Study reveals new understanding of the brain regions used when learning to speak

Learning a new language — or learning to speak again after a stroke — involves a fine-tuned set of movements requiring precise coordination in brain networks. This includes the orofacial sensory system (input like touch and position from the lips, tongue, jaw, and face) and motor system (commands that move the muscles in the right way at the right time).

New study findings from researchers at Yale School of Medicine challenge a long-held assumption in neuroscience: that speech motor learning and the memory of newly learned speech movements are fundamentally driven by motor regions of the brain. Instead, the study findings indicate that retention of newly learned speech movements relies chiefly on sensory brain processes.

The work has implications for rehabilitation and emerging neurological technologies, pointing to the sensory cortex as a potential neural target for rehabilitation after a stroke or brain injury affecting speech. The findings may also inform brain-computer interfaces, by highlighting the relevance of sensory cortical activity for movement control. The results also suggest that speech-processing and -recognition technologies could improve by more explicitly integrating auditory and somatic sensory signals.

The study was led by Nishant Rao, an associate research scientist at Yale Child Study Center. “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,” Rao said.

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

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

The researchers assessed retention of learning 24 hours later. Disrupting activity in the sensory cortex  — auditory or somatosensory — significantly impaired participants’ ability to retain newly learned speech changes, while disrupting activity in the motor cortex did not.

“Sensorimotor neuroscience has traditionally focused on frontal motor areas as the principal drivers of movement,” David Ostry, a professor adjunct in the Child Study Center and senior author of the findings, said. “This study changes that understanding by showing that human motor learning is extensively sensory in nature.”