The cochlea is the receiver of sound in the inner ear. It is composed of three adjacent fluid-filled canals that coil in two and one-half windings encased in a bony shell. Sound waves set the tympanic membrane in the outer ear in vibration. The vibrations are conducted by three small bones in the middle ear to the oval window of the cochlea, stirring waves in the cochlear fluid. The waves move a membrane stretching from base to apex at the center of the cochlea. The membrane deflects hairs on sensory receptor cells nestled beneath in the organ of Corti. The hair deflections are transduced into nerve cell signals and conveyed to the brain through the auditory nerve. High pitch exerts the greatest membrane movements at the cochlea's base and low pitch at the apex. Because the auditory nerve fibers innervate the organ of Corti sequentially, each fiber is excited by a specific sound frequency.
Exposure to high volume noise may shear off sensory receptor hair. Exposure to high doses of antibiotics triggers hair cell death. The loss of sensory hair in as much as the loss of sensory receptor cells lead to early deafness. Hardening of the middle ear bones with advancing age is another common cause. Today, hearing can be restored with cochlear implants as long as the auditory nerve fibers in the cochlea remain intact. That is, wire electrodes are implanted into the cochlea. Ambient sound is recorded with a microphone. Computer algorithms transform the recording into electrical pulses that stimulate the auditory nerve fibers according to the pitch and the volume of the recorded sound. Intriguingly, the nerve cells in the brain can use the artificially generated input to interpret sound in a meaningful fashion. Through practice and optimization of the algorithms, hearing improves. The process constitutes a striking accomplishment of engineering and medicine. However, even greater achievement lies with the nerve cells in the auditory pathway. They must re-adjust their connections such that the novel sensory input can be successfully utilized.
In today's Morning Edition of National Public Radio (NPR), Robert Krulwich spoke with Oliver Sacks (pod cast) about the neurologist's latest book entitled Musicophilia: Tales of Music and the Brain. In the book, the author observes people who hear imaginary music that ensembles of nerve cells seem to generate in the auditory parts of their brains. Robert Krulwich interviews two persons described. One is a lady who lost her hearing with advancing age. Though she fell completely deaf, she reported frequently hearing pieces of music flawlessly performed loud and clear. She opted for a cochlear implant. Although she effectively regained hearing after implantation, the imaginary music persisted to emerge. The music does not bother her. In her judgment, the quality of the brain-generated pieces is far superior to the performances she hears with the implant. She must have had a keen sense of hearing and a great appreciation of music earlier in life, and the auditory inputs produced with the implant can not rival the quality of the memory trace. The great Ludwig van Beethoven managed to compose extraordinary music having fallen entirely deaf.
- Musicophilia: Tales of Music and the Brain, Revised and Expanded Edition has just appeared in paperback. For the occasion, NPR's Fresh Air replayed an interesting interview Terry Gross conducted with Oliver Sacks today. You can listen to the interview here (10/3/08).
- Professor Sacks explores Musicophilia in a show entitled "Musical Minds" on Public Broadcasting Service's Nova tonight (06/30/09).