How Do We Hear While We Sleep?
Using electrodes implanted directly on the human cortex, a Johns Hopkins University undergraduate has located the part of the brain that appears to process sounds while people sleep. This site, in the frontal lobe, may be part of a vigilance system that, for instance, rouses a mother when her baby cries but lets the woman sleep when a truck rumbles by.
Serena J. Gondek, a 21-year-old junior majoring in biomedical engineering, is slated to present her findings on Tuesday, April 28, at the annual meeting of the American Academy of Neurology in Minneapolis, Minn. About 8,000 people are expected to attend the world's largest gathering of neurologists and neuroscience professionals. Administrators at the academy said it is unusual for an undergraduate to be chosen to make an oral research presentation at the event.
Previous studies on hearing during sleep have relied on electrodes attached to a subject's shaved scalp. Gondek's experiment is believed to be the first of its type to use electrodes implanted directly on the brain, a technique that yields far more precise information about which parts are activated by sounds during sleep.
Gondek is from the Chicago suburb of Oak Brook, Ill. She conducted her experiment under the supervision of Gregory L. Krauss, an assistant professor of neurology at the Johns Hopkins School of Medicine. Krauss is co-author of the paper, titled, "How do we hear while we sleep?"
"It is controversial how we monitor our environment while we sleep," says Krauss. "It's a pretty big part of our lives, but sleep is poorly understood. The main thing that Serena did was to show where on the cortex we hear while we sleep. She did a terrific job, particularly in the brain mapping."
Gondek is the second Hopkins undergraduate in recent months to present research findings at a major medical conference in collaboration with Krauss. "Undergraduates are great for these kind of projects because they're very enthusiastic," says Krauss, "and they work very hard."
Hopkins professors often encourage undergraduates to take part in high-level scientific studies. "That's why I really looked forward to coming to Hopkins," says Gondek, "It's been as easy as knocking on a few doors to become involved in research projects, even during my freshman year."
For her experiment, Gondek obtained the cooperation of five patients who were about to undergo brain surgery to curtail epileptic seizures. To find the focal points of these seizures, a surgeon had cut open the patients' skulls and implanted electrode grids directly on their brains.
Prior to their surgery, Gondek placed special plugs in the patients' ears. The plugs blocked out room noise but allowed the patients to hear sequences of two tones emitted by Gondek's equipment, one tone pitched at 500 hertz, the other at 1,000.
She played various tone patterns while the patients were awake, during light sleep and during deep sleep. The electrodes detected which portions of the brain were activated. Gondek analyzed the results and mapped them onto MRI and CT scans made of the patients' brains.
"We found that during waking, only areas around primary auditory cortex are activated by the tones," she says. "Then, during light and deep sleep, you find not only primary auditory activation, but the frontal lobe also responds."
The frontal lobe is believed to play a key role in vigilance functions, such as screening new stimuli and preparing the body to react. During sleep, Gondek speculates, this part of the brain may analyze sounds to decide whether the person needs to be awakened to respond. This mechanism would allow a camper in the woods to sleep through non-threatening cricket chirps. But it might awaken the camper quickly to the growl of a bear.
Gondek and Krauss are planning follow-up experiments to learn more about how the brain processes specific environmental sounds during sleep. Their research may also shed light on connections between sleep disorders and mental illnesses such as depression and dementia.
Finding the place where the brain processes sounds during sleep has been a critical first step to cracking a tough physiological puzzle. "At this point," Gondek explains, "we can say that this is where it's happening. But it's still very unclear what's going on there, how this processing works."
Her project was supported by a General Electric Foundation Undergraduate Engineering Research Stipend.
Photos by Doug Hansen, Homewood Photographic Services
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