lecture 8 - auditory perception Flashcards
what is sound?
Sound is caused by movement / vibration of an object.
The movement alternately squeezes air molecules together and pulls them apart.
Creates a longitudinal pressure wave in the air.
how do we represent sound?
High points of pressure - molecules are squished up together
what are properties of sound?
sound intensity level and perception of loudness
what are physical properties?
amplitude, period
what are derived properties?
intensity and frequency
what are perceptual effects?
loudness and pitch
how are properties of sound represented?
as a spectrum
what is timbre?
Refers to that quality which can make two sounds with the same pitch and loudness seem dissimilar.
Sound quality, tone.
Related to complexity.
what is in the outer ear?
has a perceptual effect
visible part of the ear - auricle and pinna
eardrum
what is in the middle ear?
eardrum - ossicles - oval window
what is in the ossicles?
Hammer, anvil and stirrup.
why do we need the middle ear?
Funnelled down through the ear cannel - dont just get vibrations straight into the inner ear
what is the inner ear?
cochlea
works as a frequency analyser and works as a transducer
what does the inner ear do?
Breaks sounds down into components, converts mechanical energy into electrical activity
what is the cochlea?
Stapes connected to the overall window - 2 fluid filled chambers separated by the cochlear partition filled with perilymphic fluid - sets up a series of vibrations - vibrations travel up and down, round and back
what is the transduction process?
BM moves in response to vibrations in the perilymph.
BM vibrates at the same frequencies as the incoming sound!
These vibrations bend the stereocilia of the inner hair cells.
Allows positively-charged ions to enter the cell.
Triggers the release of neurotransmitters and an electrical signal is sent to the brain.
what is place coding?
Different resonant frequencies at different points
Points of maximum BM displacement => frequencies of incoming sound.
Stimulates specific sets of inner hair cells.
Activates specific auditory nerve fibres.
what is temporal coding?
BM moves in response to vibrations in the perilymph
Stereocilia stimulated by peaks in BM vibration.
Firing occurs at the period of the incoming waveform
what is the difference between place vs timing?
place - where firing is coming from
temporal - when firing occurs
when is place coding less reliable?
low frequencies
what do inner hair cells do?
detects motion on the BM
what do outer hair cells do?
amplifies motion of the BM
what is intensity coding?
Relies on the fact that there are low- and high-threshold auditory nerve fibres.
Low-threshold fibres discriminate quiet and moderate (low- vs. medium-amplitude) sounds.
High-threshold fibres kick in to discriminate moderate and loud (high-amplitude) sounds.
The loudness of a sound ≈ total neural activity.
what is the auditory pathway?
Cochlear nucleus, superior olive, inferior colliculus, medial geniculate and primary auditory area
what does the cochlear nucleus do?
Sends neural activity to other nuclei in the brainstem
what does the superior olive do?
analyses location
what does the primary auditory area do?
analyses higher-order features
why do we need to localise sound?
The location itself may be important.
It may orient our attention to other things.
Helps with source separation
what is the location of the azimuthal plane?
sounds from the left - reaches the left ear first
what is interaural timing difference?
a sound off to the side sounds diff to those that are in front of us
what is interaural level difference?
When the sound is directly from the side
Sound diffracts (bends) around objects smaller than its wavelength.
Sound is blocked by objects larger than its wavelength.
do low frequencies diffract around the head?
yes - high frequencies do not
why does the pinnae have a funny shape?
amplifies some of the frequencies – changing the timbre of the sound slightly
what is the neural coincidence model?
Sound coming directly infront of you – specific neuron fires and tells you where sound is coming from as it hits each ear at the same time
what is the opponent process analysis?
system calculates the difference between the two sets to work out where a sound is coming from.
how do we make speech sounds?
Larynx – voice box – force air from our lungs up through the holes creating sound
Shape the spectrum of the sound – filter sound coming from our larynx, tongue, teeth, lips
what are top down effects?
Speech perception relies on more than the acoustic signal!
Top-down (cognitive) as well as bottom-up (perceptual) factors
what is coarticulation?
The “same sound” is actually different depending on the acoustic context (neighbouring sounds)
what is the McGurk effect?
Perceptual illusion: visually-presented sounds affect perception of the acoustic signal.
what is linguistic knowledge?
Our perception of speech sounds is affected by the meaning of the context.
