The Auditory System
Gross anatomy of the auditory and vestibular
systems
Tasks of the auditory system
• Resolve intensity (loudness) and
frequency (pitch, timbre) components of
sound stimuli
• Localize sound sources in space
Delivery of sound energy to the
cochlea
• Sound consists of pressure waves
• The tympanic membrane and auditory ossicles
are a mechanism for transferring sound energy
from the air medium to the liquid medium of the
cochlea
• Each sound pressure wave moves the tympanic
membrane back and forth, ultimately moving the
oval window back and forth. Since liquid in the
cochlea is incompressible, movements of the
oval window are compensated for by
movements of the round window.
Frequency segregation in the cochlea – an
initial sort of sound frequencies
Frequency sorting in the cochlea is the result of a
gradient of best resonance frequencies along the
length of the cochlea
• The best resonance frequency of a structure is
the frequency at which it vibrates most readily.
This is determined by the shape and material
properties of the structure.
• Energy transfer between two structures or from
a medium to a structure, is most efficient when
the energy source delivers vibration at the
recipient’s best resonance frequency.
Sound Transduction in
the cochlea
Arrival of a traveling
wave causes the basilar
membrane to move up
and down, bending the
stereocilia against the
tectorial membrane.
This depolarizes the hair
cells, leading to
transmitter release on
the afferent neurons that
course into the auditory
nerve.
Augmentation of sensitivity and frequency
resolution at the level of the Organ of Corti
• Hair cells are frequency-tuned by virtue of the
fact that each hair cell undergoes both electrical
and mechanical oscillation at a characteristic
frequency: as one passes along the length of the
cochlea, the oscillation frequency – and thus the
best stimulus frequency - changes incrementally
from one hair cell to the next, so that
considerable additional frequency resolution
takes place after the traveling wave has
stimulated the hair cells.
Outer hair cells exist mainly to augment vibrations
of the basilar membrane
• Since stimulated hair cells oscillate more
vigorously than unstimulated ones, the
having the additional outer cells bouncing
on the basilar membrane trampoline
causes the membrane movements to have
greater amplitude – increasing the
stimulus intensity for the inner hair cells,
which do connect to afferents.
Central Auditory Processing
Central auditory pathways
Humans use at least two strategies
for sound localization
• Strategy 1. for frequencies below 3 kHz:
phase-locking is possible; time of arrival
differences can be detected. The
threshold of detection is as small as 10
microsec. This translates to a sensitivity
of about 1o of arc.
Processing in the brainstem: sound localization by
coincidence cells in the olivary nuclei
• Q. How can time delays as small as 10
microsec be measured by neurons that
have to operate in the msec time domain?
• A. The medial superior olive (MSO)
receives bilateral inputs from the
anteroventral cochlear nuclei. These
inputs enter a chain of coincidence cells.
Time is measured by conduction time in the network
Strategy for higher frequencies
• Strategy 2. for higher frequencies,
intensity differences between the two ears
must be used. At these frequencies, the
sound wavelength is so short that the
waves cannot bend around the head, so
the head creates a sound shadow that
enhances the effect.
Detection of intensity differences in the brainstem
The players here
are the lateral
superior olive
(LSO) and the
medial superior
nucleus of the
trapezoidal body
(MNTB).
Whichever ear
receives the loudest
stimulus can also shut
off activity in the
ascending pathway
from the less
stimulated ear
A map of auditory space is generated by
integration of sound localization information in the
inferior colliculus
• Some neurons in the inferior colliculus
respond specifically to:
• Sounds coming from particular points in 3D space
surrounding the head
And also to
• Sounds of particular frequencies, or changes in
frequency
• Sounds of particular duration
The primary auditory cortex is a map of the
contralateral cochlea
In the human brain, a specific area processes
speech sounds
(Wernicke’s area)
A key component of the normal function of this area is analysis of the time
sequence of sounds. Most commonly, during early postnatal development the
left cortex acquires dominance for spoken language processing. The right
cortex then is free to differentiate a specialization in processing musical
language.
Cortical dominance
• The “dominant” cortical half is so-called because
it dominates in
– motor control, so that the majority of the population
are right-handed and right-footed
– language control- so that most people prefer to hold
the telephone to their right ear, and injury to language
related areas (Broca’s A, Wernicke’s A) in the left
cortex results in some form of aphasia, whereas
damage to the right cortex does not carry the same
consequences.