Audition Flashcards
How do sound waves form?
object vibrates, compressing air and withdrawing it creating pressure waves of compressions and rarefactions
What is the relationship between frequency and pitch?
log relationship
double frequency increases pitch by 1 octave
what is the relationship between amplitude and loudness?
double amplitude increases loudness by 6 dB
What are broadband vs narrowband sounds?
broadband: contain energy across a large range of frequencies
narrowband: most energy concentrated within a small range of frequencies
describe the structure of the outer ear
composed of the cartilaginous pinna + external auditory canal
it is separated from the middle ear by the timpanic membrane
what is the function of the outer ear?
collects and funnels sound
filters sounds, with some features being attenuated and some amplified, depending on the direction from which the sound enters the ear
which frequencies are most boosted by the pinna?
sound with frequency 2-4kHz
How can you discern where sounds come from?
sounds off to one side arrive earlier at one ear than the other and are louder (ITDs and ILDs)
What is the composition of the middle ear?
air filled cavity between timpanic membrane and the inner ear, connected to the back of the throat
what is the function of the middle ear?
transmit sound from timpanic membrane to inner ear in a way that minimises the loss of energy
how does the middle ear accomplish the transmission of sound from air to water?
impedance matching
what is acoustic impedance?
[measure of opposition to acoustic flow]
a measure of the opposition that a system presents to acoustic flow resulting from an acoustic pressure applied to it
why is impedance matching necessary in the middle ear?
the acoustic impedance of water = higher than air so water requires more energy to vibrate it using sound waves
amplification is required
how does the middle ear achieve impedance matching?
collects the sound pressure of a large area of the timpanic membrane (60mm^2) and concentrates it on the much smaller area of the stapes (3mm^2)
the lever arm of the malleus is longer than the incus, causing a further pressure increase of 1.3x
what is the order of transmission of vibration from the eardrum to through the ossicles?
eardrum -> malleus -> incus -> stapes -> oval window of inner ear
how do the ossicles protect the ear from continuous loud sounds?
stapedius reflex
there are 2 small muscles (stapedius and tensor tympani) connected to the malleus and stapes that, when they contract, reduce the mobility of the ossicles
they can transmit less of the vibration from the eardrum to the inner ear
why can the ossicles not protect the ear from sudden loud sounds
the reflex is too slow
what is the composition of the inner ear?
fluid filled chambers
semi-circular canals = vestibular system
cochlea = coiled tube enclosed in bony shell
what happens to the frequencies detected when the source of the sound is 45 degrees above the plane of the ear?
lower frequencies amplified more
higher frequencies attenuated
differences are large enough to for us to use them to localise sound along elevation dimension (spectral cues)
what are the 2 fluids of the cochlea?
what else does the cochlea contain?
endolymph and perilymph
very sensitive neuroreceptors : hair cells
Where is the perilymph found?
scala vestibuli and scala tympani
what is the composition of perilymph?
contains Na+
where is the endolymph found?
scala media
describe a cross section of the cochlea
top compartment of perilymph = scala vestibuli, Na+
Reissner’s membrane
middle compartment of endolymph = scala media, K+, +80mV
Basilar membrane
bottom compartment of perilymph = scala tympani, Na+
what causes the composition of endolymph? how is this used?
it is more positively charged as has a higher [K+], caused by the stria vascularis allowing K+ to leak into the endolymph
generates electrical voltage gradient: endocochlear potential
What sits on the basilar membrane? What is its purpose?
the Organ of Corti: contains the hair cells which transduce sound + cochlear movement into electrical signal that is passed on to auditory nerve
What happens when the stapes pushes on the oval window?
increases pressure in the fluid-filled scala vestibuli (and other chambers) and causes waves in the fluids of the cochlea
causes round window to bulge out and causes waves in the basilar membrane
What are the mechanical properties of the basilar membrane?
narrow and stiff at basal end near the oval window
wide and floppy at apical end
[ stiffness decreases from base to apex ]
[ Von Bekesy experiments]
Why do waves reach their peaks at different positions on the basilar membrane?
due to differences in thickness along the length of the BM and the inertial gradient caused by the fluid of the cochlea
position of peak depends on frequency of sound
which frequencies are preferred at each end of the BM?
high frequency sounds move the BM most at its basal end
low frequency sounds move the BM most at its apex
How does the BM establish a place code for frequency?
by vibrating in different places depending on the frequency of the sound, BM achieves an analysis of the frequency COMPONENTS of the sound and establishes the place code for frequency
this is the basis of ttonotopy
what is tonotopy?
