Defining the response of the human basilar membrane

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Sound waves enter the ear canal and cause the eardrum to vibrate. These vibrations are transmitted through the middle ear, via three tiny bones (the ossicles), to the cochlea in the inner ear. The cochlea is a long thin tube, coiled up into a shape like a snail shell, that converts sound waves into electrical impulses in the nervous system. An important part of this process involves the basilar membrane, a thin membrane that runs the length of the cochlea. Each place on the basilar membrane is tuned to vibrate to a different frequency of sound, so that high frequencies (the bright crash of a cymbal) cause the membrane at the base of the cochlea to vibrate, and low frequencies (the dull thud of a bass drum) cause the membrane at the apex of the cochlea (the tip of the spiral) to vibrate. In this way the basilar membrane separates out the different frequency components of a sound, just as water droplets separate out the different frequencies of light (i.e. colours) to produce a rainbow. The frequency separation helps us to identify sounds, and to separate out sounds that occur together (for example, when listening to a conversation at a noisy party). The vibration of the basilar membrane is modified by special cells called outer hair cells. The outer hair cells amplify sounds at low levels but not at high levels, leading to a shallow growth of basilar membrane vibration with sound level called compression. This makes us much more sensitive to quiet sounds, without affecting our sensitivity to loud sounds. Furthermore, the amplification only affects a limited range of frequencies at each place on the basilar membrane. In this way, the amplification sharpens the tuning, making the membrane better at separating sounds. My research is concerned with measuring the effects of the outer hair cells on the vibration of the basilar membrane in humans. Although physiological experiments have been conducted on other mammals for a number of years, we have only recently developed the techniques necessary for accurate measurement of the human basilar membrane response. In these experiments, sounds are presented to participants over headphones and their responses are recorded (hence, this is a 'behavioural' experiment). Participants are asked to detect one sound (the 'signal') presented after another sound (the 'masker') with the same or a different frequency. By measuring how the detection of the signal depends on the levels of the signal and of the masker, it is possible to estimate the response of the basilar membrane without requiring a surgical procedure, a procedure that would be unethical in humans. As well as providing information about the human auditory system, the results will be relevant to mammalian hearing in general. For example, one of the aims of the research is to determine whether the basilar membrane is compressive near the apex of the cochlea spiral, the region that responds to low frequencies. Direct measurements of basilar membrane vibration in other mammals suggest that it is not, but these experiments may have been compromised because of the difficulties involved in the surgical procedure. It has been suggested that the outer hair cells were damaged, so that the amplification and compression were lost. The present behavioural experiments on humans should be able to resolve this issue because we can measure the cochlea in a healthy physiological state, and compare these results to those from hearing-impaired listeners who have damaged outer hair cells. We will use our results to develop a computational cochlear model, a computer programme that will allow us to simulate the response of the basilar membrane to any sound. Our improved understanding of how the basilar membrane works conveys information to more central structures will then help us understand how the brain uses that information to analyse and identify sounds.