Physiology
of hearing.
human ear, organ of hearing and equilibrium that detects and analyzes
noises by transduction (or the conversion of sound waves into electrochemical
impulses) and maintains the sense of balance (equilibrium).
The human ear, like that of other mammals, contains sense organs that serve
two quite different functions: that of hearing and that of postural equilibrium and coordination of head and eye movements. Anatomically the ear has three
distinguishable parts: the outer, middle, and inner ear. The outer ear consists of the visible
portion called the auricle, or pinna, which projects from the side of
the head, and the short external auditory canal, the inner end of which is
closed by the tympanic membrane, commonly called the eardrum.
The function of the outer ear is to collect sound waves and guide them to the
tympanic membrane. The middle ear is a narrow, air-filled cavity in the
temporal bone. It is spanned by a chain of three tiny bones—the malleus (hammer), incus (anvil), and stapes (stirrup), collectively called the auditory
ossicles. This ossicular
chain conducts sound from the tympanic membrane to the inner ear, which has
been known since the time of Galen (2nd century ce) as the labyrinth. It is a
complicated system of fluid-filled passages and cavities located deep within
the rock-hard petrous portion of the temporal bone. The inner ear consists of
two functional units: the vestibular apparatus, consisting of the vestibule and semicircular canals, which contains
the sensory organs of postural equilibrium; and the snail-shell-like cochlea, which contains the sensory organ of
hearing. These sensory organs are highly specialized endings of the eighth
cranial nerve, also called the vestibulocochlear nerve.
The most striking differences between the human ear and the ears of other
mammals are in the structure of the outermost part, the auricle. In humans the auricle is an almost
rudimentary, usually immobile shell that lies close to the side of the head. It
consists of a thin plate of yellow fibrocartilage covered by closely adherent
skin. The cartilage is molded into clearly defined hollows, ridges, and furrows
that form an irregular, shallow funnel. The deepest depression, which leads
directly to the external auditory canal, or acoustic meatus, is
called the concha. It is partly covered by two small
projections, the tonguelike tragus in front and the antitragus behind. Above the tragus a prominent
ridge, the helix, arises from the floor of the concha and
continues as the incurved rim of the upper portion of the auricle. An inner,
concentric ridge, the antihelix, surrounds the concha and is separated
from the helix by a furrow, the scapha, also called the
fossa of the helix. In some ears a little prominence known as Darwin’s tubercle is seen along the upper,
posterior portion of the helix; it is the vestige of the folded-over point of
the ear of a remote human ancestor. The lobule, the fleshy lower part of the auricle, is
the only area of the outer ear that contains no cartilage. The auricle also has
several small rudimentary muscles, which fasten it to the skull and scalp. In most individuals these muscles do not
function, although some persons can voluntarily activate them to produce
limited movements. The external auditory canal is a slightly curved tube
that extends inward from the floor of the concha and ends blindly at the
tympanic membrane. In its outer third the wall of the canal consists of
cartilage; in its inner two-thirds, of bone. The entire length of the passage
(24 millimetres, or almost 1 inch) is lined with
skin, which also covers the outer surface of the tympanic membrane. Fine hairs
directed outward and modified sweat glands that produce earwax, or cerumen, line
the canal and discourage insects from entering it.
Tympanic membrane and middle ear
Tympanic membrane
The thin, semitransparent tympanic membrane, or eardrum, which forms the
boundary between the outer and middle ear, is stretched obliquely across the
end of the external canal. Its diameter is about 9 millimetres
(0.35 inch), its shape that of a flattened cone with its apex directed inward.
Thus, its outer surface is slightly concave. The edge of the membrane is
thickened and attached to a groove in an incomplete ring of bone, the tympanic annulus, which almost encircles it and
holds it in place. The uppermost small area of the membrane where the ring is
open is slack and is called the pars flaccida, but the
far greater portion is tightly stretched and is called the pars tensa. The
appearance and mobility of the tympanic membrane are important for the
diagnosis of middle-ear disease, which is especially common in young children.
When viewed with the otoscope, the healthy membrane
is translucent and pearl-gray in colour, sometimes
with a pinkish or yellowish tinge.
The entire tympanic membrane consists of three layers. The outer layer of
skin is continuous with that of the external canal. The inner layer of mucous
membrane is continuous with the lining of the tympanic cavity of the middle
ear. Between these layers is a layer of fibrous tissue made up of circular and radial
fibres that give the membrane its stiffness and
tension. The membrane is well supplied with blood vessels and sensory nerve fibres that make it acutely sensitive to pain.
