MIT-Using Psychoacoustics to Explore Cochlear Function

Anyone who suffers from hearing loss as well as those less familiar with this affliction should attend closely to this lecture. Brian Moore describes different kinds of damage to the cochlea, and he plays tapes that simulate what it’s like to hear with these impairments. Moore also offers the solace that research is leading to improved technology for the hearing impaired.

Moore first takes us on an animated ride into the ear canal, through the ear drum, to look at a part of the cochlea, the basilar membrane, which plays a crucial role as a frequency analyzer. Two classes of hair cells lying on top of the membrane serve distinctive purposes, and damage to them leads to common types of hearing loss.

Injuries to the outer hair cells result in a higher than normal threshold for detecting sounds, and inability to hear high and low sounds at the same time. People have difficulty separating sounds they want to hear from background noise, especially speech. Current hearing aids don’t compensate well for this impairment. Describes Moore, “Imagine sitting there listening to a concert with a friend with normal hearing and suddenly they’re playing quietly and you can see that they’re playing. You can’t hear a damn thing, you turn up the volume control on your hearing aid, they come back to a loud passage and aiyee! That’s why people say ‘Don’t shout’ when they’re wearing hearing aids.”

Inner hair cells on the basilar membrane send signals to the auditory nerve, and if they’re impaired, then the message they send to the brain gets scrambled. “It really screws up the ability to understand speech,” says Moore. There are “dead regions” where large sections of these cells are completely nonfunctioning at different sound frequencies. Scientists are learning how to compensate for these dead regions by moving sounds to other frequencies that will be audible. Moore also discusses damage to auditory processing that affects the ability to hear changes in pitch — the “dips of normal talkers” in which people with ordinary hearing “grab information.”

Moore and other researchers are trying to tailor hearing aids that compensate for reduced frequency selectivity and for insensitivity to pitch, and which feel more comfortable. In development are open ear canal aids with refined digital feedback algorithms, directional microphones and signal processing. Farther down the road are implantable hearing aids that use mechanical vibration to transmit a wide frequency range without distortion, and beyond that, the possibility of regenerating hair cells: “Don’t bother with aids at all, let’s fix the ear,” says Moore.

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