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Transcript
The Auditory System
Sound is created by pressure waves in air; these waves
are induced by vibrating membranes such as vocal
cords. Because the membranes usually vibrate in a
regular manner, the pressure waves have a fixed
spacing.
Sound has two main characteristics: A. the spacing
between waves or period. This can also be thought of in
terms of how many wave cycles pass by in one secondthat is, the sound’s frequency in Hz=cycles/second.
B. The intensity of amplitude of the sound measured in
decibels.
Bear et al.
The Auditory Periphery
Bear et al.
Kandel et al.
The outer ear and canal guide and filter sound. The tympanic membrane and ossicles transmit
the vibrations to the cochlea itself; the vibrations enter the cochlea via the round window and
exit via the round window. As they pass through the endolymph of the scala vestibuli and
tympani, sound waves cause the basilar membrane to vibrate. This is the key to auditory
function.
The Cochlea
Sound waves cause the basilar
membrane to vibrate.
The basilar membrane is stiff at its base
and loose at its apex. Just like a guitar
string, this causes it to resonate to high
frequencies at its base and low
frequencies at its apex. There is
therefore a tonotopic map- a location
code- formed on the cochlea.This is a
fundamental feature of auditory coding
and is preserved right up to cortex.
Note that this is a separate code from
synchronization.
Bear et al.
Bear et al.
Hair Cells
Hair cells transduce vibrations into
depolarization. This in turn leads
to vesicular release that excites
auditory afferent fibers and
causes them to discharge.
Hair cells are firmly attached to the basilar membrane
and therefore move up and down with it as it vibrates.
The “hairs” or cilia of these cells are attached to a
tectorial membrane; this membrane is fixed- it does not
vibrate in response to sound. So, as you can imagine,
when the basilar membrane moves upward, the cilia will
be bent. This is the first step in the transduction process.
A scanning electron microscopic view of the beautiful
organization of hair cell cilia in the cochlea.
Kandel et al.
Hair Cells 2
When the cilia bend in one direction it
causes the hair cell to hyperpolarize;
bending in the opposite direction
causes depolarization.
Bear et al.
This effect is due to the mechanical coupling of the
cilia to K+ channels at their tips. The depolarization
causes Ca2+ entry and the fusion of vesicles and
release of glutamate from hair cells. This cause
excitation and spiking of the auditory afferent
fibers.
Auditory Afferent fibers
Each auditory afferent fiber is tuned
to a specific frequency.
The tuning is simply due to the location of its
hair cell along the cochlea. This tonotopic
mapping is preserved in the projection of
cochlear afferents to the cochlear nuclei in
the medulla.
As I mentioned earlier, cochlear afferents are also phase locked
to sound (especially low frequency sounds). So there are two
ways to code for sound. A firing rate “place” code (tonotopy)
and a temporal code. The central auditory system uses both
codes for various purposes.
This is a general principle. Sensory systems are flexible and
can use multiple coding strategies.
Bear et al.
Central Auditory Pathways
Auditory reach the cochlear nuclei of the
medulla (DCN, VCN). From these nuclei
a direct pathway goes to the inferior
colliculus, then the thalamic medial
geniculate nucleus (MGN) and then onto
the auditory primary cortex.
Note that this pathway is bilateral unlike
the contralateral somatosensory
pathway. This makes sense since sound
always reaches both ears.
On the way up to cortex axons from VCN also terminate
in nuclei of the superior olivary complex. Neurons in this
cell group use relative sound timing and intensity in the
two ears to estimate the spatial location of a sound
source.
Bear et al.
Sound Localization
Sounds coming from the right arrive
at the right ear a little earlier than the
at the left ear. This small time
difference is used by the superior
olive.
Bear et al.
A cell in the superior olive responds with an increase
in firing rate to a time difference in the arrival of
sound to the ears. This cue to the sounds location is
conveyed up to the inferior colliculus and onto cortex.
Relative sound intensity is also used as a cue in
different neurons in the superior olive region.
The neural mechanisms involved are becoming
understood but are beyond the scope of an
introductory course.
Cortical Processing of Sounds
Bear et al.
The MGN projects up to the primary
auditory cortex (there are secondary
auditory areas as welll). Notice that
the cortex still has tonotopy.
However auditory cortex neurons
also respond to complex features of
sound such as modulations of
amplitude or frequency. If you pay
attention to speech or music you will
hear many examples of both kinds of
modulation.
Auditory cortex projects to numerous secondary cortical areas including multisensory areas
(allow us to recognize animals or other humans by both sound and sight) and to regions
specifically involved in communication. Communication and environmental sounds are
separated after the AC.
It is also noteworthy that the AC and MGN project to the amygdala; as we’ll see later this
permits sounds to be linked with dangerous stimuli (fear conditioning for conditioned
avoidance).
Processing of Natural Sounds
Nelken, 2004
Acoustic input typically comes from many different
sources; they have different combinations of
frequencies, their amplitudes change
independently and they have different locations.
But the sound waves coming into the ears are just
the sum of all these different acoustic signals.
The auditory system then separates these
acoustic inputs to generate the “sounds” we hearthe “auditory scene”. This takes extensive learning
early in life but the mechanisms are not
understood.
One major point is that the both the signal fine
structure (the frequencies present) and the
envelope (amplitude modulation of the individual
frequencies) must be extracted and connected
with their location.
This process occurs in cortex.