Sound Perception Flashcards
What frequencies can we hear sounds ranging between?
20 and 20,000 Hz
20 times per second to 20,000 times per second
We are good at detecting soft sounds, perceiving very small differences
How many pure tones can we differentiate up too?
1500 pure tones
What is the main function of the outer ear?
Captures sound signals
Amplifies mid frequencies
Vertical direction coding
What is the main function of the middle ear?
Protection
Impedance matching
What is the main function of the inner ear?
Frequency analysis
Transduction
What are the 2 sections of the outer ear?
Pinna
Meatus
What does the pinna do?
Affects high frequency sounds by interference between the echoes reflected off its different structures - different frequencies are amplified by different amounts depending on the origin of the sound in the vertical place - brain interprets these changes as direction (if sound is coming from the floor or air)
What does the meatus (ear canal) do?
Links the pinna to the ear drum
Resonates at around 2kHz so that frequencies in that region are transmitted better to the cochlea - corresponds to human speech frequency region
What does the middle ear consist of?
Three ossicles (tiny bones)
Malleus - hammer
Incus - anvil
Stapes - stirrup
What do the three ossicles do?
Transmit vibrations from ear drum to the smaller oval window - the ear drum is much larger than the window so ossicles serve as amplifiers
What is impedance matching?
It enables air vibrations (sound) to be efficiently transformed into fluid vibration - turns a large amplitude vibration in air into a small amplitude vibration into fluid
What would happen without a middle ear?
Most of the sound would bounce off the cochlea (inner ear)
What is the function of the inner ear: the cochlea?
To transform a mechanical signal into neural responses in the 8th cranial nerve
Snail shaped, three chambered tube
What are the chambers in the cochlea separated by?
The basilar membrane, on which sits the organ of corti
What does the arch in the middle of the organ of corti do?
Separates the inner hair cells from the outer hair cells - on top of these sits the tectorial membrane which is attached along its inner edge
What are the stereocillia?
Outer hair cells
Where are the stereocillia embedded?
In the tectorial membrane
Where are the inner hair cells embedded?
No where, they are free and moved by movement of the fluid (endolymph) that fills the space between the hair cells and tectorial membrane
What is endolymph?
Fluid which fills the space between the hair cells and tectorial membrane - bends the hairs
What happens when inner hair cells are bend towards the stereocilium?
The cells voltage is increased
more neurotransmitter is released
auditory nerves connected to the hair cell increase firing rate
What happens when inner hair cells are bend away from the stereocilium?
the cells voyage is decreased
less neurotransmitter is released
auditory nerves connected decrease their firing rate
What is the function of inner hair cells?
Turn mechanical movement of the basilar membrane into neural firing of the auditory nerve (transduction)
What do the inner hair cells excite?
The synapses of fibres of the afferent (ear to brain) auditory nerve - each of the 3,000 inner hair cells have 10 auditory nerves attached to it with different diameters, thresholds and rates
What is the role of the outer hair cells?
Sharpening
the outer hair cells have synapses from the efferent (brain to ear) fibres, so the brain controls the mechanical properties of the ear
they exhibit motility - act as tiny motors that amplify the mechanical movement of the basilar membrane
responsible for high sensitivity (low thresholds) and sharp turning (high frequency resolution) or normal hearing
What does motility mean?
The outer hair cells act as tiny motors that amplify the mechanical movement of the basilar membrane
Where does hearing loss come from?
Damage to the outer hair cells
What does sound produce?
A travelling wave down the basilar membrane
What do different frequencies of sound give?
Maximum vibration at different places along the basilar membrane
The cochlea - base and apex
Near the basis it is wider and narrower at the apex, but inside the basilar membrane the basis is narrow and stiff while towards its apex it is wider and more flaccid - opposite
Where does the vibration to high frequency tones peak?
At the base of the membrane
Where does the vibration to low frequency tones peak?
At the apex
What does the place of maximum vibration indicate?
The frequency of the sound wave
The place theory of frequency analysis
Neurons have different characteristics depending on their position along the basilar membrane - high frequencies only excite neurons near base, low excite frequencies near the apex
Place code - tonotopic organisation of the cochlea
if 2 complex sounds, the ear can separate these by reporting where there is maximum reflection at basilar membrane
The temporal - phase locking theory of pitch perception for pure tones
Competing theory
Auditory nerves connected to inner hair cells fire in phase with the stimulating wave form - independent of position. When amplitude increases, it fires. Intervals between spikes tend to be numbers of cycles - closely spaced when the frequency is high. The brain times these intervals to estimate the frequency of the tone
Problem with the phase locking theory
Only works for low frequencies - less than 5kHz in humans
Coding of intensity
The louder a sound is, the more frequently the auditory nerve fires
Each inner hair cell has about 10 auditory nerves attached to it - different nerves have different thresholds
most have low - saturate at normal sounds
the minority have higher thresholds - don’t saturate un till sounds are loud
