A recent study reveals that learning a new language or recovering speech relies more on brain regions that process sounds and physical sensations than on those that govern motor control.
Conducted by research teams from McGill University and Yale School of Medicine, the study could contribute to evolving the theory of speech learning and to the development of speech processing and recognition technologies.
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Until now, it was thought that learning and memorizing the facial and mouth movements needed for speech depended on the brain's motor regions. The recent findings challenge this assumption and instead highlight the primary role of the auditory and somatosensory systems.
"Sensorimotor neuroscience has long placed frontal motor regions at the forefront of movement production. The study's results invite us to revise this concept, since they show that human speech learning relies largely on sensory mechanisms," explains David Ostry, professor of psychology at McGill University.
These findings could lead to the study of new approaches for developing brain-speech technologies, for example, to restore speech after a stroke; integrating sensory processes could improve the efficiency and user-friendliness of these technologies.
**Assessing retention through brain stimulation** To evaluate the role of the brain's sensory regions in learning and retaining speech movements, the researchers altered participants' speech in real time before feeding the modified signal back through headphones, thereby stimulating motor learning of speech.
They then used transcranial magnetic stimulation (TMS), a non-invasive brain stimulation technique, to disrupt neural activity in key speech-related areas: the auditory cortex, the somatosensory cortex, and the motor cortex. Retention was assessed 24 hours later.
The researchers hypothesized that if a brain region plays an essential role in acquiring and maintaining speech, disrupting it should impair retention of movements; otherwise, retention would remain intact.
They found that disrupting activity in the sensory cortex, whether auditory or somatosensory, significantly reduced participants' ability to retain new speech movements, while disrupting the motor cortex did not have this effect.
"Our results challenge the idea that new speech-related memories rely solely on changes in the brain's motor regions. Instead, we highlight the determining role of changes occurring in the brain's auditory and somatosensory areas for speech learning," notes Nishant Rao, co-author of the study and associate researcher at Yale University.
**The role of brain plasticity** The study is part of a broader research program on the role of plasticity in the brain's sensory systems in motor learning and memory. It follows recent work by the group on upper limb movements, which shows that disruption of the sensory cortex impairs learning and retention of new movements.
Future work will map the cortical brain circuits associated with learning and explore sensory interventions for treating movement disorders, particularly in the context of post-stroke rehabilitation.
**The study** The article "Sensory Basis of Speech Motor Learning and Memory," by Nishant Rao, Rosalie Gendron, Timothy Manning, and David Ostry, was published in Proceedings of the National Academy of Sciences of the United States of America.