New Insight into the Brain Regions Used When Learning to Speak May Inform Rehab After Stroke or Brain Injury

Learning a new language or relearning speech after a stroke requires coordinated movements that are controlled by networks in the brain. This includes the orofacial sensory system (input such as touch and position from the lips, tongue, jaw, and face) and the motor system (commands that move the muscles in the correct way at the correct time).

Researchers at Yale School of Medicine have found evidence that challenges the assumption that speech motor learning and the memory of newly learned speech movements are primarily driven by motor regions of the brain. The study, published in the Proceedings of the National Academy of Sciences, was led by Yale Child Study Center faculty members Nishant Rao, PhD, associate research scientist, and David Ostry, PhD, professor adjunct.

Study findings indicate that retaining newly learned speech movements depends primarily on sensory brain processes, rather than motor learning. Results suggest that speech-processing and recognition technologies could improve by more explicitly integrating auditory and somatic sensory signals.

“These findings establish a sensory basis for speech motor memory, indicating that plasticity in sensory brain areas is necessary for learning and retaining newly acquired speech movements,” says Rao.

The work has implications for rehabilitation and new neurological technologies, suggesting the sensory cortex as a possible target for rehabilitation after a stroke or brain injury that affects speech. The findings may also help improve brain-computer interfaces by showing the importance of sensory cortical activity for movement control.

“Our study challenges the assumption that new speech memories are solely reliant on changes in motor areas of the brain,” says Rao. “Instead, it underscores the importance of changes in auditory and somatosensory brain areas in shaping how we learn to speak.”

Rao, Ostry, and their team used an experimental model in which participants’ speech was changed in real time and played back to them through headphones, leading to speech motor learning. They applied transcranial magnetic stimulation (TMS), a noninvasive technique that can temporarily disrupt neural activity, to one of three important speech-related regions: the auditory cortex, the somatosensory cortex, or the motor cortex.

The researchers checked how well participants remembered their learning 24 hours later. Disrupting activity in the sensory cortex, either auditory or somatosensory, made it harder for participants to keep new speech changes, but disrupting the motor cortex did not have this effect.

“Sensorimotor neuroscience has traditionally focused on frontal motor areas as the principle drivers of movement,” says Ostry. “This study changes that understanding by showing that human motor learning is extensively sensory in nature.”