Audition Flashcards
What actually is sound
Vibrations of objects set up pressure waves in the surrounding air
What allows sound to travel
The ‘elastic’ property of air allows pressure waves to propagate
What are sine waves
Produced by pure tones, (waveform made of a single frequency) wave frequency is directly related to pitch and amplitude is directly related to perceived loudness
What is the frequency range of sine waves
20-Hz-20kHz (10 octaves)
What is the pressure range of sine waves
20uPa- 10^8 uPa (0-140dB)
What scale do octave and decibels follow
Logarithmic scale- every doubling of frequency increases pitch by one octave, every doubling of amplitude increases loudness by 6dB
Are most sounds pure tones?
No- most real-world objects vibrate at multiple frequencies
What is Fourier analysis
A mathematical procedure that makes it possible to represent waveforms as the sum of sine waves
How does the ear conduct Fourier analysis
It decodes sound into its frequency (sine) components
What is the (Fourier) spectrum
A frequency domain description of a waveform- states the amplitudes of the cosine components of the waveform rather than describing the waveform as a function of time
What are ‘narrow band’ sounds
Sounds in which a relatively small no of components contain most of the energy eg pure tone is an extreme example
Properties of ‘narrow band’ sounds
More or less periodic, may evoke an identifiable pitch
What are ‘broad band’ sounds
Contain very many components of similar amplitudes eg clicks, noises
Properties of ‘broad band’ sounds
Often don’t evoke a strong pitch
When does the Fourier spectrum work as a complete description of a soudn
Only if the frequency composition of the sound is constant over time- yet most natural sounds vary with time
What is a spectrogram
Produced by dividing sounds into short time segments, and spectra calculated for each time segment in turn
How are spectrograms relevant to the auditory system
You could say the job of the ear is producing a spectrogram of incoming sounds like a ‘neurogram’, and the brain performs further spectro-temporal analysis of the ‘neurogram’ to instantly identify sounds
What do auditory nerve fibre discharge rates depend on
The amount of soudn energy at or near the neuron’s characteristic frequency
What does the external ear consists of
Pinna, external auditory canal
What separates the external and middle ear
The eardrum/tympanic membrane
How does the pinna filter sound
Its convolutions filter sound according to the direction it enters the ear eg higher frequency sounds can enter the auditory canal more effectively when they come from an elevated source than at a lower level
What is the consequence of the pinna filtering sound based on its direction
Subtle differences in amplification between vertically low vs high high-frequency sounds allows vertical localisation of sound
How does the pinna ampify sound
Pinna collects sound and acts as a funnel, certain features of sound are amplified while others are attenuated before they enter the ear canal
What frequencies of sound are amplified by the pinna
Pinna amplifies frequencies around 2-4kHz, the range most human speech sounds fall into, 30-100 fold (Purves et al,2001)
What is the middle ear
Air filled cavity between the tympanic membrane (eardrum) and inner ear, contains 3 ossicles
What are the 3 ossicles of the middle ear
Malleus, incus, stapes
What other part of the head is the middle ear connected to
Connected to the back of the throat by the Eustachian tube
What is the role of the middle ear
Transmit sound vibrations from the tympanic membrane to the oval window of the inner ear in a way that minimises energy loss
Why are sound waves not transmitted through the air of the middle ear
A lot of energy would be lost and reflected away from the oval window when the sound waves reached it (99.9%), due to the greater acoustic impendance of the cochlear fluid that exerts pressure at the back of the oval window
What 2 mechanisms does the middle ear use to amplify sound to minimise energy loss and allow impedance matching
Ossicles, difference in SA
What 2 muscles are attached to the malleus and stapes
Stapes muscle (stapedius) and malleus muscle (tensor tympani)
What do the ossicles do to achieve impedance matching
Ossicles act as levers to transform the large movements of the tympanic membrane into smaller but stronger vibrations at the oval window, plus the lever arm formed by the malleus is slighlty longer than that of the incus, increasing pressure by a factor of 1.3
How is impendace matching achieved in the middle ear by SA differences
The sound pressure across the relatively large eardrym is concentrated on the much smaller oval window
By how much does our middle ear increase our sensitivity to sound
By about 30dB, the same amount that would otherwise be lost- pressure at the oval window is 20x greater than at the tympanic membrane
How can the middle ear muscles have a protective function
Attenuation reflex- can protect the ear from damage by very large sounds, protect delicate structures from damage (however, 50-100msec delay means it can’t protect against sudden loud sound eg explosions)
