Why Your Brain’s Speech Center Shuts Off When You Talk
The brain’s language circuits are even more complex than were previously thought. New findings show evidence that Broca’s Area in fact switches off during the time we talk out loud.
In the early 1860’s, French physician Pierre Paul Broca discovered that speech-impaired stroke patients of his all had damage in the same brain region. Broca’s Area, as the region became known, has since been recognized as the command center for human speech, including vocalization.
People with Broca’s aphasia are characterized as having suffered damage to the brain’s frontal lobe and tend to speak in short, stilted phrases that often omit short connecting words such as “the” and “and.”
The new findings from researchers at UC Berkeley and Johns Hopkins University paint a more multifaceted picture than previously thought of the frontal brain regions associated with speech production. The implications for diagnoses and treatments of stroke, epilepsy and brain injuries that result in language impairments are significant.
Adeen Flinker, lead author and postdoctoral researcher at New York University said of the study:
“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. Our results could help us advance language mapping during neurosurgery as well as the assessment of language impairments.”
Broca’s Area and Speech Production
According to the classical neurobiological language models, damage to Broca’s Area is associated with the inability to form speech, or Broca’s aphasia. Damage to Wernicke’s Area, discovered by German physician Carl Wernicke, is associated with inability to understand spoken language.
That belief drives how we map out language during neurosurgery and classify language impairments. This new finding helps us move towards a less dichotomous view where Broca’s area is not a center for speech production, but rather a critical area for integrating and coordinating information across other brain regions.
Multiple Complex Language Circuits
In 2007, researchers at the University of California, Davis, re-examined the preserved brains of Broca’s patients using state of the art MRI scanning. What they saw was surprising.
The scans showed that the greatest damage wasn’t in the brain region known as Broca’s Area. Rather, it was in an area just in front of it.
The damage was not restricted to the surface of the brain, as was noted by Broca in his reports, but extended deep into the brain. Other brain scan studies have shown that stroke patients with Broca’s aphasia sometimes show damage to the basal ganglia or white matter underlying the frontal lobe, as well as a brain structure called the insula.
The emerging picture is that our brains have multiple complex systems dedicated to language functions.
Event-related Causality Technology
In their research, Flinker and his team have found that Broca’s area, which is located in the frontal cortex above and behind the left eye, does engage with the brain’s temporal cortex, which organizes sensory input. It also engages later with the motor cortex, as we process language and plan which sounds and movements of the mouth to use, and in what order.
But, 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.
The study involved seven hospitalized epilepsy patients. as they repeated spoken and written words aloud. Electrical signals emitted from the brains were tracked and followed using event-related causality technology.
The signals were tracked 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.
Adeen Flinker, Anna Korzeniewska, Avgusta Y. Shestyuk, Piotr J. Franaszczuk, Nina F. Dronkers, Robert T. Knight, Nathan E. Crone. Redefining the role of Broca’s area in speech. PNAS, 2015; 201414491 DOI:10.1073/pnas.1414491112