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 pharmacological effects (e.g. ototoxic drugs)
o infections & diseases
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 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 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
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.
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.