How can the Stapedius reflex allow filtering of sound
Suppresses intense, low-frequency sounds or continuous sounds, or when we vocalise, allowing us to discern high-frequency sounds in a noisy environment eg speech, and prevent us hearing our own voice too loudly
What comprises the inner ear
Fluid filled chambers- semi circular canalds, and a coiled tube in a hard bony shell aka the cochlea
What 2 fluids does the cochlea contain
Perilymph and endolymph
How long would the cochlea be if uncoiled
30 mm long
Where is perilymph located
Scala vestibuli and scala tympani
Where is endolymph located
Scala media
What is the ionic composition of perilymph
Main ionic component is Na+
What is the ionic composition of endolymph
Contains lots of K+ due to it leaking in from the stria vascularis, much more positively charged than perilymph
What separates the scala media and scala tympani
Basila membrane
What is the organ of corti
Sits on the basilar membrane, contains hair cells that connect to the auditory nerve leaving the cochlea
What is the result in the cochlea of the stapes pushing the oval window membrane inwards
It increases pressure in the fluid-filled space of the cochlea in waves, causing the round window to bulge out and the basilar membrane to move in waves (von Bekesy)
What is the effect of complex sounds on the basilar membrane
Cause deflections at several positions along the membrane as they contain multiple frequencies
How many hair cells are in the human ear
15-20,000
How does the basilar membrane have a place code for frequency
By vibrating in different places depending on the frequency of the sound, the membrane achieves an analysis of the frequency components of sound- tonotopy
How does structure of the basilar membrane change across it
Stiffness decreases as you move from base to apex, and gets wider (like a flipper)
At which end of the basilar membrane do high frequency vs low frequency sounds cause vibration
Stiff base- high frequency
Less stiff apex- low frequency
The position of the peak of the travelling wave depends on the sound frequency
How does movement of the basilar membrane cause movement of the hair cell bundles
Up-down movement of the basilar membrae causes the tectorial membrane to slide sideways over the membrane, causing a sideways displacement of the stereocilia on the cochlear hair cells
What is the tectorial membrane
A membrane covering the organ of corti
How does displacement of the hair cell bundle cause the opening of K+ channels in the hair cells
Movement of hair cell bundles changes the tension on tip links joining the stereocilia of the cell, opening/closing stretch sensitive K+ channels
What is the result of opening K+ channels in hair cells
Influx of K+ from the endolymph, causing a depolarisation of the hair cell membrane, which opens voltage-gated Ca2+ channels and increases probability of glutamate release
What do hair cells release glutamate onto
Auditory nerve fibre
What is movement of the basilar membrane directly related to
Deflection of stereocilia, amount of K+ influx, amount of membrane potential depolarisation
What do hair cells do instead of firing APs
Change their membrane potential to release glutamate upon stereocilia deflection
What is an AC response for hair cells
At low frequencies, the hair cell membrane potential follows every cycle of the stimulus
What is a DC response for hair cells
At high frequencies, membrane potential is unable to follow individual cycles, so instead remains depolarised throughout duration of stimulus
Why does DC mode response for hair cells come about
Due to a slight asymmetry in effects of displacing stereocilia- opening channels can depolarise the membrane more than closing them hyperpolarises it (because only a small proportion of stereocilia channels are open at rest)
How can the relationship between membrane potential and sound input be tested experimentally
Sticking an intracellular recording electrode into a hair cell and measuring its membrane potential
How does the outer hair cell change in response to hyperpolarisation and depolarisation
It changes its length in resposne to depolarisation as depolarisation activates prestin
What mediates the length change of OHCs
A motor protein called prestin in the OHC membrane
What is the result of OHC motility
Produced localised amplification of the basilar membrane motion, leading to higher sensitivity and sharper frequency tuning
Also a source of non-linearity as weak stimuli are amplified more effectively than strong ones
The basilar membrane (in the way it uses place coding) acts as a mechanical…
Frequency analyser
What do the auditory nerve fibres do after leaving the cochlea
The auditory nerve fibres join the 8th cranial nerve and branch, terminating in the anteroventral and dorsal cochlear nuclei
Where do the cochlear nuclei project to
The ipsilateral and contralateral superior olivary complex
What does the nuclei of lateral lemniscus receive input from
