A brain area, long recognised as the command centre for human speech, actually shuts down when we talk, a new study has found.
Broca's area of the brain has been recognised as the command centre for human speech, including vocalisation.
Scientists at University of California, Berkeley and Johns Hopkins University in Maryland have now challenged this long-held assumption after they found that Broca's area actually switches off when we talk out loud.
The discovery has major implications for the diagnoses and treatments of stroke, epilepsy and brain injuries that result in language impairments.
"Every year millions of people suffer from stroke, some of which can lead to severe impairments in perceiving and producing language when critical brain areas are damaged," said study lead author Adeen Flinker, a postdoctoral researcher at New York University who conducted the study as a UC Berkeley PhD student.
"Our results could help us advance language mapping during neurosurgery as well as the assessment of language impairments," Flinker said.
Flinker said that neuroscientists traditionally organised the brain's language centre into two main regions: one for perceiving speech and one for producing speech.
"That belief drives how we map out language during neurosurgery and classify language impairments," he said.
"This new finding helps us move towards a less dichotomous view where Broca's area is not a centre for speech production, but rather a critical area for integrating and coordinating information across other brain regions," he said.
Flinker and fellow researchers found that Broca's area - which is located in the frontal cortex above and behind the left eye - engages with the brain's temporal cortex, which organises sensory input, and later the motor cortex, as we process language and plan which sounds and movements of the mouth to use, and in what order.
However, the study found, it disengages when we actually start to utter word sequences.
"Broca's area shuts down during the actual delivery of speech, but it may remain active during conversation as part of planning future words and full sentences," Flinker said.
The study tracked electrical signals emitted from the brains of seven hospitalised epilepsy patients as they repeated spoken and written words aloud.
Researchers followed that brain activity - using event-related causality technology - from the auditory cortex, where the patients processed the words they heard, to Broca's area, where they prepared to articulate the words to repeat, to the motor cortex, where they finally spoke the words out loud.
The findings are published in the Proceedings of the National Academy of Sciences.
