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week13 outline

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McMaster University
Paul Faure

This outline summarizes major points covered in lecture. It is not intended to replace your own lecture notes. The Abnormal Auditory System  The auditory system can become (or start out) dysfunctional in a number of ways: o repeated exposure to loud noise o ageing o pharmacological effects (e.g. ototoxic drugs) o infections & diseases o accidents o heredity  Audiologists and Otorhinolaryngologists (ENT; ear, nose & throat doctors) use clues from behaviour and psychophysics to aid in diagnosis and treatment of auditory abnormalities.  One important distinction is between SYMPTOM and CAUSE – abnormalities in behavioural and psychophysical tests are symptoms, but causes are physiological origins of problem. Hearing Loss  Hearing loss is symptom of auditory dysfunction.  Many other problems arise as a result of hearing loss o communication disorders, tinnitus, hyperacusis, etc.  Hearing loss measured with audiometry by an audiologist; by looking for a threshold shift.  Hearing losses divided into two types by their cause: o Conductive hearing loss (CHL) o Sensorineural hearing loss (SNHL)  Clinically it’s very important to identify type and cause of hearing loss; affects treatment strategy. Threshold Shift (TS)  There are several different levels of hearing loss.  You can have different levels of hearing loss at different frequencies. 1. Conductive hearing Loss (CHL)  CHL: increase in hearing threshold associated with diminished conduction of sound.  Diminished conduction in outer or middle ears results in decreased stimulation of basilar membrane.  Has several common causes: o Wax buildup o damage to tympanic membrane (e.g. torn or ruptured eardrum) o Otitis media o Otosclerosis o banana stuck in ear, etc CHL: Otitis Media & Ear Tubes  Middle ear infection (Otitis media) can cause painful swelling and conductive hearing loss in the middle ear.  Repeated ear infections and severely blocked Eustachian tubes in children are not uncommon, and sometimes require surgical treatment to restore middle ear function. o In severe case, tympanostomy tubes are inserted into the tympanic membrane to allow pressure equalization and drainage of middle ear. CHL: Otosclerosis & Stapedectomy  Otosclerosis: calcification of the ossicular chain can cause conductive hearing loss.  One solution: surgically remove the stapes and insert a small pin, attached to the incus and grafted through the oval window – essentially an artificial replacement for the stapes. 2. Sensorineural Hearing Loss (SNHL)  Hearing loss caused by change in hair cell or auditory nerve function is sensorineural.  Causes of sensorineural hearing loss are many and not always clear.  Most common cause: excessive mechanical stimulation of basilar membrane & hair cells. Psych 3A03 02 December 2011 Week 13 Dr. Paul A. Faure o Most common cases: oxidative stress (physiological) due to metabolic exertion can cause hair cells to die. o Extreme cases: stimulation exceeds elastic limit of the Organ of Corti, resulting in mechanical or physical damage hair cells and their stereocilia.  Cytocochleogram charts the number of haircells remaining along length of cochlea. SNHL: Presbycusis  Presbycusis: progressive decline in hearing with age.  As we age we first experience slight threshold shifts primarily at high frequencies, then gradually larger threshold shifts and at lower sound frequencies.  Age-related; very common among people >55 yrs old.  Cause is sensorineural – repeated, gradual damage to Cochlea & Organ of Corti.  Haircells and neurons dying as a result of age and wear. Presbycusis Treatment: Hearing Aids  Hearing aids used to compensate for SNHL by selectivity amplifying frequencies of hearing loss.  Primarily used to treat presbycusis. Threshold Shift (TS) Summary  Different types of auditory Threshold Shift (TS): o Temporary Threshold Shift: TTS o Asymptotic Threshold Shift: ATS (limit of TTS) o Permanent Threshold Shift: PTS o Compound Threshold Shift: CTS = TTS+PTS  Threshold shift caused by exposure to noise is termed Noise-Induced Threshold Shift (NITS) o Noise-Induced Temporary Threshold Shift: TNTTS o Noise-Induced Permanent Thershold Shift: TNPTS  Exposure to loud sounds can cause hearing to be temporarily diminished (TTS).  TTS varies with the loudness, duration, frequency spectrum and temporal pattern of a sound.  Maximal threshold shift on audiogram occurs approximately ½ to 1 octave above the inducing sound – the half-octave shift.  TTS increases rapidly in first few hours, but eventually asymptotes.  Called asymptot
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