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Sent: Thursday, April 12, 2007 11:17
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Subject: Wired for Sound: How the Brain Senses Visual Illusions

The following news release and any accompanying images can be accessed on the
web at http://ucsdnews.ucsd.edu/newsrel/health/04-07Wired.asp


Media contact: 
Debra Kain
619-543-6163
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Wired for Sound: How the Brain Senses Visual Illusions

In a study that could help reveal how illusions are produced in the
brain's visual cortex, researchers at the University of California, San
Diego (UCSD) School of Medicine have found new evidence of rapid
integration of auditory and visual sensations in the brain.  Their
findings, which provide new insight into neural mechanisms by which
visual perception can be altered by concurrent auditory events, will be
published online in the April 12 edition of the Journal of Neuroscience.


When subjects were shown a single flash of light interposed between two
brief sounds, many subjects reported seeing two distinct flashes of
light.  Investigating the timing and location of the brain processes
that underlie this illusory effect - the illusion of seeing two flashes
in the presence of two auditory signals, when only one flash actually
occurs - can reveal how information from different senses are integrated
in the brain.  

The study of 34 subjects was carried out in the laboratory of Steven A.
Hillyard, Ph.D., UCSD professor of neurosciences.  "This type of
perceptual illusion has been described before," said first author Jyoti
Mishra, graduate student in the Hillyard lab.   "The surprising finding
we made is that the illusion depends on a rapidly timed sequence of
interactions between the auditory and visual cortical areas." 

"This is part of a set of new findings by scientists in the field that
show how integration of multiple sensations can happen much more rapidly
than we thought before," said Mishra.  "We show physiological evidence
that visual and auditory stimulation might not be processed separately,
then merged together, as previously assumed, but that an
almost-simultaneous integration of the sensations may actually take
place in the brain."

The UCSD scientists measured event-related potentials (ERPs), brain
responses that are directly related to the perceptual experiences
induced by sensory stimuli, using an electrophysiological or EEG
recording procedure that measures electrical activity of the brain
through the skull.  

"In subjects who reported seeing a second flash, the ERP measurements
showed a boost of activity within the visual cortex of the brain
immediately after hearing the second sound," said Mishra, adding that
the second sound amplified the brain activity stimulated by the first
sound.  Perception of the second illusory flash was also marked by a
rapid enhancement of processing in the auditory cortex of the brain.  By
observing the auditory boost, the researchers could predict when
subjects would report seeing the visual illusion of a second flash.  

"Our results provide evidence that perception of the illusory second
flash is based on a very rapid and dynamic interplay between the
auditory and visual cortices of the brain - on a time scale less than
one tenth the blink of an eye." Mishra said.  Interestingly, the pattern
was very different between individuals who did or didn't see the second
flash, indicating that the brain's wiring and the strength of
integration between the different sensory cortices may differ between
individuals, or even vary over time.  "It suggests that there are
consistent differences in the neural connectivity that are possibly
shaped during one's development and through experience," she said.

Next, the researchers plan to look at whether or not attention affects
these illusory sensations. These studies could shed light on how people
deprived of one sensation often compensate by developing another - for
instance, blind people with a more acute sense of hearing. 

Additional contributors to the study include Antigona Martinez, UCSD
Department of Neurosciences and Nathan S. Kline institute for
Psychiatric Research, and Terrence J. Sejnowski, UCSD Department of
Neurosciences, the Howard Hughes Medical Institution and Computational
Neurobiology Laboratory at the Salk Institute.

UCSD news on the web at http://ucsdnews.ucsd.edu.
                                                  

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