Princeton Weekly Bulletin March 1, 1999
How brain senses sounds
By Steven Schultz
Research staff member Michael Graziano, working in the lab of psychology professor Charles Gross, has identified a portion of the brain that controls our finely tuned ability to judge the distance of sounds that are very close to our heads.
The research offers an insight into the complex processes that connect seeing, hearing and touching with doing -- for example, how a person hears a sound and then ducks his head to avoid a nearby obstacle.
Graziano's research focused on an area of the brain called the premotor cortex, which plays a role in perception as well as in preparing the commands that result in physical movement; it is a region that links input with output. He and his collaborators discovered that some cells, or neurons, in this area are involved with three functions at once: seeing, touching and hearing. And some of these "trimodal" neurons have the uncanny ability to respond according to how far the ear is from the source of the sound, regardless of how loud or soft the sound is.
Until recently, scientists could observe only the sensory cues that go into the brain and the behavior that comes out; the process in between had been a mystery, a "black box," said Graziano, who is lead author of the paper "A neuronal representation of the location of nearby sounds" in the February 4 issue of Nature. In the last decade, however, new brain imaging techniques have led to precise mapping of the neurons that process sensory inputs. Most of that work has been in studying vision. Some research has been done on hearing, but most of that has focused on how we perceive the direction of sounds, a relatively simple process having to do with sound reaching one ear sooner than the other. How we perceive the distance of sounds has baffled scientists.
Senior thesis project
Graziano and Lina Reiss '97, who wrote her senior thesis on this project, tackled the problem by studying neurons in the premotor cortex of macaque monkeys. The scientists found that certain neurons respond only to sounds within 30 cm. of the head. Each of these distance-sensing neurons was responsible for perceiving sounds that come from one very specific direction relative to the animal's head. One cell may respond to sounds directly in front of the nose, while another may be triggered by a sound a few inches off to one side.
The study did not reveal what cues tip these cells off to the distance of a sound. Graziano suspects that it may have something to do with subtle variations in the timbre of a sound; a distant sound has a complex quality that comes from tiny echoes and distortions, while a nearby noise is more clean and crisp.
Stroke treatment
The Princeton study adds to a view that the premotor cortex has the critical function of guiding a person's movements toward and around objects close to the body.
Understanding this ability could lead to applications for Graziano's work in diverse areas. Stroke victims, for example, often lose their ability to judge the proximity of objects. These patients may see, hear or touch an object, but if it is in the "bad" or "neglected" part of space near the body, the patient ignores or fails to react to it. Knowing how neurons in the premotor cortex evaluate nearby objects could lead to better ways to diagnose and treat stroke.
The question may also be important in making artificial limbs that can closely mimic the function of natural ones, or in making better hearing or visual aids that create more life-like perceptions.
Graziano's research is currently funded by the National Institutes of Health and the McDonnell-Pew Program in Cognitive Neuroscience. NASA also supported some of the work, he said, because scientists who design robots want to know how a computer like the brain translates inputs to outputs.