New Brain Mechanism Identified For Interpreting Speech
Editor: Don't ever think that interpreting speech is a simple task.
Scientists are constantly finding out more about this surprising complex
process. A recent discovery is described in this press release from the
University of Chicago.
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In conversation, humans recognize words primarily from the sounds they
hear. However, scientists have long known that what humans perceive goes
beyond the sounds and even the sights of speech. The brain actually
constructs its own unique interpretation, factoring in both the sights and
sounds of speech.
For example, when combining the acoustic patterns of speech with the
visual images of the speaker's mouth moving, humans sometimes reconstruct
a syllable that is not physically present in either sight or sound.
Although this illusion suggests spoken syllables are represented in the
brain in a way that is more abstract than the physical patterns of speech,
scientists haven't understood how the brain generates abstractions of this
sort.
In a study published in the December 20th issue of Neuron, researchers
at the University of Chicago identify brain areas responsible for this
perception. One of these areas, known as Broca's region, is typically
thought of as an area of the brain used for talking rather than listening.
"When the speech sounds do not correspond exactly to the words that are
mouthed, the brain often conjures a third sound as an experience and this
experience may often vary from what was actually spoken," explains Uri
Hasson, lead author of the study and a post-doctoral scholar at the
university's Human Neuroscience Laboratory.
"As an example, what would happen if a person's voice says 'pa,' but
the person's lips mouth the word 'ka?' One would think you might hear 'pa'
because that is what was said. But in fact, with the conflicting verbal
and visual signals, the brain is far more likely to hear 'ta,' an entirely
new sound," he explains.
This demonstration is called the McGurk effect (named after Harry
McGurk, a developmental psychologist from England who first noticed this
phenomenon in the 1970s). In the current study, scientists used functional
magnetic resonance imaging (graphic depiction of brain activity) to
demonstrate that Broca's region is responsible for the type of abstract
speech processing that underlies this effect.
Although we experience speech as a series of words like print on a
page, the speech signal is not as clear as print, and must be interpreted
rather than simply recognized, Hasson explains.
He says this paper provides a glimpse into how such interpretations are
carried out in the brain. These types of interpretations might be
particularly important, when the speech sounds are unclear, such as when
conversing in a crowded bar, listening to an unfamiliar accent, or coping
with hearing loss. "In all these cases, understanding what is said
requires interpreting the physical speech signal to determine what is
said. And scientists now know the Broca's region is plays a major role in
this process."
The National Institute of Mental Health supported this research
(#R01-DC03378). Additional authors include Jeremy Skipper, Howard Nusbaum
and Steven Small of the University of Chicago.
University of Chicago Medical Center
MC 6063, 5841 S. Maryland Ave.
Chicago, IL 60637
United States
http://www.uchospitals.edu
