Jack Freeman worked for four decades in a noisy brick-making factory, but for years his wife found it hard to believe that he had a hearing loss. He would often stay up late watching TV–always with the volume turned low. “How can he claim not to hear me, too, when I come in to talk to him?” she would ask.
The Freemans’ son, Dennis, SM ‘76, PhD ‘86, an MIT professor of electrical engineering, has been studying the inner ear for more than 30 years. But only recently has he gotten to the bottom of his mother’s question. Freeman’s lab, in the Research Laboratory of Electronics’ Auditory Physiology Group, has made a fundamental discovery about the inner ear, one that helps explain why Freeman’s father has trouble with sounds from different sources.
Scientists have long known that people lose the ability to discriminate between sounds when exposure to excessive noise damages the delicate structures of the inner ear. (The problem can also be congenital.) But they have yet to uncover why the inner ear is normally such an extraordinary sensor–allowing us to hear everything from a low whisper to the roar of a jet engine, and to distinguish up to 30 tones between the frequencies of adjacent keys on a piano.
These remarkable abilities are thought to arise from cochlear amplification, a process by which the inner ear’s response to sounds is amplified as much as a thousandfold by the collective action of 12,000 sensory receptor cells. Many researchers have studied how individual sensory cells–particularly those known as outer hair cells–work to magnify sounds, either making them loud enough to hear or enabling detection of minute changes in frequency. But scientists are just beginning to understand how different parts of the ear interact with those hair cells.
“There are 12,000 sensory cells in each ear, and they’re talking to each other in a feedback system,” Freeman says. “And that system is what we’re trying to understand. “
Freeman’s interest is personal as well as academic: when he got rheumatic fever in fourth grade, the streptomycin used to treat it weakened his hearing. Then, after his freshman year at Penn State, his hearing was further damaged by a summer job in the same thundering factory where his father worked. Even so, Freeman didn’t come to MIT in the 1970s to study the ear. He came to build computers. Then he met Professor Campbell Searle–author of his first circuitry textbook–and realized that he could apply electrical engineering to the study of hearing. Freeman worked with Searle and others to try to develop hearing aids that made speech sounds easier to understand by using signal processing to do some of the ear’s work for it. But that approach, Freeman says, “just didn’t work.”
By the early 1980s, Freeman had concluded that existing models of the ear were incomplete. So instead of trying to build a better hearing aid using those models, he embarked on a crash course in neurophysiology and cell physiology, so he could do his doctoral research on cochlear hydrodynamics. Over the last two decades, Freeman has refined his models to reflect new evidence, such as the discovery, by William Brownell of the Baylor College of Medicine, that sensory receptor cells act as mechanical amplifiers, actually generating motion in inner-ear structures in response to sound instead of simply reporting sound-induced motions to the brain.
Now Freeman’s lab has uncovered a key role played by a little-understood part of the inner ear. Using a clever experimental setup designed by graduate student Roozbeh Ghaffari ‘01, Mng ‘03, Freeman’s team demonstrated that the tectorial membrane, a structure traditionally thought to be inert, in fact moves, transmitting waves that travel at a precise speed, and in a direction perpendicular to that of other wave motion in the ear. Interaction between the two kinds of waves appears to make the hair cells more sensitive.
“It’s a very fundamental piece of work,” says Rahul Sarpeshkar ‘90, an MIT associate professor of electrical engineering who works on bionic ears and cochlear implants. “People have suspected that the tectorial membrane could be part of a resonant system. But until now, no one has ever shown it experimentally.”