"We have tens of millions of neurons in our auditory cortex, and we've looked at how they might allow us to distinguish different pitches," says lead author Mark Jude Tramo, MD, PhD, a neurologist at Massachusetts General Hospital. Past research has shown that a particular neuron in the brain will only respond to a few, distinct notes or tones. How all of the neurons coordinate together has been somewhat of a mystery. "Now we know that there's no question that you need the neurons in the auditory cortex for fine-tuned pitch discrimination," says Tramo.
There are differences in the sounds of two voices or two musical instruments even if they hit the same note, and somehow the brain knows that. In this study, Tramo and his colleagues looked at how people perceive these differences in pitch. "Something in the auditory cortex allows us to decipher all of this so easily," says Tramo, who also is on staff at Massachusetts Eye and Ear Infirmary.
The research team studied five patients with neurological abnormalities in different parts of the brain, ranging from strokes to head trauma, and they compared the patients' brain activity and pitch perception to that of normal volunteers. Through their battery of tests, the researchers found that particular subdivisions of the auditory cortex make essential contributions to identifying fine distinctions in pitch.
While the current study looked at pure tones, each of which corresponds to one note, Tramo is now looking into how the auditory cortex deals with complex tones, such as those in speech and musical compositions. "We'd like to figure out how the brain computes all of the complex information it receives when a musician plays an elaborate song," he says. "Subtle manipulations go a long way in forming the nuances that are perceived by the listener."
The study's co-authors are Gaurav Shah, MD, of MGH Neurology and the Department of Electrical Engineering and Computer Science at Massachusetts Institute of Technology, and Louis Braida, PhD, also of MIT. Support for the study was provided by the National Institute on Deafness and Other Communication Disorders, the McDonnell-Pew Program in Cognitive Neuroscience, and the Henry Ellis Warren Fund at MIT.
The Massachusetts General Hospital, established in 1811, is the original and largest teaching hospital of Harvard Medical School. The MGH conducts the largest hospital-based research program in the United States, with an annual research budget of more than $300 million and major research centers in AIDS, the neurosciences, cardiovascular research, cancer, cutaneous biology, transplantation biology and photomedicine. In 1994, the MGH joined with Brigham and Women's Hospital to form Partners HealthCare System, an integrated health care delivery system comprising the two academic medical centers, specialty and community hospitals, a network of physician groups and nonacute and home health services.
Journal
Journal of Neurophysiology