when sounds with similar frequency content are processed in topographically neighbouring regions of the auditory nerve and brain
it is maintained throughout the auditory system
How are the movements of the BM converted to electrical signals?
by the Organ of Corti
when the BM vibrates up and down, the organ of corti moves with it
OoC has 3 rows of OHCs and 1 row of IHCs touching the tectorial membrane above
on top of the hair cells is the tectorial membrane, which slides sideways over the hair bundles (stereocilia) that stick out of the top of the hair cells
when the BM goes up, hair bundle deflected by TM 1 way, when BM goes down, bundle goes other way
How are the stereocilia and the hair cells structured?
about 3-5 micrometres long
connected by ‘tip links’
hair cell bodies (sit in perilymph and) have RMP of -50mV
stereocilia sit in endolymph which has +80mV
How does movement of the bundles translate to neurotransmitter release?
pushing bundle towards longest stereocilium causes tension on tip links
pushing it away will release the tension
tip links are linked to stretch-sensitive K+ channels that allow K to flow from endolymph when bundles pushed towards longest SC
K flows into the hair cells causing depol and the opening of v.g. Ca2+ channels
increases the probability of Glut release onto ANF (spiral ganglion)
what is an AC hair cell membrane potential response?
at low frequencies
membrane potential of hair cell follows every cycle of stimulus
what is a DC hair cell membrane potential response?
at high frequencies
membrane potential cannot follow individual cycles, instead remains depol throughout duration of stimulus
How does an outer hair cell transfer to DC response mode?
there is slight asymmetry in the effects of displacing stereocilia either way
opening K channels can depolarise the membrane more than closing them hyperpolarises it
: neuron keeps firing
What happens as AC is injected into hair cell?
hair cell contracts and expands, corresponding to the AC frequency
What does the motor protein prestin cause OHCs to do?
shorten by 4% every time they are depolarised and lengthen when they are hyperpolarised
What does the contraction and relaxation of OHCs serve to do?
localised amplification of movement of BM, causes stereocilia to be deflected more, causes more depolarisation, causes further contraction of OHCs
causes greater sensitivity, for lower intensity sounds, and sharper frequency tuning
weak stimuli amplified more effectively than strong ones
what is the order of conveying sound information to the auditory cortex?
sound > cochlea > 8th cranial nerve > cochlear nucleus > superior olivary complex > nuclei of the lateral lemniscus > inferior colliculus > medial geniculate body (thalamus) > auditory cortex
what is the principal auditory structure of the midbrain?
inferior colliculus > it receives input from the cochlear nucleus, superior olivary complex and nuclei of lateral lemniscus
how do hair cells work? do they send action potentials?
have no axons/dendrites and don’t fire APs but form glutamatergic synapses with neurons of teh spinal ganglion
what are spinal ganglion neurons and where do they connect to the brain?
have long axons (auditory nerve fibres) that travel through the auditory nerve to connect to the cochlear nucleus
what are the properties of inner hair cells?
thick, myelinated, fast transmission
10x-20x more than OHCs
has 10-20 ANFs innervating it
what are the properties of outer hair cells?
thin, unmyelinated slow transmission
only minor role in auditory processing
has to share ANFs with other OHCs
amplify quiet sounds and sharp the tuning curve through prestin motor
what are the efferent innervations to the hair cells?
neurons sent down from SOC, contact hair cells, protect from high intensity sounds + ‘turn up’ sound for improvement of detection
how do our ears express loudness?
on a log scale with decibels for units
smallest pressure = 20 micropascals = 0 decibels
define sound intensity?
the amount of power carried by sound waves per unit area
how is sound coded for in neurons? what increases discharge rate?
different auditory fibres fire in different thresholds, distinguished by their spontaneous firing rate
all increase their discharge rate with increasing sound intensity
what are low spontaneous fibres?
have higher thresholds and saturate firing rates at very high sound levels (dB)
what are intermediate spontaneous fibres?
have an intermediate threshold and saturate at an intermediate sound intensity
what are high spontaneous AN fibres?
have the lowest thresholds and are saturated by 40 dB
how do differences in ANF sensitivity allow for population coding of sound intensity (volume)?
each ANF on its own has a dynamic range that covers part of the total auditory range
to process sound intensity, the brain takes into account the firing rates of many neurons with different sensitivities that together will encode the entire range of sound levels (dB)
what coding is used for low frequency sounds?
phase locking
what coding is used for higher frequency sounds?
place coding
what determines whether a given nerve fibre responds to a sound of a particular frequency?
