Speech learning may depend more on sensory brain regions, study finds

Speech learning may rely more on how the brain processes sound and physical sensation than on the areas that command mouth movement, according to new research from McGill University and the Yale School of Medicine. The finding could affect work on speech rehabilitation after stroke and future brain-based communication systems, McGill said.

The study examined how people learn and keep new speech patterns. Researchers reported that disrupting sensory regions of the brain weakened retention of newly learned speech movements, while disrupting a motor region had little effect.

The work was published in the Proceedings of the National Academy of Sciences under the title “Sensory basis of speech motor learning and memory.” The authors are Nishant Rao, Rosalie Gendron, Timothy F. Manning and David J. Ostry, according to the journal reference released by McGill.

How the experiment tested speech learning

For many years, researchers have treated motor areas in the frontal part of the brain as central to learning the movement patterns needed for speech, McGill said. Those areas help control the face, mouth and vocal tract.

To test that view, the research team changed participants’ speech in real time and fed the altered sound back to them through headphones. McGill said that setup prompted participants to adjust how they spoke, creating a test of speech motor learning.

The researchers then used transcranial magnetic stimulation, a noninvasive brain stimulation method, to temporarily interfere with activity in three areas involved in speech: the auditory cortex, the somatosensory cortex and the motor cortex. They checked 24 hours later to see how well participants retained the speech patterns they had learned.

The logic of the test was direct, according to McGill. If a brain area was needed to form or store the new speech memory, interfering with that area should reduce retention; if it was less central, retention should be largely unchanged.

Sensory regions showed the stronger effect

The results pointed to the auditory and somatosensory systems, McGill said. Participants retained less of the learned speech pattern after stimulation disrupted either the auditory cortex, which processes sound, or the somatosensory cortex, which processes physical sensation.

By contrast, disrupting the motor cortex had little effect on retention, according to the researchers. McGill psychology professor David Ostry said the findings challenge the traditional emphasis in sensorimotor neuroscience on frontal motor regions as the main drivers of movement, and indicate that speech learning is strongly sensory.

Study co-author Nishant Rao, an associate research scientist at Yale University, said the results challenge the idea that new speech memories depend only on changes in motor areas. He said the study highlights the role of auditory and somatosensory areas in learning to speak, according to McGill.

Possible uses in rehabilitation and technology

McGill said the study is part of broader work on how plasticity in sensory systems supports learning and long-term memory. The same research group previously reported similar findings in studies of arm and hand movement, where interfering with sensory brain regions reduced learning and retention of new motor skills.

The researchers said future work will look for the specific cortical circuits involved in learning and examine sensory-based treatments for movement disorders. McGill said the findings may be relevant to stroke rehabilitation, speech recovery and the design of brain-speech technologies intended to help restore communication.

The research was funded by the U.S. National Institute on Deafness and Other Communication Disorders, according to McGill.

This story draws on original reporting from ScienceDaily.