The cochlear nucleui and superiori olivary complex (2 previous structures)
What is the principle aufitory structure of midbrain
Inferior colliculus
Where does the inferior colliculus receive input from
All brainstem nuclei (previous structures)
Where does the medial geniculate body receive intput from
Inferior colliculus
What is the medial geniculate body
The auditory part of the thalamus
What does the auditory cortex receive input from
The medial geniculate body
Summary of auditory pathway
Cochlea -> cochlear nuclei -> superior olivary complex -> nuclei of lateral lemniscus->inferior colliculus ->medial geniculate body-> auditory cortex
How many rows of outer hair and inner hair cells are there
3 rows of OHCs, 2 rows of IHCs
What nerves form the axons that travel through the auditory nerve to connect hair cells to cochlear nucleus
Spiral ganglion neurons
What are the 2 types of auditory nerve fibres that inner vs outer hair cells connect to
IHCs- type 1 fibres
OHCs- type 2 fibres
What are the properties of type 1 AN fibres
Connect to IHCs, myelinated, thick, fast signal transmission, form more specific connections
What are the properties of type 2 AN fibres
Connect to OHCs, unmyelinated, slow, play a minor role in auditory processing
What is the relative no of type 1 and type 2 AN fibres
Type 1 fibres outnumber type 2 fibres by a factor of 10
How many fibres are IHCs typically innervated by
10-20 type 1 fibres to one IHC
How many fibres are OHCs typically innervated by
6 type 2 fibres connect to each OHC, typically have to share each fibre with 10 other OHCs (so less specific info)
What axons does the superior olivary complex send down to connect with earlier structures
SOC sends axons connecting with AN dendrites and OHC- likely function is protection against damage from high intensity noise and improvement of signal-noise ratio
What power does 100dB SPL (sound pressure level) exert
10mW/m^2
What is the size of the eardrum in m
0.0001m^2
How much power does sound of 100dB exert on each eardrum
1uW
What is the effect of a 0dB sound on the eardrum
A 0dB sound (20uPa) moves the eardrum by less than the diameter of one H molecule
What code do AN fibres use to encode intensity
AN fibres use a rate code- they increase their firing rate as a function of sound intensity
Population code- due to sensitivity differences between low/intermediate/high SR fibres
What different types of AN fibres are there (3 types, not 1 and 2)
Low spontaneous rate fibres, intermediate spontaneous rate, high spontaneous rate- differ in their sensitivity and dynamic range
What are the properties of low spontaneous rate fibres
Have the highest threshold, saturate only at high sound levels (above 90dB)
What are the properties of intermediate spontaneous rate fibres
Intermediate thresholds, saturate by about 60dB
What are the properties of high spontaneous rate fibres
Most sensitive fibres aka lowest threshold, may saturate by 40dB
What is the lowest sound level that will elicit AP firing
eg 40db for high SR fibres
What is the effect of increasing sound intensity on the range of frequencies a nerve fibre responds to
As intensity increases, larger areas of the basilar membrane will vibrate, meaning nerve fibres respond to a greater range of frequencies
What does the tuning band of a auditory nerve fibre show
The range of tone frequencies a nerve responds to over a range of dB (as dB increases, the band gets wider)
What is the range at which we have greatest sensitivity to frequency
Between 2-5kHz
At what frequencies does sensitivity decrease
At particularly high and particularly low frequencies- outside of 2-5kHz range
Through which structures is tonotopic organisation maintained
Auditory nerve, cochlear nuclei, medial superior olive, medial nucleus
What type of code does tonotopy create
Place code and population code for sound frequency
How do auditory nerve fibres encode sound frequency
ANs fire APs to low frequency sounds at particular times via phase locking, and the spike time intervals encode temporal stimulus features aka sound frequency
What limits the phase locking of spikes of a single neuron
Refractory period of at least 1ms sets an upper limit of 1kHz for phase locking
Why are individual nerve fibres not particularly informative at high frequencies w phase locking
Neurons in practice can’t sustain firing rates over 600Hz, and even at lower rates, they tend not to fire at every stimulus cycle as the nerve fibre response is stochastic
How does a population of neurons encode frequency via phase locking
If spikes from a no of nerve fibres are combined, it provides info about the temporal structure of the sound from the temporal distribution of the spikes up to frequencies that surpass the phase locking limit of individual nerve fibres
What sets the absolute limit for the temporal resolution of frequency that can be encoded by phase locking
The AC mode limit of the hair cell membrane potential, no AC response for frequencies above 3kHz
How many auditory nerve afferents are in the auditory nerve
10,000, 95% are type 1
