Linguistics Encyclopedia.pdf

The linguistics encyclopedia 58
The outer ear includes the pinna and the external meatus—the visible cartilaginous
structures—and the external auditory canal which terminates at the tympanic
membrane or eardrum. The outer ear ‘collects’ auditory signals which arrive as sound
waves or changing acoustic pressures propagated through the surrounding medium,
usually air. The outer ear also serves as protection for the delicate middle ear, provides
some amplification and assists in sound localization, i.e. in determining where a sound
originates.
The middle ear is bounded on one side by the tympanic membrane and on the other
by a bony wall containing the cochlea of the inner ear. In addition to the tympanic
membrane, the middle ear contains three ossicles; these are the malleus, incus, and
stapes, a set of three tiny interconnected bones extending in a chain from the tympanic
membrane to the oval window of the cochlea. The tympanic membrane vibrates in
response to the sound waves impinging upon it; the ossicles greatly amplify these
vibratory patterns by transferring pressure from a greater area, the tympanic membrane,
to a much smaller one, the footplate of the stapes attached to the oval window of the
cochlea.
The inner ear contains the vestibule, the semicircular canals—which primarily
affect balance—and the cochlea, a small coiled passage of decreasing diameter. Running
the length of the cochlea are the scala tympani and scala vestibuli, two fluid-filled
canals which are separated from the fluid-filled scala media or cochlear duct. The
vibratory patterns of sound-pressure waves are transferred into hydraulic pressure waves
which travel through the scala vestibuli and scala tympani and from the base to the apex
of the scala media.
One surface of the scala media contains a layer of fibres called the basilar
membrane. This tapered membrane is narrow and taut at its base in the larger vestibular
end of the cochlea, and wide and flaccid at its terminus or apex in the smaller apical
portion of the cochlea. On one surface of the basilar membrane is the organ of Corti
which contains thousands of inner and outer hair cells, each supporting a number of cilia
or hairs. When the basilar membrane is displaced in response to the travelling waves
propagating throughout it, the tectorial membrane near the outer edge of the organ of
Corti also moves. It is believed that the shearing effect of the motion of these two
membranes stimulates the cilia of the hair cells, thereby triggering a neural response in
the auditory-receptor cells. These cells, in turn, relay electrochemical impulses to a fibre
bundle called the auditory nerve, or the VIIIth cranial nerve. Information about the
spatial representation of frequencies on the basilar membrane is preserved in the auditory
nerve, which is thus said to have tonotopic organization.
The precise nature of the information received on the basilar membrane and encoded
in the auditory nerve has been a matter of much investigation. The fact that the basilar
membrane changes in width and rigidity throughout its length means that the amplitudes
of pressure waves peak at specific loci or places on the membrane. Hence, the peak
amplitudes of low-frequency sounds occur at the wider and more flaccid apex while the
peak amplitudes of high-frequency sounds occur at the narrower and tauter base, which
can, however, also respond to low-frequency stimulation. This was demonstrated in a
series of experiments conducted by von Békésy in the 1930s and 1940s (see von Békésy,
1960).