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The role of the cochlea in auditory perception

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  • 1. Review Assignment CS4005Student: Tony Gunning (11140054)
  • 2. The role of the Cochlea in Auditory perceptionAuditory perception is a complex combination of physical and biological process which takeplace 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 theCochlea and then on into the auditory cortex where the brain interprets these messages. Inmy review assignment I will put most of my concentration into the role of the Cochlea inauditory perception. The physiology of the auditory system does not just involve theCochlea but various other parts of the ear and the brain that detects and encodes soundpressure waves, which we perceive as normal everyday sounds. Although the Cochlea is notthe first stage in auditory perception it does play an integral part in the process. The Cochleais located in the inner part of the ear and is a snail shaped figure, and the main job of theCochlea is to distinguish between the different frequency’s that are passed through theauditory canal and are then filtered through the Cochlea by their intensity in Hz by thelocation of the matching frequency found along the basilar membrane were they rangefrom 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 theconcha located on the outer part of the ear before it turns its focus onto the ear canal. Thenthe small bones (ossicles) transfer the sound wave through the ear drum and onto the ovalwindow 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 coiledtube and is filled with fluid (perilymph) and it is the displacement of this fluid in which thesound wave finds the matching frequency. In the cochlea there is a long division down thecentre of the coiled tube (basilar membrane). On either side of the membrane there is achamber, the scala vestibuli and the scale tympani. At the end of the scala vestibuli is theoval window in which the sound waves enter from the stapes and on the corresponding endof the scala tympani is the round window. The helicotrema is small opening at the apex ofthe basilar membrane which allows the two chambers to share the same fluid. Also on thebasilar membrane contains the cochlear sensory receptor cells also known as the cochleahair cells. The cochlear hair cells produce graded receptor response to the displacement oftheir corresponding stereocilia. Transduction may occur over as small a time period as 10microseconds. The fluctuation in receptor cells has a reflection on the fluid displacementwithin the cochlear tube. Most of the information taken through the cochlea is found in theinner hair cells on the basilar membrane.When the sound wave is sent from the stapes through the oval window, the vibrationscause’s the fluid to move in the upper chamber the scala vestibuli. The pressure from here isthen in turn transferred across the cochlear partition and in to the scala tympani and sendsthese 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 theform 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 thevibration. Bèkèsy’s research was the first to lead to this conclusion in 1928. This specificform of frequency-dependant displacement pattern is imperative in the ability of thecochlea to encode sound frequency. Also we know that in a normal ear each particular pointon the basilar membrane is sharply tuned to a specific frequency, the position of this peakon the basilar membrane is a function of the frequency of the sound. As the hair cells areevenly distributed throughout the basilar membrane, the same hair cell distribution activityprovides place code for frequency, (maximum cell activity in relation to the frequency of thesound input). While each hair cell connects to only a limited number of auditory nerves, thisso called place code is preserved in a response pattern of the auditory nerves. As a largesection 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” (Mather2011), although we know that it may respond at best to the specific characteristicfrequency.From earlier reading and research we now know that there are two pathways from thecochlea to the auditory cortex, the ascending auditory pathway and the descending auditorypathway both of which receive information from the cochlea. When signals are triggeredand processed by the cochlea and auditory nerve fibres, they then start making their way tothe auditory cortex. The route in which the messages take is one of quite complexity, in theway that the messages from the right cochlea send most of the fibres to the left side of thebrain while some of them stay on the right side and go through and by-pass various stagesof the pathway while travelling to the primary auditory cortex and visa-versa in relation tomessages that are travelling from the left cochlea they cross over the right side and somestay on the left side travelling towards the primary auditory cortex. Were as in thedescending auditory pathway the nerve fibres start their journey in the primary auditorycortex going back towards the cochlea in pretty much the same way as the ascendingauditory pathway but in the opposite direction. Also the descending auditory pathway itcontains more excitatory and also inhibitory connections, where they are believed to act asa control system for the sensory input. “The complexity of the auditory system becomesmore apparent from the fact that, in addition to the pathway from the cochlea to thecortex, there are also connections between nuclei on the opposite sides of thebrain”(Goldstein 1996).As with any part of the body we know that damage can occur and the cochlea is no differentto this, problems with your hearing etc. may be known as conductive disorders as opposedto conductive hearing loss which is slightly different. The cochlea and its associated partsmay be susceptible to damage by extensive noise levels, infection, genetic disorders andalso age. All of which can have perceptual consequences. As stated earlier conductivehearing disorders are not the same as conductive hearing losses, and cannot be treated thesame 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’lltalk about later. But with hearing damage in this case people who suffer they may findsounds unclear and distorted. These problems stem from damage in frequency tuning in theauditory nerve. As with damage to the outer hair cells the sufferers’ may also be unable todifferentiate a background noise form something like a specific speech sound. As people tryto 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 cellsare strained and stumble on hearing low frequency’s and sound vibrations. To counter actall these problems sometimes an amplifier is used in the form of a hearing aid, but wherethis is not possible the use of a cochlear implant may be used. While they may be only usedin severe cases, it is the process of surgically implanting an electronic device which acts asan amplifier and is sometimes referred to as a bionic ear. The implant works when a smallmicrophone on the external part of the head picks up frequency’s and transfers them downthrough the speech processer and into the receiver then the electrodes which are wrappedaround 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 interpretthat sound and we know exactly what that sound is. Although we now know that this is notso true and that it’s a long complex process from sound wave to cortex. But without somany theories and experiments published we would not have known to such extent theprocess 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, PsychologyPress 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.aspxAdvanced Bionics (2011) Cochlear Implant division [online] available:http://www.advancedbionics.com/com/en/your_journey/getting_a_cochlearimplant.htmlExton, C (2011) “Lect 1_Chap4”, CS4005 Perceptual systems and multimedia, 24 Sept 2011,University of Limerick, Unpublished.