understand how plasticity in the brain's sensory systems contributes to learning and long-term memory.

It also builds on previous studies by the same research group involving arm and hand movements. Those studies similarly found that disrupting sensory regions of the brain interfered with the ability to learn and retain new motor skills.

Future work will focus on identifying the specific cortical circuits involved in learning and investigating sensory-based treatments for movement disorders. The researchers are particularly interested in applications for stroke rehabilitation and speech recovery.

About the Study

The study, "Sensory Basis of Speech Motor Learning and Memory," by Nishan Rao, Rosalie Gendron, Timothy Manning and David Ostry, was published in Proceedings of the National Academy of Sciences of the United States of America.

The research was funded by the (U.S.) National Institute on Deafness and Other Communication Disorders.