Which parts of the cochlear nucleus do different parts of the basilar membrane project to
Base of basilar membrane projects to medial CN (HF), apex of basilar membrane projects to lateral CN (LF)
What are the different parts of the cochlear nucleus that fibres project to
Dorsal, posteroventral and anteroventral
What do auditory nerve fibres do when they enter the cochlear nucleus
Bifocate, and send fibres to the different parts of the cochlear nucleus to form synapse with multiple cochlear nucleus neurons
What cell types are contained in the anteroventral part of the cochlear nucleus
Globular bushy cells and spherical bushy
What do globular and spherical bushy cells (AVCN) cells receive input from
A small no of very large excitatory synapses from auditory nerve fibres
What responses to globular and spherical bushy cells (AVCN) SHOW
Primary-like responses, meaning their firing patterns are almost identical to the firing patterns of their auditory nerve input and preserve into contained in phase locking
What cell types are contained in the posteroventral cochlear nucleus
Octopus cells and multipolar cells
What do octopus cells and multipolar cells in the PVCN receive input from
Octopus cells receive convergent input from many AFs
Multipolar cells receive input from multiple ANs mostly on the dendrites
What responses to octopus and multipolar cells of the PVCN show
Octopus cells are very broadly tuned to to their convergent input and fire a single AP in response to a sudden burst
Multipolar cells show chopper responses when stimulated with pure tones
What are the chopper responses shown by multipolar PVCN cells
Regular, rhythmic bursts, but burst frequency is unrelated to tone stimuli frequency
What cells are contained in the dorsal cochlear nucleus
Pyramidal cells
What response do pyramidal cels in the DCN show
Pauser response aka fire strongly to onset of sound, followed by an inhibitory period, then ramping up of firing rate again
How do the different cell types in the cochlear nucleus differ
Each cell type differs in morphology and response properties- attracts different aspects of the incoming acoustic info, and passes this on to different points in the auditory pathway
What are the tuning curves of globular/spherical bushy cells AND octopus cells
Tuning curves excited by a range of frequency that becomes wider with increasing sound intensity (look like AN tuning curves)
Why do the tuning curves of multipolar and pyramidal cells in the CN differ from those of the others/ANs?
More complex tuning curves with inhibitory regions where firing rates are suppressed below spontaneous rates because of the tuning of their input by inhibitory CN neurons
What do multipolar cells have that provides lateral inhibition to affect their tuning curve
Inhibitory side bands
How may pyramidal cell tuning curves have very complex shapes
May have large inhibitory regions and only small excitatory pitches due to lateral inhibition
Where/what do globular and spherical bushy cells in the ACVN project to
Primarylike info is projected to the superior olivary nuclei
Where/what do multipolar PVCN cells and pyramidal DCN cells project to
Multipolar- chopper info
Pyramidal- pauser info
May use lateral inhibition to extract spectral contrast, project to inferior colliculus
What are onset cells (in CN)
An inhomogenous class- some are multipolar and some are stellate
What do onset cells do in the CN
May encode temporal pattern info across many AN fibres or encode sound intensity
Where do onset cells project to (from CN)
Project to inferior colliculus or lateral lemniscus
What is the first stage of binaural convergence of info from the 2 ears
The superior olivary nuclei
What are the 2 parts of the superior olivary nuclei
Lateral superior olive (LSO) and medial superior olive (MSO)
What does the MSO receive input from
Excitatory input from CN of both sides
What does the LSO receive input from
Excitatory input from ipsilateral CN, inhibitory input from the contralateral CN via medial nucleus of trapezoid body (MNTB)
What is the synapse between the LSO and medial nucleus of trapezoid body
Calyx of Held, because it is the biggest synapse in our brain
What is the concept of interaural time/level differences
Sounds arrive earlier and are louder at the near ear to the sound
What is specialised for the processing of interaural level differences
Lateral superior olive
When will an LSO neuron only fire when processing interaural level differences and why
LSO neuron will fire strongly only when a loud sound is received in the ipsilateral ear and a quiet sound in the contralateral ear, because of the inhibitory connection from the contralateral side by the MNTB
What is the MNTB
Medial nucleus of trapezoid body
How does the LSO encode sound source direction using the interaural level difference
Rate code- fire much more strongly when I>C
How are interaural level differences affected by frequency
ILDs are highly frequency dependent- at higher sound frequencies, ILDs tend to become larger, more complex, and potentially more informative
Study into effect of frequency on ILD- procedure?
