  Most sounds of interest consist of complex, time-dependent admixtures of
tones of diverse frequencies and variable amplitudes. To detect and process
these signals, the ear employs a highly nonlinear, adaptive, real-time spectral
analyzer: the cochlea. Sound excites vibration of the eardrum and the three
miniscule bones of the middle ear, the last of which acts as a piston to
initiate oscillatory pressure changes within the liquid-filled chambers of the
cochlea. The basilar membrane, an elastic band spiraling along the cochlea
between two of these chambers, responds to these pressures by conducting a
largely independent traveling wave for each frequency component of the input.
Because the basilar membrane is graded in mass and stiffness along its length,
however, each traveling wave grows in magnitude and decreases in wavelength
until it peaks at a specific, frequency-dependent position: low frequencies
propagate to the cochlear apex, whereas high frequencies culminate at the base.
The oscillations of the basilar membrane deflect hair bundles, the mechanically
sensitive organelles of the ear's sensory receptors, the hair cells. As
mechanically sensitive ion channels open and close, each hair cell responds
with an electrical signal that is chemically transmitted to an afferent nerve
fiber and thence into the brain. In addition to transducing mechanical inputs,
hair cells amplify them [...]
