Audition Outline

•  Perceptual dimensions

•  Ear Anatomy •  Auditory transduction•  Pitch Perception

– by Place Coding– by Rate coding

•  Sound Localization– by phase difference– by intensity difference

Perceptual Dimensions

Stimulus Vision Audition

Frequency Hue (nm)

(‘color’)

Pitch (Hz)(e.g. 440Hz)

Amplitude Brightness Loudness (dB)

Purity (vs. complexity)

Saturation

Timbre

complexity

Sound: Variation of pressure over time

Ear Anatomy• Peripheral Structures

– Outer ear– Middle ear– Inner ear– Auditory nerve

• Central Structures– Brainstem– Midbrain– Cerebral

Air Bones Liquid

Eardrum >> oval window

Ear Anatomy

Tympanic Membrane (ear drum)

semi-transparent cone shaped

http://www.qub.ac.uk/cskills/Ears.htmHow to use an otoscope

http://medweb.uwcm.ac.uk/otoscopy/Default.htm

Pearly gray1=Attic (pars flaccida) 2= Lateral process of malleus 3=Handle of malleus 4=End of the malleus 5=Light reflex

http://medweb.uwcm.ac.uk/otoscopy/common.htmVirtual otoscope & common conditions

http://icarus.med.utoronto.ca/NeuroExam/

normalAcute 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

Membrane

                                 

Opaque with Inflammation

                               

  Bulging Membrane

                  

               Chronic

Inflammation

                                 

Resolving Infection

                              

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)

Middle ear

Inner ear

www.iurc.montp.inserm.fr/cric/audition/english/cochlea/fcochlea.htm

Transduction of sound

- Basilar membrane oscillates- Outer Hair cell cilia bends - Cations inflow- Depolarization- Increased firing rate

• Bend on opposite direction• Reduced firing rate

020

500

2000

4000

20,000

30,000

HUMANRANGE

Volley Code

Place Code

Hz

language

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.

Pitch perception: Place coding

• The cochlea has a tonotopic organization

• For high frequencies

Pitch Perception: Rate code • 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).

Sound Localization

Interaural Intensity Difference (high frequency)

Interaural Time Difference (low frequency)

Delay Lines – Interaural Time Difference (ITD)

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

• 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)

Central Auditory Mechanism

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’

inferior colliculus

medial geniculate body

cortex

superior olivary complex

cochlea

cochlear nucleuscomplex

• Primary Auditory cortex:– Tonotopic Organization– Columnar Organization– Cells with preferred

frequency, and – cells with preferred inter-

aural time difference

Anatomy (part 3) source : Palmer & Hall, 2002

• Primary & non-primary auditory cortex

Sylvian Fissure

Superior Temporal Gyrus

Superior Temporal Sulcus

Medial Temporal Gyrus

Right hemisphere

Heschl’s gyrus (primary AC)

planum temporale (nonprimary AC)

planum polare (nonprimary AC)

Spare slides

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

Auditory Nerve Tuning Curves (receptive fields)

Inner Ear - Labyrinth

Inner Ear – Organ of Corti