Tiny microphones places into the ear canals of humans, different frequency sounds from diferent locations in space all aruond the subject presented
Study into effect of frequency on ILD- results
LF- ILD only varies over a range of 30db
HF- ILD varies over a range of 80dB, more useful
Why do low frequency stimuli create lower ILD than high frequency stimuli?
LF sound waves can more easily wrap around the head without being attenuated
What is specialised for processing of interaural time differences
Medial superior olive, neurons are sensitive to changes in ITD
What code was originally thought to be used for sound source direction in the MSO
Place code- now disputed for mammals
What does the brain need to compare in order to process iTDs
Compare the arrival time of sounds at the 2 ears
At which ITD is firing of MSO neurons highest
Firing peaks when sound is slightly on the contralateral side (accounts for longer contralateral delay)
What innervates the MSO
AVCN neurons project to the MSO via spherical bushy cells connected to end bulb of Held synapses
What are the properties of the end bulb f Held synapses that join the AVCN to th MSO
Like the calyx of Held- unusually large, operate with very high temporal precision
How does the end bulb of Held synapse between the AVCN and MSO operate with such high temporal precision
A single presynaptic AP at an end bulb of Held synapse can trigger an AP in the postsynaptic cell, allowing the MSO to be provided with info that reflects the timing of the sound arriving at both ears
What model has been proposed to explain ITD analysis at the MSO
Jeffress (1948) Delay Line and Coincidence Detector Model
When do MSO neurons fire maximally in detecting ITD
Fire maximally ONLY if they receive simultaneous input from both ears
What is the result of input from different ears being delayed by different amounts on the way to the MSO which detects ITD
Because the input from different ears is delayed by different amounts (eg one side’s afferent axons are longer), the MSO neuron will fire maximally only if an interaural delay in the arrival time at the ears exactly compensates for the transmission delay
What different types of MSO neurons are there as the result of the coincience detection property of MSO neurons in detecting ITDs
Different MSO neurons become tuned to characteristic interaural delays
How does the MSO use a place code for sound source direction
Different neurons in different areas of the MSO will be activated by different ITDs that result from sound stimuli from different directions
How are ITDs affected by frequency?
We are bad at extracting ITDs from high frequency sounds as our ANs can’t phase lock to high frequency stimuli, so the high temporal info needed for processing ITDs can’t be extracted
How big are normal ITDs in humans
‘Unaided’ ITDs are max 0.7ms in humans
What is the smallest detectable difference in ITD
0.01ms
What scientists have donw recent work that sheds doubt on whether the Jeffress (1948) Coincidece Detector Model is a good description for ITD in the mammalian MSO
McAlpine, Palmer, Grothe
What does the inferior colliculus do
Major (+ big) processing centre- collects and integrates input from all auditory brainstem nuclei, obligatory relay for all ascending auditory info
How does the input to the IC affect the excitability of its neurons
Most inputs to the IC are from the other hemisphere, so most neurons in the IC and above are most strongly excited by sounds presented in the contralateral ear
How is the IC divided
Has a number of anatomical subdivisions- central nucleus, dorsal cortex and external nucleus in the shell
Which subdivisinos of the IC are tonotopically organised
The largest, the central nucleus, is tonotopically organised
The dorsal cortex and external nucleus (in the shell) are NOT
How is the central nucleus of the IC tonotopically organised
High frequency neurons are in the ventral part, low frequency neurons are in the dorsal part
Where does integration of acoustic with contextual info start
The inferior colliculus
Study showing how neurons in the IC can be impacted by non-acoustic signals eg behaviour
Chen and Song (2019)- speed of firing in the IC increased as a mouse increases its running speed
Where does the inferior colliculus project to other than the medial geniculate body (aka outside the main auditory pathway)
Superior colliculus
What is the superior colliculus
Multisensory nucleus- mostly concerned with control of eye and head movements
How is the superior colliculus involved in audition
Only place in the brain we have found a map of auditory space- in the deeper layers of SC, aligned with map of visual space found in upper layers
What does the medial geniculate body consist of
3 major nuclei, the ventral, medial and dorsal MGB
What do the different parts of the medial geniculate body receive input from
Ventral MGB receives input from central nucleus of the IC
Dorsal and medial MGB receive input from shell of IC AND non-auditory structures
