Berkeley University. US: For 150 years, the iconic Broca’s area of the brain has been
recognized as the command center for human speech, including
vocalization. Now, scientists at UC Berkeley and Johns Hopkins
University in Maryland are challenging this long-held assumption with
new evidence that Broca’s area actually switches off when we talk out
loud.
The findings, reported today (Feb. 16) in the Proceedings of the National Academy of Sciences
journal, provide a more complex picture than previously thought of the
frontal brain regions involved in speech production. 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 Ph.D. student. “Our results could help us advance
language mapping during neurosurgery as well as the assessment of
language impairments.”
Flinker said that neuroscientists traditionally organized the brain’s
language center 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 center for
speech production, but rather a critical area for integrating and
coordinating information across other brain regions.”
In the 1860s, French physician Pierre Paul Broca pinpointed this
prefrontal brain region as the seat of speech. Broca’s area has since
ranked among the brain’s most closely examined language regions in
cognitive psychology. 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.”
Specifically, Flinker and fellow researchers have 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 organizes 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
hospitalized 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.
In addition to Flinker, other co-authors and researchers on the study
are Robert Knight and Avgusta Shestyuk at the Helen Wills Neuroscience
Institute at UC Berkeley, Nina Dronkers at the Center for Aphasia and
Related Disorders at the Veterans Affairs Northern California Health
Care System, and Anna Korzeniewska, Piotr Franaszczuk and Nathan Crone
at Johns Hopkins School of Medicine.