if the cochlear connects to the hair cells at the basal end: ANF responds to higher frequency
if connects at the apical end: ANF responds to lower frequency
as basilar membrane vibrates at different positions depending on sound frequency
what effect does sound level have on the range of frequencies detected?
at low sound level, range is smaller
at high sound intensity, larger areas of the basilar membrane vibrate, so more easily picked up
what is does the triangle of the frequency response graph indicate?
that neuron’s frequency response area/ the frequencies it is tuned to
what does tonotopy mean for tones close to each other?
tones close to each other (in terms of frequency) are represented in topologically neighbouring regions in the brain
how do the mechanical properties of the basilar membrane relate to tonotopy?
different frequencies cause neighbouring parts of the membrane to vibrate, these spatial positions are preserved in neighbouring ANFs, the auditory nerve and auditory system
major organisational principle of the AS
which ANFs lie anatomically next to each other in the AN ?
those with overlapping tuning curves
(from 100Hz to 40kHz)
what is phase locking?
membrane potential fluctuations of hair cells follow movement of BM very closely
ANFs fire APs: the sequence of APs fired due to a stimulus waveform isn’t random, the spikes occur at particular phases of the waveform
spikes occur at a phase of the waveform close to one of the cycling peaks
intervals between spikes encode general features of the sound stimulus
why can we not fire APs at an infinitely high rate?
refractory period = >1ms
what is the maximum sustained firing rate?
600 Hz
beyond which becomes difficult to fire at every cycle of a stimulus
what happens beyond sustained firing rates of 600 Hz?
ANFs don’t fire at every cycle of the stimulus (stochastic)
nerve fibres are combined to learn the temporal fine structure of sound due to temporal distribution of spikes
allows interpretation of frequencies above the phase locking limit of an individual neuron
what limits the ability of temporal information to be encoded through temporal distribution of spikes?
the AC limit of the hair cell membrane
(the potential fluctuations of the hair cells)
where does AC response stop? why? what does this mean for sound perception?
above 3 kHz : membrane potential on hair cells doesn’t follow individual cycles of the stimulus anymore
frequencies become encoded in place code not temporal code
different neurons tuned to different frequencies are tonotopically ranged within the brain at different places
what neural codes are used for sound intensity?
rate and population
how are ANFs frequency tuned?
firing (discharge) rate depends on amount of acoustic energy at or near the neuron’s characteristic frequency
tuning occurs due to basilar membrane mechanics
what neural codes are used for sound frequency?
temporal (phase locking)
place (tonotopy)
which part of the CN does the BM attuned to high frequencies (basal) project to?
medial regions of anteroventral and posteroventral
which part of the CN does the BM attuned to low frequencies (apical) project to?
lateral regions
what are the purposes of having different cell types in the CN?
each extracts dif aspects of acoustic information and passes it to different points in the auditory pathway
what cells are in the anteroventral CN?
globular + spherical bushy cells
what are the properties of globular + spherical bushy cells?
small number of large excitatory synapses with primarylike responses:
firing patterns almost identical to firing patterns of ANF innervating them
preserve temporal information contained in phase locking of ANFs + project to SOC
what cells are in the posteroventral CN?
octopus + multipolar cells
what are the properties of octopus + multipolar cells?
O: fire single AP in response to a single total firing pattern from ANF
MP: have multiple ANF inputs on the dendrites
show ‘chopper response’ with regular rhythmic bursts, size of which is unrelated to stimulus
what cells are in the dorsal CN?
pyramidal cells
what are the properties of pyramidal cells?
‘pauser response’
fire strongly on onset of sound followed by inhibitory period and then increased firing rate (similar to multipolar)
which cells are responsible for inhibition in the CN?
type IV and V (pyramidal)
why does lateral inhibition occur in the CN cells?
multipolar cells give tuning input to other cells, which suppresses firing rates below spontaneous rates
so if a cells picks up a sound that occurs at a certain volume and frequency, firing rates from that cell will be suppressed
what happens at the SO nuclei?
first stage of binaural convergence
what does the medial superior olive receive?
excitatory input from CN of both ears
when does the MSO fire strongly?
only when receiving direct, excitatory, temporally coincident input from both ears
what does the lateral superior olive receive?
excitatory input from ipsilateral CN and inhibitory input from contralateral CN
what is the layout of the inhibitory connection from the contralateral side to the LSO?
contralateral CN sends excitatory projection to medial nucleus of trapezoid body and synapses with inhibitory neurons at the calyx
whereas ipsilateral CN has no inhibitory synapse so arrives at the LSO first
what are the 2 types of binaural localisation cues?
