Week 6 Flashcards
Auditory System
• Transmit sound to the sensory organ
• Transduce sound energy into a neural signal
• Transmit the neural signal to the brain
• Processing of the neural signal to provide
meaningful (and useful) auditory information
Sound
physical dimension- amplitude, frequency, complexity
physical stimulus- high, low, pure, rich
perceptual dimension- loudness, pitch, timbre
ear anatomy
Inner Ear Middle Ear Outer Ear Cochlear Ossicles External Auditory Canal Auricle or pinna
Auricle (Pinna)
• Collect sound waves and channel them into the
auditory canal
• Important role in localising sounds - folds
selectively reflect sounds of various frequencies
around the ear and into the auditory canal
• As a sound source changes its location relative to
the head, the frequency profile of these reflections
changes - offering a cue to the location of the
source
Auditory Canal
• Channel sound energy to the tympanic membrane
Tympanic Membrane (ear drum)
Vibrates in response to air pressure changes of the
sound waves
• Middle ear ossicles are attached to the TM
Ossicles
Middle ear is for impedance matching – sounds in air but sensory in fluid
• If TM transmitted directly – air to fluid – almost all sound energy would be lost (reflected back)
• Concentrate the vibrations of the tympanic membrane on a very small area on the oval window
• Think of how pressure is increased by concentrating a given mass on a small area - like when a woman stands on your foot with a stiletto heel compared to a wide boot
• In the case of the middle ear, this is a 17 fold increase
• The lever action of the ossicles amplify the vibrations by approximately 1.3 times
• Combined, this accounts for a 22 fold increase in the strength of vibrations hitting the tympanic membrane
Inner Ear
Action of the stapes at the oval window produces pressure changes that propagate through cochlear • Pressure causes basilar membrane to vibrate
Transduction
At auditory threshold, the hair cell
displacement is 100 picometers
Equivalent to 10mm at the top of the
Eiffel Tower
Pitch Perception
Auditory processing is tonotopic
• Basilar membrane is a mechanical analyser of
sound frequency
• Structure of the membrane changes continuously
along its length
• Much wider at the apex than the base
• Each point along the membrane responds
preferentially to a different frequency – high at the
base, low at the apex
• Preserved throughout early processing
Auditory Pathways
No major pathway (cf retina-geniculate-striate of vision) – complex network • First ipsilateral cochlear nuclei • Ultimately medial geniculate nucleus of thalamus (MGN) then primary auditory cortex (A1)
Subcortical - Sound Localisation
Localisation of sound sources mediated subcortically at superior olives (SO) • 2 ears - sound impinging on each ear slightly different depending on where the sound is coming from • Differs in 2 detectable ways: • Interaural time difference • Interaural intensity difference
Interaural Time Difference
As sound source moves left or right of centre, time to each ear differs • Medial SO generates a map of time differences • Coincidence detectors
Interaural Intensity Difference
• Head acts to block sound reaching one ear • Lateral SO - intensity comparison • Cross Inhibition
Subcortical - Sound Localisation
Teng et al. (2012):
• Human expert echolocators can discriminate target
offsets of as little as 1.2 degrees (similar to bats)
• Acuity is similar to visual acuity in the far periphery
Auditory Cortex
Tonotopic Columnar organisation (like V1) but based on frequency (rather than orientation) Both ears contribute to processing from early
Auditory Dysfunction – Hearing Loss
- Conduction deafness
- Sensorineural deafness
- Central deafness
Conduction Deafness
Damage to the tympanic membrane and ossicles
• E.g. ossicles become fused and no longer transmit sound vibrations from the
outer ear to the cochlea
• Does not involve the
nervous system
• Treatment – hearing aid or bone conduction implants
Sensorineural Deafness
Auditory nerve fibres are not stimulated properly • Deafness is permanent • Infection, trauma, exposure to toxic substances • Loud sounds (e.g. noise pollution, personal headsets) Streptomycin (antibiotic) has ototoxic properties • Tuberculosis patients treated with streptomycin had cochlear damage • In some cases, all the hair cells in the cochlea were destroyed - leading to total deafness
Cochlear Implant
Bypass hair cells and stimulate auditory nerve fibres directly • External processor converts sound into digital code • Internal electrode array (in the cochlear) stimulate the nerve accordingly • Uses tonotopic principle • Time and training to learn to interpret the signals
Central Deafness
Caused by brain lesions in the temporal cortex (e.g. stroke) (also brainstem)
• Results in loss of specific faculties -
like language processing (left lobe) or discrimination of non-language sounds (right lobe)
• Unilateral lesions may result in unilateral hearing loss; bilateral lesions for bilateral loss
• Remapping may improve hearing with time and rehab
Vestibular System
Proprioception – information about the movement and position of body parts
• Especially important – movement and position of the head – position of whole body; balance; control of
vision
• 5 receptor organs that sense accelerations of the head
• 3 semicircular canals (sense head rotations)
• 2 otolith organs (utricle and saccule) sense linear acceleration – horizontal movement and tilt
• NOTE – measure accelerations (i.e. changes in speed) and not constant motion
• Each has a cluster of hair cells that transduce head
motion/position into vestibular signals
