Matching Sound Perception to Brain Activity
Scientists from Georgetown University and Massachusetts Institute of Technology (MIT) have devised a neurophysiological model that explains how a person can pick out individual voices in a noisy room -- a complex puzzle known among brain scientists as the "cocktail party problem.".
They say their model, reported in the October 6 issue of Neuron, may ultimately provide insights into common neurological disorders that involve focused hearing, such as attention-deficit disorder (ADD), and problems resulting from age-related hearing loss.
"Attention plays a big role in the perceptual effects we've been studying, so it will be interesting to see if this disorder [ADD] is related to discerning streams of auditory information within the brain," said the study's principal investigator, Josef P. Rauschecker, Ph.D., of the Department of Physiology and Biophysics and the Georgetown Institute for Cognitive and Computational Sciences.
The results also may help reveal how hearing declines later in life, Rauschecker said. "There are common deficits of auditory processing with aging, related to the 'cocktail party' problem, that are often attributed to a simple hearing loss but have more to do with the higher-order central processing areas in the brain which we studied," he said.
The study was conducted both at GUMC and at MIT's Research Laboratory of Electronics, in collaboration with the study's first author, Christophe Micheyl, Ph.D.
Their model helps expose the neural mechanisms of perception, and determine where in the brain these "percepts" are generated, said Rauschecker.
"The neural mechanisms underlying the perception of sound sequences, as they occur, for instance, in speech and music, are still poorly understood," he said. In fact, the issue is so complex that no computer program has yet been devised that can explain it, the scientists say.
In this study, the research team specifically sought to model the "source separation" that occurs when the brain organizes myriad sounds into separate auditory streams as a way to process them. They know, for example, that two sounds that have frequencies close to each other are perceived by the brain as a single sound, akin to the galloping hoofs of a horse. Frequencies that are far apart are regarded as two separate streams. But something interesting happens with two sounds that have intermediate frequencies ? the brain perceives them as one sound, the galloping horse, but then separates them into distinct sounds after several seconds. To understand why this happens, the scientists wanted to match what occurs in the brain to how a number of sounds the listener perceives.
So they turned to human participants, at MIT, and to monkey subjects, at GUMC. The same auditory stimuli were given to both groups, but the monkeys were wired with neural electrodes that recorded activity in their auditory cortex.
Using human perceptual responses, and monkey brain mapping, the scientists then devised a model that accurately predicted, given different frequencies and tones, if human listeners would report one or two auditory streams.
"We demonstrate a striking correspondence between neural responses in the primary auditory cortex of the monkeys, when listening to repeating tone sequences, and perceptual judgments on these same sequences by humans," said Rauschecker. "This means that neural habituation in the auditory cortex can explain the observed tendency of sound sequences to split perceptually into multiple streams after several seconds of uninterrupted listening.".
The study was funded by the National Institutes of Health (National Institute of Deafness and Other Communication Disorders) and the Engineering and Physical Sciences Research Council of the United Kingdom. Other co-authors include GUMC investigator Biao Tian, Ph.D.and Robert Carlyon, Ph.D., of the MRC Cognition and Brain Sciences Unit of Cambridge, England.
About Georgetown University Medical Center
Georgetown University Medical Center is an internationally recognized academic medical center with a three-part mission of research, teaching and patient care (through our partnership with MedStar Health). Our mission is carried out with a strong emphasis on public service and a dedication to the Catholic, Jesuit principle of cura personalis-or "care of the whole person." The Medical Center includes the School of Medicine and the School of Nursing and Health Studies, both nationally ranked, and the world renowned Lombardi Comprehensive Cancer Center. For more information, go to http://gumc.georgetown.edu.