2. The role of the Cochlea in Auditory perception
Auditory perception is a complex combination of physical and biological process which take
place on the outside of the body in the form of sound vibrations in the air (physical noises),
all the way through the ear and into the brain through the hammer, the anvil and the
Cochlea and then on into the auditory cortex where the brain interprets these messages. In
my review assignment I will put most of my concentration into the role of the Cochlea in
auditory perception. The physiology of the auditory system does not just involve the
Cochlea but various other parts of the ear and the brain that detects and encodes sound
pressure waves, which we perceive as normal everyday sounds. Although the Cochlea is not
the first stage in auditory perception it does play an integral part in the process. The Cochlea
is located in the inner part of the ear and is a snail shaped figure, and the main job of the
Cochlea is to distinguish between the different frequency’s that are passed through the
auditory canal and are then filtered through the Cochlea by their intensity in Hz by the
location of the matching frequency found along the basilar membrane were they range
from 20,000Hz at the base of the Cochlea, to 200Hz at the apex of the Cochlea.
From when the sound wave enters the ear it firstly starts to focus the wave energy on the
concha located on the outer part of the ear before it turns its focus onto the ear canal. Then
the small bones (ossicles) transfer the sound wave through the ear drum and onto the oval
window in the inner ear once the sound wave or frequency has gone through the outer,
middle and some parts of the inner ear it reaches the cochlea. The cochlea is a small coiled
tube and is filled with fluid (perilymph) and it is the displacement of this fluid in which the
sound wave finds the matching frequency. In the cochlea there is a long division down the
centre of the coiled tube (basilar membrane). On either side of the membrane there is a
chamber, the scala vestibuli and the scale tympani. At the end of the scala vestibuli is the
oval window in which the sound waves enter from the stapes and on the corresponding end
of the scala tympani is the round window. The helicotrema is small opening at the apex of
the basilar membrane which allows the two chambers to share the same fluid. Also on the
basilar membrane contains the cochlear sensory receptor cells also known as the cochlea
hair cells. The cochlear hair cells produce graded receptor response to the displacement of
their corresponding stereocilia. Transduction may occur over as small a time period as 10
microseconds. The fluctuation in receptor cells has a reflection on the fluid displacement
within the cochlear tube. Most of the information taken through the cochlea is found in the
inner hair cells on the basilar membrane.
When the sound wave is sent from the stapes through the oval window, the vibrations
cause’s the fluid to move in the upper chamber the scala vestibuli. The pressure from here is
then in turn transferred across the cochlear partition and in to the scala tympani and sends
these sound vibration waves in the opposite direction and through the round window.
These travelling waves cause the displacement of the basilar membrane fluid to take the
form of the vibrating wave that is already in place along the basilar membrane. As we know
3. that the displacement along the basilar membrane varies as it peaks at a particular point,
the position of maximum displacement of the membrane depends on the frequency of the
vibration. Bèkèsy’s research was the first to lead to this conclusion in 1928. This specific
form of frequency-dependant displacement pattern is imperative in the ability of the
cochlea to encode sound frequency. Also we know that in a normal ear each particular point
on the basilar membrane is sharply tuned to a specific frequency, the position of this peak
on the basilar membrane is a function of the frequency of the sound. As the hair cells are
evenly distributed throughout the basilar membrane, the same hair cell distribution activity
provides place code for frequency, (maximum cell activity in relation to the frequency of the
sound input). While each hair cell connects to only a limited number of auditory nerves, this
so called place code is preserved in a response pattern of the auditory nerves. As a large
section of the basilar membrane is displaced in response to a single sound wave frequency
“Any one auditory nerve fibre will respond to a broad range of sound frequencies” (Mather
2011), although we know that it may respond at best to the specific characteristic
frequency.
From earlier reading and research we now know that there are two pathways from the
cochlea to the auditory cortex, the ascending auditory pathway and the descending auditory
pathway both of which receive information from the cochlea. When signals are triggered
and processed by the cochlea and auditory nerve fibres, they then start making their way to
the auditory cortex. The route in which the messages take is one of quite complexity, in the
way that the messages from the right cochlea send most of the fibres to the left side of the
brain while some of them stay on the right side and go through and by-pass various stages
of the pathway while travelling to the primary auditory cortex and visa-versa in relation to
messages that are travelling from the left cochlea they cross over the right side and some
stay on the left side travelling towards the primary auditory cortex. Were as in the
descending auditory pathway the nerve fibres start their journey in the primary auditory
cortex going back towards the cochlea in pretty much the same way as the ascending
auditory pathway but in the opposite direction. Also the descending auditory pathway it
contains more excitatory and also inhibitory connections, where they are believed to act as
a control system for the sensory input. “The complexity of the auditory system becomes
more apparent from the fact that, in addition to the pathway from the cochlea to the
cortex, there are also connections between nuclei on the opposite sides of the
brain”(Goldstein 1996).
As with any part of the body we know that damage can occur and the cochlea is no different
to this, problems with your hearing etc. may be known as conductive disorders as opposed
to conductive hearing loss which is slightly different. The cochlea and its associated parts
may be susceptible to damage by extensive noise levels, infection, genetic disorders and
also age. All of which can have perceptual consequences. As stated earlier conductive
hearing disorders are not the same as conductive hearing losses, and cannot be treated the
same way. With hearing loss the general solution is to amplify the signal, were as with a
4. disorder this may not work and could require something like a cochlear implant, which I’ll
talk about later. But with hearing damage in this case people who suffer they may find
sounds unclear and distorted. These problems stem from damage in frequency tuning in the
auditory nerve. As with damage to the outer hair cells the sufferers’ may also be unable to
differentiate a background noise form something like a specific speech sound. As people try
to talk to someone who suffers from hearing dysfunction, they tend to raise their voice,
hence where the “no need to shout”(Mather 2011), comes into play as the outer hair cells
are strained and stumble on hearing low frequency’s and sound vibrations. To counter act
all these problems sometimes an amplifier is used in the form of a hearing aid, but where
this is not possible the use of a cochlear implant may be used. While they may be only used
in severe cases, it is the process of surgically implanting an electronic device which acts as
an amplifier and is sometimes referred to as a bionic ear. The implant works when a small
microphone on the external part of the head picks up frequency’s and transfers them down
through the speech processer and into the receiver then the electrodes which are wrapped
around the cochlea transfer the frequency’s to the primary auditory cortex.
Conclusion:
Auditory perception may be conceived as a simple process, we hear something we interpret
that sound and we know exactly what that sound is. Although we now know that this is not
so true and that it’s a long complex process from sound wave to cortex. But without so
many theories and experiments published we would not have known to such extent the
process and how delicate it is, but also how easily we can damage and loose this process.
Bibliography:
Goldstein, E.B (1996) Sensation and Perception, ed. 4, USA, Cole Publishing Company.
Mather, G (2011) Foundations of Sensation and Perception, ed. 2, East Sussex, Psychology
Press Ltd.
Handel, S (1993) Listening An introduction to the perception of auditory events, England,
The MIT Press.
American Psychological Association (2002) Cochlea [online], available:
http://www.apa.org/research/action/glossary.aspx
Advanced Bionics (2011) Cochlear Implant division [online] available:
http://www.advancedbionics.com/com/en/your_journey/getting_a_cochlearimplant.html
Exton, C (2011) “Lect 1_Chap4”, CS4005 Perceptual systems and multimedia, 24 Sept 2011,
University of Limerick, Unpublished.
