Exploring Hearing

Dennis Freeman’s an expert on the ear’s sound-processing machinery, and one of the factors that drew him to the field was a desire to improve hearing aids.

Freeman, an assistant professor of electrical engineering, believes much of the stigma associated with hearing aids would disappear if they worked better.

“There’s no stigma attached to eyeglasses,” he notes. “I think that’s because eyeglasses improve your vision in almost all circumstances.” Hearing aids, by contrast, often work well in some environments but are virtually useless in others–cocktail parties, for example.

Freeman started educating himself about hearing not long after he arrived to do graduate work at MIT with a freshly minted SB degree from Pennsylvania State University. He first began probing how ears localize sounds, revealing crucial information like the direction from which a car is approaching. That interest evolved into a concern with aiding speech reception.

One approach he and associates tried involved a device that lowered pitches artificially–a strategy aimed at compensating for the fact that sensitivity to high-pitched sounds tends to fade first. “It made everyone sound like the Jolly Green Giant,” says Freeman, “but it didn’t help people who were hard of hearing.”

Freeman thought he could figure out why by studying the ear. “It was supposed to be a summer diversion,” he says. “Then I found out that nobody really knows how hearing works.”

The Melody Stays the Same

So began a quest that has large potential payoffs. Roughly 28 million Americans are hearing impaired, with the elderly being most at risk. Moreover, many with hearing loss pay a high penalty in terms of personal isolation, difficulties in the workplace, and threats to personal safety.

If the potential benefits are great, though, so is the challenge of making sense of hearing.

Our auditory system vaguely resembles a micro-sound studio: A source makes a sound, the ear drum vibrates, three tiny, interconnected bones in the middle ear transmit these motions to the inner ear–the structure that harbors our sound-sending hair cells–these cells convert their own motions into electrical signals, neural cells convey these signals to the brain.

Engineers, though, can explain how sound in a studio gets converted into electrical signals, and why these signals yield the output they do. They can’t tell us how the ear accomplishes the task, says Freeman, a member of the Research Laboratory of Electronics and the Harvard/MIT Division of Health Sciences and Technology (HST).

A core mystery is how our hearing apparatus and associated brain regions translate sound signals into the experience of hearing. If you turn up the volume on your stereo while listening to music, the notes sound louder but the melody doesn’t change. Studies of neural signals from the ear, however, suggest that we should experience a drastic change in the melody, with many new notes added and others eliminated.

“The neural signals look totally different,” notes Freeman, “yet the only perceived change is the loudness.”

The Choreography of Hair Cells

Freeman’s group has created a sophisticated system to try to crack what he calls the neural code. Using images of inner-ear hair cells taken with a high-powered microscope, they create digital movies of such cells in motion.

The movies, which show the miniscule cells from above, reveal what appear to be blow-ups of a flower’s center–actually, they’re collections of individual sensory hairs–rocking back and forth ever so slightly. Although hardly exciting by Hollywood standards, the films are in fact a technological tour de force: the movements recorded cover as little as 5-billionths of a meter, or about one one-hundredth the diamater of a sensory hair.

Analysis of the videos has led to significant discoveries about hearing. For example, it used to be thought that all the strands in the tufts of hair atop inner-ear hair cells sway in concert, like clumps of seaweed under the waves. In fact, they move semi-independently.

“There’s kind of an accordion effect,” notes Freeman. “We want to see what significance that has in terms of the cells’ responses to different sounds.”

It probably wouldn’t be wise to bet against his succeeding. “Denny’s one of the best biomedical engineers I know,” says his colleague Martha Gray, professor of electrical engineering and acting co-director of the Harvard/MIT Health Sciences and Technology Division. “He took a very difficult problem, and by bringing tools from various fields together showed how you can image these movements that have everything to do with how we hear.”

For the longer term, Freeman’s aim is to probe the relationships between movements of the inner ear’s sound-sensing hairs and the neural code for sound. The ultimate goal would be to foster advances like truly effective hearing aids.

“The neural code is so unusual,” says Freeman, “that it’s a wonder we can make sense of anything we hear. I’m hoping our studies will help us comprehend how this complex system works.”

(Some of the Freeman group’s images are available on the Web.)

by Richard Anthony

On Topic: electrical engineering+electronics, faculty, health science+technology