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Wired for sound: How the brain senses visual illusions
April 12, 2007In a study that could help reveal how illusions are produced in the brain's visual cortex, researchers at the 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.
University of California-San Diego
"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.
University of California-San Diego
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Can we 'learn to see?': Study shows perception of invisible stimuli improves with training
Although we assume we can see everything in our field of vision, the brain actually picks and chooses the stimuli that come into our consciousness.
Scans show learning 'sculpts' the brain's connections
Spontaneous brain activity formerly thought to be "white noise" measurably changes after a person learns a new task, researchers at Washington University School of Medicine in St. Louis and the University of Chieti, Italy, have shown.
Babies see it coming
Do infants only start to crawl once they are physically able to see danger coming? Or is it that because they are more mobile, they develop the ability to sense looming danger?
Perceptual learning relies on local motion signals to learn global motion
Researchers have long known of the brain's ability to learn based on visual motion input, and a recent study has uncovered more insight into where the learning occurs.
Study Finds Needle Biopsies Safe in 'Eloquent' Areas of Brain
After a review of 284 cases, specialists at the Brain Tumor Center at the University of Cincinnati (UC) Neuroscience Institute have concluded that performing a stereotactic needle biopsy in an area of the brain associated with language or other important functions carries no greater risk than a similar biopsy in a less critical area of the brain.
MIT: Long-distance brain waves focus attention
Just as our world buzzes with distractions - from phone calls to e-mails to tweets - the neurons in our brain are bombarded with messages.
'Singing brains' offers epilepsy and schizophrenia clues
Studying the way a person's brain 'sings' could improve our understanding of conditions such as epilepsy and schizophrenia and help develop better treatments.
Preclinical work shows how one gene causes severe mental retardation
Researchers at Duke University Medical Center and the University of North Carolina have discovered in mice how a single disrupted gene can cause a form of severe mental retardation known as Angelman syndrome.
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