A team of Harvard and University of California,
San Diego (UCSD), researchers report having pinpointed an area of the brain
where three essential components of language — word identification, grammar,
and word pronunciation — are processed.
Ned T. Sahin, a postdoctoral fellow in both
Harvard's Psychology Department and UCSD’s Radiology
Department, and colleagues at both schools, have used a technique called
intracranial electrophysiology (ICE) to gain access, with unprecedented
precision, to Broca's area, a region of the cerebral cortex
long-suspected to be the seat of language.
This sandwiched image of two X-rays shows electrodes that surgeons use to find and remove the source of seizures (to cure epilepsy), while sparing the source of mental functions like language. Composite illustration Ned T. Sahin/Department of Psychology
Their results are reported in the journal Science.
In the ICE method, “direct recordings of brain signals are
made using electrodes placed inside the human brain, allowing extremely high
spatial, temporal, and physiological resolution, concurrently,” says Sahin. For
years, neurobiologists had lacked the techniques to pinpoint the areas of the
brain responsible for language processing.
Because there are no animal models for human language
processing and because it would be ethically unacceptable to insert electrodes
into the brains of healthy human subjects, scientists have up to now been
stymied in their attempts to locate the precise area of the brain responsible
for processing the various components of language.
Sahin and his colleagues have done their ICE studies using
patient volunteers who were undergoing neurosurgery for epilepsy.
"What captures my interest is being able to record what
sets of brain cells are saying to each other while the person [in whose brain
the cells reside] is saying something to me. It's amazing," Sahin says. He
points out, however, that it is not yet possible to interpret what the brain
cells are saying. Sahin likens the difficulty to that of a French-speaking
child listening to a conversation in Greek. “He knows something important is
being communicated, but he just can't unlock it yet."
Sahin and his team listened to signals from the "forest
of neurons" required to compute language as they came "yelling and
screaming down" electrodes that passed through or near Broca’s area of the
brain. Each electrode recorded input from a collection of about 10,000 cells.
Careful analysis of the brain signals allows the researchers to determine which
signals come from the local area as opposed to echoes from signals originating
elsewhere.
With the electrodes in place, the researchers asked patients
to read a word on a laptop computer screen while they are in their hospital
beds and connected to the recording computers. Words like “come” are first
read, then used to complete a sentence like “Yesterday they _____.” In this
case, the participants would need to supply the word “came.” "Subjects do
not speak out loud, but rather think the word to themselves, then press a
button to signal completion," Sahin says.
Sahin’s task requires the patients to recognize both
frequent and infrequent words, to find the appropriate tense of a verb or
plural/singular form of a noun, and to change or retain the pronunciation of
the base form of the word (as presented). Patients repeat the exercises in
different combinations. "If the word is ‘house,’ for instance, and the
sentence requires the plural form, a patient's brain must compute the abstract
concept we call plural, and additionally must prepare and articulate the added
syllable for the ‘-es’ ending," Sahin explains. The idea, he says, is to
get a person to either repeat the words they see verbatim, or make a
transformation to use them properly in context.
Within a fifth of a second, information about the word's
identity arrives at the patient's Broca's area, Sahin and colleagues reported.
The experiment has yielded evidence that language processing
takes place in a small part of Broca's area as a sequence of three different
stages, each ending before the next one begins. All three are necessary in
order to complete a simple task such as looking at a word and uttering it,
though there may be other stages beyond these three. "We found a tightly
timed sequence of brain activity associated with aspects of identifying the
word, grammatically transforming it into the right form for the context, and
then preparing to pronounce the final sound form," Sahin explains.
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