Middle-ear cavity
The cavity of the middle ear is a narrow, air-filled space. A slight
constriction divides it into an upper and a lower chamber, the tympanum (tympanic cavity) proper below and the epitympanum above. These
chambers also are referred to as the atrium and attic, respectively. The
middle-ear space roughly resembles a rectangular room with four walls, a floor,
and a ceiling. The outer (lateral) wall of the middle-ear space is formed by
the tympanic membrane. Its ceiling (superior wall) is a thin plate of bone that
separates it from the cranial cavity and brain above. The floor (inferior wall)
is also a thin bony plate separating the cavity from the jugular vein and
carotid artery below. The back (posterior) wall partly separates it from
another cavity, the mastoid antrum, but an opening in
this wall leads to the antrum and to the small air
cells of the mastoid process, which is the roughened, slightly bulging portion
of the temporal bone just behind the external auditory canal and the auricle.
In the front (anterior) wall is the opening of the eustachian,
or auditory, tube, which connects the middle ear with the nasopharynx
(see Eustachian tube). The inner (medial) wall, which
separates the middle ear from the inner ear, or labyrinth, is a part of the
bony otic capsule
of the inner ear. It has two small openings, or fenestrae, one above the other.
The upper one is the oval window, which is closed by the footplate of
the stapes. The lower one is the round window, which is covered by a thin membrane.
Crossing the
middle-ear cavity is the short ossicular chain formed
by three tiny bones that link the tympanic membrane with the oval window and
inner ear. From the outside inward they are the malleus (hammer), the incus (anvil), and the stapes (stirrup). The malleus more closely
resembles a club than a hammer, and the incus looks more like a premolar tooth
with uneven roots than an anvil. These bones are suspended by ligaments, which
leave the chain free to vibrate in transmitting sound from the tympanic
membrane to the inner ear.
The malleus
consists of a handle and a head. The handle is firmly attached to the tympanic
membrane from the centre (umbo) to the upper margin. The head of the malleus
and the body of the incus are joined tightly and are suspended in the epitympanum just above the upper rim of the tympanic
annulus, where three small ligaments anchor the head of the malleus to the
walls and roof of the epitympanum. Another minute
ligament fixes the short process (crus) of the incus in a shallow depression,
called the fossa incudis, in the
rear wall of the cavity. The long process of the incus is bent near its end and
bears a small bony knob that forms a loose, ligament-enclosed joint with the
head of the stapes. The stapes is the smallest bone in the body. It is about 3 millimetres (0.1 inch) long and weighs scarcely 3
milligrams (0.0001 ounce). It lies almost horizontally, at right angles to the
process of the incus. Its base, or footplate, fits nicely in the oval window
and is surrounded by the elastic annular ligament, although it remains free to
vibrate in transmitting sound to the labyrinth.
Two minuscule
muscles are located in the middle ear. The longer muscle, called the tensor tympani, emerges from a bony canal just
above the opening of the eustachian tube and runs backward then outward as it changes
direction in passing over a pulleylike projection of
bone. The tendon of this muscle is attached to the upper part of the handle of
the malleus. When contracted, the tensor tympani tends to pull the malleus
inward and thus maintains or increases the tension of the tympanic membrane.
The shorter, stouter muscle, called the stapedius, arises from the
back wall of the middle-ear cavity and extends forward and attaches to the neck
of the head of the stapes. Its reflex contractions tend to tip the stapes
backward, as if to pull it out of the oval window. Thus it selectively reduces
the intensity of sounds entering the inner ear, especially those of lower
frequency.
The seventh
cranial nerve, called the facial nerve, passes by a somewhat circuitous
route through the facial canal in the petrous portion of the temporal bone on
its way from the brain stem to the muscles of expression of the face. A small but important branch, the chorda tympani nerve, emerges from the canal into
the middle ear cavity and runs forward along the inner surface of the pars tensa of the membrane, passing between the handle of the
malleus and the long process of the incus. Since at this point it is covered
only by the tympanic mucous membrane, it appears to be quite bare. Then it
resumes its course through the anterior bony wall, bringing sensory fibres for taste to the anterior two-thirds of the tongue
and parasympathetic secretory fibres to salivary
glands.
