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Audition Outline • Perceptual dimensions • Ear Anatomy • Auditory transduction • Pitch Perception • by Place Coding • by Rate coding • Sound Localization • by phase difference • by intensity difference
Sound: Variation of pressure over time
Ear Anatomy • Peripheral Structures • Outer ear • Middle ear • Inner ear • Auditory nerve • Central Structures • Brainstem • Midbrain • Cerebral
Ear Anatomy Air Bones Liquid Eardrum >> oval window
Tympanic Membrane (ear drum) semi-transparent cone shaped
http://icarus.med.utoronto.ca/NeuroExam/ Pearly gray 1=Attic (pars flaccida) 2= Lateral process of malleus 3=Handle of malleus 4=End of the malleus 5=Light reflex http://www.qub.ac.uk/cskills/Ears.htm How to use an otoscope http://medweb.uwcm.ac.uk/otoscopy/Default.htm http://medweb.uwcm.ac.uk/otoscopy/common.htm Virtual otoscope & common conditions
Acute otitis media with effusion. There is: - distortion of the drum, - prominent blood vessels in the upper half - dullness of the lower half. - bulging of the upper half of the drum - the outline of the malleus is obscured. normal
Middle Ear • Eustachian Tube: connects to pharynx • Ossicles: 3 bones, which transmit acoustic energy from tympanic membrane to inner ear
Ossicles’ functions • To amplify sound waves, by a reduction in the area of force distribution (Pressure = Force/Area) • To protect the inner ear from excessively loud noise. Muscles attached to the ossicles control their movements, and dampen their vibration to extreme noise. • to give better frequency resolution at higher frequencies by reducing the transmission of low frequencies (again, the muscles play a role here)
Inner ear Middle ear
Transduction of sound • Basilar membrane oscillates • Outer Hair cell cilia bends • Cations inflow • Depolarization • Increased firing rate • Bend on opposite direction • Reduced firing rate
Hz 0 20 500 2000 4000 20,000 30,000 Volley Code language HUMAN RANGE Place Code Pitch Perception: Place vs. Rate Coding
Place Coding: Tonotopic representation • Base • High Freq • Apex • Low Freq.
Traveling wave • High frequencies have peak influence near base and stapes • Low frequencies travel further, have peak near apex • A short movie: • www.neurophys.wisc.edu/~ychen/auditory/animation/animationmain.html • Green line shows 'envelope' of travelling wave: at this frequency most oscillation occurs 28mm from stapes.
The cochlea has a tonotopic organization For high frequencies Pitch perception:Place coding
Used for low frequency sounds ( <1500 Hz ) Mechanism: The rate of neural firing matches the sound's frequency. For example, 50 Hz tone (50 cycles per sec) -> 50 spikes/sec, 100 hz -> 100 spikes/sec Problem: even at the low frequency range, some frequencies exceed neurons’ highest firing rate (200 times per sec) Solution: large numbers of neurons that are phased locked (volley principle). Pitch Perception: Rate code
Sound Localization Interaural Time Difference (low frequency) Interaural Intensity Difference (high frequency)
Deafness • Conduction deafness • outer or middle ear deficit • E.g. fused ossicles. No nerve damage • Sensori-neural • Genetic, infections, loud noises (guns & roses), toxins (e.g. streptomicin) • Potential Solution: Cochlear implants • Central • E.g. strokes
Central Auditory Mechanism • Bilateral projection to auditory cortex (stronger contralateral). • Also, efferent fibers from inferior colliculus back to ears: • they attenuate motion of the middle ear bones (dampen loud sounds)
Anatomy and function • Many sound features are encoded before the signal reaches the cortex - Cochlear nucleus segregates sound information - Signals from each ear converge on the superior olivary complex - important for sound localization - Inferior colliculus is sensitive to location, absolute intensity, rates of intensity change, frequency - important for pattern categorization - Descending cortical influences modify the input from the medial geniculate nucleus - important as an adaptive ‘filter’ cortex medial geniculate body inferior colliculus cochlear nucleus complex cochlea superior olivary complex
Primary Auditory cortex: • Tonotopic Organization • Columnar Organization • Cells with preferred frequency, and • cells with preferred inter-aural time difference
Heschl’s gyrus (primary AC) Anatomy (part 3) source : Palmer & Hall, 2002 Right hemisphere • Primary & non-primary auditory cortex Sylvian Fissure Medial Temporal Gyrus planum polare (nonprimary AC) Superior Temporal Gyrus Superior Temporal Sulcus planum temporale (nonprimary AC)
Steps to Hearing: A summary • Sound waves enter the external ear • Air molecules cause the tympanic membrane to vibrate, which in turn makes vibrate the ossicles on the other side • The vibrating ossicles make the oval window vibrate. Due to small size of oval window relative to the tympanic membrane, the force per unit area is increased 15-20 times • The sound waves that reach the inner ear through the oval window set up pressure changes that vibrate the perilymph in the scala vestibuli • Vibrations in the perilymph are transmitted across Reissner’s membrane to the endolymph of the cochlear duct • The vibrations are transmitted to the basilar membrane which in turn vibrates at a particular frequency, depending upon the position along its length (High frequencies vibrate the window end and low frequencies vibrate the apical end where the membrane is wide) • The cilia of the hair cells, which contact the overlying tectorial membrane, bend as the basilar membrane vibrates Displacement of the stereocilia in the direction of the tallest stereocilia is excitatory and in the opposite direction is inhibitory • The actions are transmitted along the cochlear branch of the vestibulocochlear nerve, activating auditory pathways in the central nervous system, eventually terminating in the auditory area of the temporal lobe of the cerebral cortex