Chapter 6--Audition/Hearing

sound

• air vibrations between 20-20,000 Hz are perceived as “this”

• pitch

• loudness

• timbre

nature of the stimulus

• sound is categorized along physical and perceptual dimensions

timbre

• complexity of a sound that consists of the fundamental (lowest) frequency plus all other associated vibrations

• overtones are multiples of the fundamental frequency (harmonics are overtones that are integer multiples)

hearing in nature

• elephants hear low frequencies that humans don’t

• cats hear high frequencies that humans don’t

17th century aids for hearing

• “ear trumpets" had a wider end that collected sound and a tapering end placed in ear

  • a strategy similar to one adopted by animals

19th century aids for hearing

• elegant means of concealment and bone conduction devices were developed

20th century

• before radar distant enemy movements were detected by sound

outer ear

• pinna (ear lobe) and auditory canal

  • folds in the pinna direct sound reflections to the canal

middle ear

• the tympanic membrane and ossicles in an air-filled chamber

inner ear

• cochlea

  • fluid filled

role of middle ear

• alternating compressions and rarefactions push and pull the tympanic membrane

• lever action of the ossicles causes footplate to push in at oval window

role of ossicles

• usually there is a loss of amplitude going from air (middle air) to liquid (inner ear), but this helps to overcome

role of tympanic membrane

• pressure at footplate is amplified relative to “this”

structure of inner ear: cochlea

• bony and coiled

• 3 fluid-filled chambers (‘scala’) with flexible membranes at each ‘end’

  • oval window at stapes

  • round window at end of coil

• basilar membrane extends the entire length

  • hole at the apex (helicotrema)

3 fluid-filled chambers

• scala vestibula and tymani

  • perilympth (low K+)

• scala media

  • endolymph (high K+)

organ of corti

• sits on basilar membrane

• where ‘mechanoelectric transduction’ takes place

• location of auditory receptors: hair cells

inner hair cells

• 1 row of ~3500

• perception of pitch and timbre

• absolutely required for hearing

outer hair cells

• 3 rows of ~12,000

• amplify and modulate

• not needed for “hearing”

  • loss leads to lower sensitivity

hair cell structure

• stereocilia at the apex

  • transduction occurs here

• base: cell body where neurotransmitter is released

  • innervated by dendrites of spiral ganglion cells

frequency detection begins

• different frequencies are coded by the cochlea based on the rate of hair cell action potentials produced AND where the hair cells are located

temporal code (low frequencies)

• based on number/timing of action potentials generated by hair cells

  • fewer/spaced = lower frequency

  • more/close = higher frequency

  • limited by the max. num of APs the auditory nerve can generate (~1000/sec)

place code (all but lowest frequencies)

• higher frequencies cause vibration at the stiff and narrow base

• relatively lower frequencies vibrate the wider and floppier apex

• vibration causes the hair cells in that location to fire

placing code along the basilar membrane

• as the stapes move in and out, it causes endolymph to flow

• this generates a traveling wave

• at 3000 Hz, the fluid and membrane movement end abruptly about halfway between the base and apex

stereocilia of hair cells

• when sound causes the basilar membrane to move up and down, hair cell stereocilia will move correspondingly

  • waves of motion cause back and forth movement in the inner/outer hair cells

inner hair cells in stereocilia

• stereocilia attached to tectorial membrane by fine filaments

outer hair cells in stereocilia

• stereocilia in contact with tectorial membrane

cilia of hair cells

• stereocilia from frog hair cell, where tip links attach to insertional plaques

transduction by inner hair cells

• signals to the brain are mostly carried by “this”

channels to open

stretching of tip links cause

influx of K+ and Ca++

these chemicals cause depolarization of the cell and results in release of neurotransmitter by the hair cell (transduction of IHCs)

firing in the cochlear nerve

• more neurotransmitter releases ___

• (transduction by IHCs)

hair cells form synapses with

with dendrites of bipolar cells of the VCN (vestibulocochlear nerve)

IHCs (release)

glutamate is released by

OHCs (release)

ACh is released by

release of cochlear nerve is from

95% transmits input from inner hair cells, 5% from outer hair cells

role of OHCs

• like IHCs, they depolarize due to vibrations of basilar membrane

  • these changes cause this to change its length and amplify BM movement

• alter sound sensitivity to both weak and loud sounds via feedback from the brain

encoding sound: location

• vertical sound localization can be acquired from reflections off the pinna

  • the timber of sounds differs depending on how the pinna is oriented

    • delays in direct vs reflected path

  • “spectral filtering”

spectral filtering

• sounds of different frequencies bounce off the folds differently and in varied directions depending on the orientation of the head

interaural intensity difference

• for higher frequencies

• ears experience different level of sound

• the “head shadow” effect

  • if wavelength >/= width of the head (ie: low frequencies), the sound will diffract around the head and be heard with almost equal intensity in the opposite ear

interaural time difference

• for all frequencies

• onset disparity

• phase disparity

onset disparity

• works only when a sound is first heard

phase disparity

• differences in compressions and rarefactions at each ear

encoding location for lower frequencies

• localization from the left or right can be acquired from phase disparity in sound arriving at the two ears

audio pathway to the brain

• from the auditory nerve to the brain

tonotopy:

• topographically organized mapping of different frequencies in auditory brain regions

cells in primary auditory cortex

• sensitive to temporal and directional characteristics

primary auditory cortex

• mostly areas 41 and 42

• not necessary for sound detection

  • more important for hearing “biological sounds” (ex: speech) than for pure tones

association areas

• receive input from primary auditory cortex

• ex: Wernicke’s

streams

• flow of information goes in two basic directions

  • anterior/ventral

  • posterior/dorsal

anteroventral stream

• decodes what the sounds are (pitch)

• ‘what’

• recognition and meaning

posterodorsal stream

• decodes location

• ‘where’

conduction deafness

• usually involves a middle ear problem that blocks sound vibrations from reaching the inner ear

• inability to process sound coming in at initial

• no flow to the ossicles

sensorineural deafness

• there is a problem with the structures—especially the cochlea—that converts sound vibrations into neural activity and project to the brain

• no flow to the cochlea (deficit)

central deafness

• damage to auditory brain structures can affect hearing in various ways

• now flow to the brain

extreme acoustic trauma

• repeated trauma can cause permanent and profound hearing loss or deafness