Lecture 17: Audition - how do we hear

Describe how sound is created/behaves

• changes in sound/air pressure experienced perceptually as sound

• sound moves forward (compression) and backwards (rareification)

• compression represents high air pressure - peaks

• rarefication represents low pressure - troughs

What are the three features that account for the perception of sound

Amplitude - extent of rareifcation + compressions

• accounts for loudness

• greater difference in rarefication + compression means more loud sound

frequency - number of cycles of rarefication + compression per second

• accounts for pitch - high (high frequency) or low (slow)

• complexity/timbre - complex sounds have more combinations

• The perceptual dimensions due to processing in brain however originates due to differences in physical properities of sound waves

What constitutes the outer, middle and inner ear.

outer ear:

• ear +pinna + ear cannal

• sound travels and hits pinna

Sound travels through ear canal - End of ear cannal is the eardrum (tympaic membrane) - behind eardrum is middle ear

middle ear

• ossicles

• stapes/stirrups

inner ear

• cochlea (has oval window and round window) - contains fluid

• loops things - critical for vestibular (balance) response - contains fluid

Describe how sound reaches the inner ear.

• The forwards and backwards motion of sound, causes the eardrum to resonate (also move backwards and forward in the same way the sound was produced)

• Eardrum moving causes bones (ossicles) to lever backwards anf forwards

• stapes or stirrups

  • connected to ossicles - moves backwards and forwards in response against the oval window

    • As oval window is soft (as compared to rest of cochlea), can cause the membrane to move inward

    • Inside cochlea there is fluid - so when stapes moves forward, the fluid moves along the cochlea (around the basilar membrane from the base to the apex) and come back down to the round window  (at the base)

      • Round membrane is soft - so allows fluid to move backwards (back toward base) as well

      • thus through windows- fluid moving backwards + forwards

Describe the basiliar membrane and how it responds differently to a particular physical characterisitc of sound.

• spiraling membrane (membrane follows spiral of cochlea)

• fluid moves along here from base to apex

• higher frequency causes more vibration at base (near oval window) vs low frequency at the apex

• more vibration = more hair cell stimulation = more spiral ganglion stim. at this location

• each spiral ganglion can resond to a range of frequencies, they will respond most to their preffered frequency at the specific location

• known as place coding

Describe place coding

• High frequency causes more vibration at the base whereas low frequency causes more vibration at the apex

  • More vibration -> more response from spiral ganglion cells at this location

    • the brain can use the location of the spiral ganglion cell (with most activity) to determine the frequency of the sound

• Each spiral ganglion cell will respond to a range of frequenices, but has a preffered

• Increasing intensity of sound, has a higher response at all frequencies, BUT the peak/preffered frequency does NOT change

• Relative output -> how much of each frequency there is

• Total activity across all cells -> tells us how intense the sound is

i.e. basiliar membrane INFORMS frequency of the sound to the brain!