The eustachian tube, about 45 millimetres
(1.75 inches) long, leads downward and inward from the tympanum to the nasopharynx, the space that is behind and continuous with
the nasal passages and is above the soft palate. At its upper end the tube is
narrow and surrounded by bone. Nearer the pharynx it widens and becomes
cartilaginous. Its mucous lining, which is continuous with that of the middle
ear, is covered with cilia, small hairlike
projections whose coordinated rhythmical sweeping motions speed the drainage of
mucous secretions from the tympanum to the pharynx.
The eustachian tube helps to ventilate the middle ear and to
maintain equal air pressure on both sides of the tympanic membrane. The tube is
closed at rest and opens during swallowing so that minor pressure differences
are adjusted without conscious effort. During a dive or a rapid descent in an
airplane the tube may remain tightly closed. The discomfort that is felt as the
external pressure increases can usually be overcome by attempting a forced
expiration with the mouth and nostrils held tightly shut. This maneuver, which
raises the air pressure in the pharynx and causes the tube to open, is called Valsalva’s maneuver and is named
for the Italian physician-anatomist Antonio Maria Valsalva
(1666–1723), who recommended it for clearing pus from an infected middle ear.
There are
actually two labyrinths of the inner ear, one inside the other—the membranous labyrinth contained within the bony labyrinth. The bony labyrinth consists of a
central chamber called the vestibule, the three semicircular canals, and the
spirally coiled cochlea. Within each structure, and filling only a fraction of
the available space, is a corresponding portion of the membranous labyrinth:
the vestibule contains the utricle and saccule, each semicircular canal its semicircular duct, and the cochlea its cochlear duct. Surrounding the membranous
labyrinth and filling the remaining space is the watery fluid called perilymph. It is derived from blood plasma and
resembles but is not identical with the cerebrospinal fluid of the brain and
the aqueous humour of the eye. Like most of the
hollow organs, the membranous labyrinth is lined with epithelium (a sheet of
specialized cells that covers internal and external body surfaces). It is
filled with a fluid called endolymph, which has a
markedly different ionic content from perilymph. Because the membranous
labyrinth is a closed system, the endolymph and
perilymph do not mix.
The cochlea
contains the sensory organ of hearing. It bears a striking resemblance to the
shell of a snail and in fact takes its name from the Greek word for this
object. The cochlea is a spiral tube that is coiled two and one-half turns
around a hollow central pillar, the modiolus. It forms a cone
approximately 9 millimetres (0.35 inch) in diameter
at its base and 5 millimetres in height. When stretched
out, the tube is approximately 30 millimetres in
length; it is widest—2 millimetres—at the point where
the basal coil opens into the vestibule and tapers until it ends blindly at the
apex. The otherwise hollow centre of the modiolus
contains the cochlear artery and vein, as well as the twisted trunk of fibres of the cochlear nerve. This nerve, a division of the very
short vestibulocochlear nerve, enters the base of the modiolus
from the brain stem through an opening in the petrous portion of the temporal
bone called the internal meatus. The spiral ganglion cells of the cochlear nerve are
found in a bony spiral canal winding around the central core.
A thin bony
shelf, the osseous spiral lamina, winds around the modiolus like the thread of a screw. It projects about
halfway across the cochlear canal, partly dividing it into two compartments, an
upper chamber called the scala vestibuli (vestibular ramp) and a lower chamber
called the scala tympani
(tympanic ramp). The scala vestibuli
and scala tympani, which are filled with perilymph,
communicate with each other through an opening at the apex of the cochlea,
called the helicotrema, which can be
seen if the cochlea is sliced longitudinally down the middle. At its basal end,
near the middle ear, the scala vestibuli
opens into the vestibule. The basal end of the scala
tympani ends blindly just below the round window. Nearby is the opening of the
narrow cochlear aqueduct, through which passes the perilymphatic duct. This duct connects the interior of the
cochlea with the subdural space in the posterior cranial fossa (the rear
portion of the floor of the cranial cavity).
A smaller scala, called the cochlear duct (scala
media), lies between the larger vestibular and tympanic scalae;
it is the cochlear portion of the membranous labyrinth. Filled with endolymph, the cochlear duct ends blindly at both
ends—i.e., below the round window and at the apex. In cross section this duct
resembles a right triangle. Its base is formed by the osseous spiral lamina and
the basilar membrane, which separate the cochlear duct
from the scala tympani. Resting on the basilar
membrane is the organ of Corti, which
contains the hair cells that give rise to nerve signals in response to sound
vibrations. The side of the triangle is formed by two tissues that line the
bony wall of the cochlea, the stria vascularis, which lines the outer wall of the
cochlear duct, and the fibrous spiral ligament, which lies between the stria and the bony wall of the cochlea. A layer of flat
cells bounds the stria and separates it from the
spiral ligament. The hypotenuse is formed by the transparent vestibular membrane of Reissner,
which consists of only two layers of flattened cells. A low ridge, the spiral limbus, rests on the margin of the osseous spiral lamina. Reissner’s membrane stretches from the inner margin of the limbus to the upper border of the stria.
