FoundationalConceptsNeuroscience

The interior of the bony labyrinth is filled with fluid (water and
ions), and the vibration of the oval window sets the fluid inside the
cochlea into vibration. The structure of the ossicles is such that the
vibrational energy is very efficiently transferred from the medium of
air into the medium of fluid. As the oval window vibrates, it pushes
on the fluid inside the cochlea and creates a wave that propagates
through the interior of the cochlea—essentially, the fluid is set into
vibration. Running the length of the cochlea, down the central core of
its spiral interior, is a thin tissue called the basilar membrane. As the
surrounding fluid vibrates, the basilar membrane vibrates, too.
The basilar membrane varies in thickness, being thickest at the end
nearest the oval window and thinnest at the other end. This variation
in thickness causes different regions of the basilar membrane to be
set into vibration by different frequencies, a phenomena called reso-
nance. The thicker end vibrates resonantly with higher frequencies,
and the thinner end vibrates resonantly with lower frequencies.
Because most sounds are mixtures of many different frequencies, an
incoming sound will set into vibration multiple regions of the basilar
membrane. Thus, the basilar membrane creates a spatial represen-
tation of the component frequencies of sound entering the ear—a
Fourier analysis of the sound.
To summarize: different frequencies of sound vibration—when
transferred into the cochlea via the eardrum, ossicles, and oval win-
dow—set different regions of the basilar membrane into vibration.
The basilar membrane performs a Fourier analysis of the incoming
sound and represents the result spatially, along the length of the
membrane—exquisite biophysics!