ITDs: interaural time differences
ILDs: interaural level differences
ILDs: what will be the LSO neuron response when the sound intensity on the ipsilateral side is greater than the contralateral side?
very large response
ILDs: what will be the LSO neuron response when the sound intensity on the contralateral side is greater than the ipsilateral side?
very small response
what does the inhibitory connection in the MNTB (at calyx) prevent from happening?
prevents LSO neuron firing if the sound in the contralateral ear is louder than in the ipsilateral ear
LSO neuron doesn’t fire, conveyed to IC
what happens to ILDs at higher frequencies?
they become larger and more complex
(and more informative)
why do ILDs not occur so much at low frequency?
low freq sounds can ‘wrap themselves around your head’ without being so attenuated as the wavelength is large enough
what would the ILD be at 700 Hz
+-15dB
what would the ILD be at 11000 Hz
+-40 Hz
which area of the brain processes interaural time differences?
medial superior olive
when does an LSO neuron fire strongly?
when a loud sound is received in the ipsilateral ear and a quiet sound in the contralateral ear
how do we ensure efficient transmission of temporal information to the MSO for ITD processing?
ANFs enter AVCN and neurons project from there to MSO through endbulb of Held synapses
large synapses that operate with extremely high temporal precision
what is the transmission pathway from ANF to MSO for ITD info?
ANF > endbulb of Held > triggers AP in bushy cell > precisely reflects the timing info from the 2 ears to the > MSO
how do different MSO neurons encode different ITDs?
MSO neurons only fire maximally when there’s coincident excitation from both sides
dif MSO neurons receive input from delay lines of different lengths and thus encode different ITDs by comparing the difference between ipsi and contra ears
in what cases will MSO neurons fire maximally?
if the input from either ear is delayed by some amount (as afferent axons are longer) then will fire maximally only if the ITD exactly compensates for the transmission delay
(‘allow A and E to combine to make ideal C’)
why are we not good at extracting ITDs from high frequency sounds?
ANFs cannot phase lock onto high freq stimuli and thus fine temporal info required for ITD processing cannot be extracted
are unaided ITDs smaller in cats than humans?
yes
0.3ms vs 0.7ms due to head diameter
what is the function of the inferior colliculus?
connects + integrates all auditory brainstem nuclei
obligatory relay for all ascending auditory information to thalamus + cortex
what is the organisation of the IC?
central nucleus is tonotopically organised
shell (dorsal + external cortex) is not?
what sort of integration may be happening in the IC?
acoustic info with contextual info i.e. less about analysing sound properties
(no unifying theory)
what else modulates IC neuron activity?
behaviour
neuron firing varies with speed at which mouse runs, although IC is not interested in motor output signals it is involved in integration
allows IC to make sense of acoustic input in context it is experienced
where does the main projection from the IC go?
medial geniculate body (thalamus)
what part of the MGB receives input from the central nucleus of the IC?
ventral
also tonotopically organised
what determines whether an auditory thalamic neuron responds to acoustic stimulation?
whether there is simultaneous stimulation of another sensory modality (aspect of a stimulus)
what is the example of MGB integration with a mouse?
response to an isolated noise burst is increased by ~50% if the whiskers also move
multimodal integration
how is the auditory cortex organised?
primary (A1) tonotopically
secondary and higher : less so
what are examples of the complex response properties of auditory cortical neurons
neurons can signal mismatches between expected sensory feedback from vocalisation and actual sensory feedback : tune out vocal production
what is the experiment to back up the idea of signal mismatches to eliminate vocalisation?
e.g. Eliades + Wang 2008 : when frequency of monkey’s voice is shifted and played back, firing rate of neurons increases: neurons signalling mismatches
what is the key role role of the auditory cortex?
integrate contextual and auditory information to make sense of acoustic input, even more so than IC & SC
cortical circuits are plastic therefore cortex essential for perceptual learning
how is IC related to SC?
a map of auditory space in the deep layers of the SC is aligned with a map of visual space found in the upper layers
where do the descending projections from the auditory cortex target ? where do they originate from?
L5 and L6
target thalamus, IC and brainstem nuclei (mostly the shells)
[might control and gate the flow of information to subscortical structures]
through which loop does the somatosensory system impact auditory activity in thalamus + cortex?
cortico-colliculo-thalamo-cortical loop
inhibitory interconnected neurons receive input from S1, suppressing AC activity due to somatosensory activity
how does place coding work above 3kHz?
different neurons tuned to different frequencies are tonotopically ranged within the brain at different places