The spiral ligament extends above the attachment of Reissner’s membrane and is in contact with the perilymph in
the scala vestibuli.
Extending below the insertion of the basilar membrane, it is in contact with
the perilymph of the scala tympani. It contains many
stout fibres that anchor the basilar membrane and
numerous connective-tissue cells. Behind the stria
the structure of the spiral ligament is denser than near the upper and lower
margins. The spiral ligament, like the adjacent stria,
is well supplied with blood vessels. It receives the radiating arterioles that
pass outward from the modiolus in bony channels of the
roof of the scala vestibuli.
Branches from these vessels form a network of capillaries above the junction
with Reissner’s membrane that may be largely
responsible for the formation of the perilymph from the blood plasma. Other
branches enter the stria, while still others pass
behind it to the spiral prominence (see below). From these
separate capillary networks, which are not interconnected, small veins
descending below the attachment of the basilar membrane collect blood and
deliver it to the spiral vein in the floor of the scala
tympani.
At the lower
margin of the stria is the spiral
prominence, a low ridge parallel to the basilar membrane that contains its own
set of longitudinally directed capillary vessels. Below the prominence is the outer sulcus. The floor of the outer sulcus is
lined by cells of epithelial origin, some of which send long projections into
the substance of the spiral ligament. Between these so-called root cells,
capillary vessels descend from the spiral ligament. This region appears to have
an absorptive rather than a secretory function, and it may be involved in
removing waste materials from the endolymph.
In humans the
basilar membrane is about 30 to 35 millimetres in
length. It widens from less than 0.001 millimetre
near its basal end to 0.005 millimetre near the apex.
The basilar membrane is spanned by stiff, elastic fibres
that are connected at their basal ends in the modiolus.
Their distal ends are embedded in the membrane but are not actually attached,
which allows them to vibrate. The fibres decrease in calibre and increase in length from the basal end of the
cochlea near the middle ear to the apex, so that the basilar membrane as a
whole decreases remarkably in stiffness from base to apex. Furthermore, at the
basal end the osseous spiral lamina is broader, the stria
vascularis wider, and the spiral ligament stouter
than at the apex. In contrast, however, the mass of the organ of Corti is least at the base and greatest at the apex. Thus,
a certain degree of tuning is provided in the structure of the cochlear duct
and its contents. With greater stiffness and less mass, the basal end is more
attuned to the sounds of higher frequencies. Decreased stiffness and increased
mass render the apical end more responsive to lower frequencies.
Beneath the fibrillar layer of the basilar membrane is the acellular
ground substance of the membrane. This layer is covered in turn by a single
layer of spindle-shaped mesothelial cells, which have
long processes arranged longitudinally and parallel, facing the scala tympani and forming the tympanic lamella that is in
contact with the perilymph.
Capillary blood
vessels are found on the underside of the tympanic lip of the limbus and, in some species, including the guinea pig and
humans, within the basilar membrane, beneath the tunnel. These vessels, called spiral vessels, do not enter the organ of Corti but are thought to supply most of the oxygen and
other nutrients to its cells. Although the outer spiral vessel is seldom found
in adult animals of certain species such as the dog, cat, and rat and is not
found in the basilar membrane of every adult human, it is present in the human
fetus. Its impressive diameter in the fetus suggests that it is an important
channel for blood delivery to the developing organ of Corti.
Arranged on the
surface of the basilar membrane are orderly rows of the sensory hair cells,
which generate nerve impulses in response to sound vibrations. Together with
their supporting cells they form a complex neuroepithelium
called the basilar papilla, or organ of Corti. The
organ of Corti is named after the Italian anatomist Alfonso Corti, who first
described it in 1851. Viewed in cross section the most striking feature of the
organ of Corti is the arch, or tunnel,
of Corti, formed by two rows of pillar cells, or
rods. The pillar cells furnish the major support of this structure. They
separate a single row of larger, pear-shaped, inner hair cells from three or more rows of
smaller, cylindrical, outer hair cells. The inner hair cells are
supported and enclosed by the inner phalangeal cells, which rest on the thin
outer portion, called the tympanic lip, of the spiral limbus.
On the inner side of the inner hair cells and the cells that support them is a
curved furrow called the inner sulcus. This is lined with more or less
undifferentiated cuboidal cells.
Each outer hair
cell is supported by a phalangeal cell of Deiters, or supporting cell, which holds the base of the
hair cell in a cup-shaped depression. From each Deiters’
cell a projection extends upward to the stiff membrane, the reticular lamina,
that covers the organ of Corti. The top of the hair
cell is firmly held by the lamina, but the body is suspended in fluid that fills
the space of Nuel and the tunnel of Corti. Although this fluid is sometimes referred to as cortilymph, its composition is thought to be similar, if
not identical, to that of the perilymph. Beyond the hair cells and the Deiters’ cells are three other types of epithelial cells,
usually called the cells of Hensen, Claudius, and Boettcher, after the
19th-century anatomists who first described them. Their function has not been established,
but they are assumed to help in maintaining the composition of the endolymph by ion transport and absorptive activity.
Each hair cell
has a cytoskeleton composed of filaments of the protein actin, which imparts
stiffness to structures in which it is found. The hair cell is capped by a
dense cuticular plate, composed of actin filaments,
which bears a tuft of stiffly erect stereocilia, also
containing actin, of graded lengths arranged in a staircase pattern. This
so-called hair bundle has rootlets anchored firmly in the cuticular
plate. On the top of the inner hair cells 40 to 60 stereocilia
are arranged in two or more irregularly parallel rows. On the outer hair cells
approximately 100 stereocilia form a W pattern. At
the notch of the W the plate is incomplete, with only a thin cell membrane
taking its place. Beneath the membrane is the basal body of a kinocilium, although no
motile ciliary (hairlike)
portion is present as is the case on the hair cells of the vestibular system.
The stereocilia are about three to five micrometres
in length. The longest make contact with but do not penetrate the tectorial membrane. This membrane is an acellular, gelatinous structure that covers the top of the
spiral limbus as a thin fibrillar
layer, then becomes thicker as it extends outward over the inner sulcus and the
reticular lamina. Its fibrils extend radially and
somewhat obliquely to end at its lateral border, just above the junction of the
reticular lamina and the cells of Hensen. In the
upper turns of the cochlea, the margin of the membrane ends in fingerlike
projections that make contact with the stereocilia of
the outermost hair cells.
The myelin-ensheathed fibres of the vestibulocochlear nerve fan out in spiral fashion from the modiolus to pass into the channel near the root of the
osseous spiral lamina, called the canal of Rosenthal. The
bipolar cell bodies of these neurons constitute the spiral ganglion. Beyond the
ganglion their distal processes extend radially outward in the bony lamina
beneath the limbus to pass through an array of small
pores directly under the inner hair cells, called the habenula
perforata. Here the fibres
abruptly lose their multilayered coats of myelin and continue as thin, naked, unmyelinated fibres into the
organ of Corti. Some fibres
form a longitudinally directed bundle running beneath the inner hair cells and
another bundle just inside the tunnel, above the feet of the inner pillar
cells. The majority of the fibres (some 95 percent in
the human ear) end on the inner hair cells. The remainder cross the tunnel to
form longitudinal bundles beneath the rows of the outer hair cells on which they
eventually terminate.
The endings of
the nerve fibres beneath the hair cells are of two
distinct types. The larger and more numerous endings contain many minute
vesicles, or liquid-filled sacs, containing neurotransmitters, which mediate
impulse transmission at neural junctions. These endings belong to a special
bundle of nerve fibres that arise in the brain stem
and constitute an efferent system, or feedback loop, to the cochlea. The
smaller and less numerous endings contain few vesicles or other cell
structures. They are the terminations of the afferent fibres
of the cochlear nerve, which transmit impulses from the hair cells to the brain
stem (see The physiology of hearing: Cochlear nerve and central
auditory pathways).
The total number
of outer hair cells in the cochlea has been estimated at 12,000 and the number
of inner hair cells at 3,500. Although there are about 30,000 fibres in the cochlear nerve, there is considerable overlap
in the innervation of the outer hair cells. A single fibre
may supply endings to many hair cells, which thus share a “party line.”
Furthermore, a single hair cell may receive nerve endings from many fibres. The actual distribution of nerve fibres in the organ of Corti has
not been worked out in detail, but it is known that the inner hair cells
receive the lion’s share of afferent fibre endings
without the overlapping and sharing of fibres that
are characteristic of the outer hair cells.
Viewed from
above, the organ of Corti with its covering, the
reticular lamina, forms a well-defined mosaic pattern. In humans the
arrangement of the outer hair cells in the basal turn of the cochlea is quite
regular, with three distinct and orderly rows; but in the higher turns of the
cochlea the arrangement becomes slightly irregular, as scattered cells form
fourth or fifth rows. The spaces between the outer hair cells are filled by
oddly shaped extensions (phalangeal plates) of the supporting cells. The double
row of head plates of the inner and outer pillar cells cover the tunnel and
separate the inner from the outer hair cells. The reticular lamina extends
from the inner border cells near the inner sulcus to the Hensen
cells but does not include either of these cell groups. When a hair cell
degenerates and disappears as a result of aging, disease, or noise-induced
injury, its place is quickly covered by the adjacent phalangeal plates, which
expand to form an easily recognized “scar.”
Analysis of sound by the auditory nervous system
Evidence of orderly spatial representations of the organ of Corti at the lower levels of the auditory pathway has been
reported by many investigators. These patterns seem to be in accord with the
place theory of the cochlear analysis of sound. Physiological evidence of
tuning of the auditory system also has been obtained by recording with the
electrical potentials from individual neurons at various levels. Most neurons
of the auditory pathway show a “best frequency”—i.e., a frequency to which the individual neuron responds
at minimal intensity. This finding is entirely compatible with experimental
evidence of frequency tuning of the hair cells (see Transmission of
sound within the inner ear). With each increase in the intensity of the sound
stimulus, the neuron is able to respond to a wider band of frequencies, thus
reflecting the broad tuning of the basilar membrane. With sounds of lower
frequency, the rate of impulses fired by the neuron reflects the stimulus
frequency, and the response often reveals phase-locking with the stimulus; that
is, the nerve fibres are stimulated at regularly
recurring intervals, corresponding to a particular position or phase, of each
sound wave. Increased intensity of stimulation causes a more rapid rate of
responding. In general, the pitch of a sound tends to be coded in terms of
which neurons are responding, and its loudness is determined by the rate of response and
the total number of neurons activated.
Although extensive studies have been made of the responses of single
cortical neurons, the data do not yet fit any comprehensive theory of auditory
analysis. Experiments in animals have indicated that the cortex is not even
necessary for frequency recognition, which can be carried out at lower levels,
but that it is essential for the recognition of temporal patterns of sound. It
appears likely, therefore, that in humans the cortex is reserved for the analysis of more
complex auditory stimuli, such as speech and music, for which the temporal
sequence of sounds is equally important.
Presumably it is also at the cortical level that the meaning of sounds is
interpreted and behaviour is adjusted in accordance
with their significance. Such functions were formerly attributed to an
“auditory association area” immediately surrounding the primary area, but they
probably should be thought of as involving much more of the cerebral cortex,
thanks to the multiple, parallel interconnections between the various areas.
The localization of sounds from a stationary source in the horizontal
plane is known to depend on the recognition of minute differences in the
intensity and time of arrival of the sound at the two ears. A sound that
arrives at the right ear a few microseconds sooner than it does at the left or
that sounds a few decibels louder in that ear is recognized as coming from the
right. In a real-life situation the head may also be turned to pinpoint the
sound by facing it and thus canceling these differences. For low-frequency
tones a difference in phase at the two ears is the criterion for localization,
but for higher frequencies the difference in loudness caused by the sound
shadow of the head becomes all-important. Such comparisons and discriminations
appear to be carried out at brain stem and midbrain levels of the central
auditory pathway. The spectral shapes of sounds have been shown to be most
important for determining the elevation of a source that is not in the
horizontal plane. Localization of sound that emanates from a moving source is a
more complicated task for the nervous system and apparently involves the
cerebral cortex and short-term memory. Experiments in animals have shown that
injury to the auditory area of the cortex on one side of the brain interferes
with the localization of a moving sound source on the opposite side of the
body.
Each cochlear nucleus receives impulses only from the ear of the same side.
A comparison between the responses of the two ears first becomes possible at
the superior olivary complex, which receives fibres from both cochlear nuclei. Electrophysiological
experiments in animals have shown that some neurons of the accessory nucleus of
the olivary complex respond to impulses from both
ears. Others respond to impulses from one side exclusively, but their response
is modified by the simultaneous arrival of impulses from the other side.