What properties do dorsal and medial medial geniculate bodies have as a result of non-auditory structures
MGB neurons may, to a greater extent than the IC, represent non-acoustic features of sensory stimuli and be even more modulated by other sensory/motor systems
Evidence of multimodal integration in the medial geniculate body
Neurons in the MGB of a mouse showed an attenuated response to acoustic info following somatosensory stimulation (whiskers)
What is another name for the medial geniculate body
The auditory thalamus
What type of integration do we see more and more as we move up through the hierarchy of the auditory pathway
More and more multimodal integration eg info from motor system and sensory systems integrated with auditory info
Which lobe is the auditory cortex located in
Temporal lobe
What is the primary auditory cortex also called
A1, Brodmann’s area 41
Which sections of the auditory cortex are tonotopically organised
A1 is strictly tonopically organised
Organisation starts to break down in A2 and higher cortical areas beyond
How do primary and higher order cortical areas show increasingly complex response properties
They integrate acoustic info with contextual info (other sensory input, motor output, memories, expectations) to make sense of it
Example of integration of memories/expectations with acoustic info
Eliades and Wang (2008)- monkeys neurons in A1 signal mismatches between the expected sensory feedback from a vocalisation and the actual sensory feedback, a type of error signal crucial for fine tuning our vocal production
How is the auditory cortex divided up
Divided into 6 different layers with different functions
What is the flow of information through the auditory cortex
Thalamus -> L4 -> L2/3 ->L5 ->L6-> out
What descending projections of the auditory cortex are there
Deep layer neurons (L5 and L6) descend to the thalamus, inferior colliculus and brainstem through MASSIVE projections
What are the descending projections from the auditory cortex thought to be responible for
Controlling and gating the flow of info in subcortical structures
From which direction does the thalamus auditory nucleus receive more inputs
Receives more inputs from the cortex than it receives ascending input
How are cortical circuits highly plastic
Can readily change their response properties as a result of new experiences, suggesting the cortex is essential for perceptual learning
Which areas of the auditory system are not well understood
Auditory midbrain, thalamus and cortex
What is the frequency of sound
The no of compressed patches of air that pass by our ears per second (Hz)
What is the intensity/amplitude of sound
The difference in pressure between compressed and rarefied patches of air
What is the attenuation reflex
A loud noise triggers a neural response that causes the tensor tympani and stapedius muscles to contract, making the ossicle chain more rigid and decreasing sound conduction to the inner ear
What 3 chambers is the cochlea divided into
Scala vestibuli, scala media, scala tympani
What does Reissner’s membrane separate
Scala vestibuli from the scala media
Where is the round window
At the base of the cochlea, the scala tympani meets the round window
What is the stria vascularis
The endothelium lining one wall of the scala media contacting the endolymph
What does the stria vascularis do
Absorbs Na+ and secretes K+ into the endolymph
Why can low frequency sounds travel further up the basilar membrane
They can travel further before all of their energy is dissipated, compared to higher frequencies that dissipate most of their energy at the stiff base
Why are the auditory receptors called hair cells
They each have 10-300 stereocilia extending from the top
What type of cells are hair cells
Specialised epithelial cells, not neurons
Where are the outer hair cells
Further out than the rods of Corti
Where are the inner hair cells
Between the rods of Corti and the modiolus (central axis of cochlea)
What moves the OHC cilia vs IHC cilia
OHC cilia moved by the bending of the tectorial membrane over them
IHC cilia probably pushed by moving endolymph
What makes it so that stereocilia only bend at their base
Aligned actin filaments make stereocilia rigid rods that only bend at their base
Who pioneered an approach of studying hair cells in isolation
Hudspeth (1980s)- a sound wave causing stereocilia to bend back and forth causes the hair cell to generate a receptor potential that alternately hyperpolarises and depolarises from the resting potential of -70mV
How does the opening of K+ cause depolarisatino of the hair cells since for most cells opening K+ channels hyperpolarises cells
Because of the unusually high K+ conc in the endolymph
How many spiral ganglion neurites does each IHC feed
Each IHC feeds about 10 spiral ganglion neurites
What is the cochlear amplifier
The action of OHCs as tiny motors to amplify the movement of the basilar membrane during low-intensity sound stimuli
Evidence for the importance of prestin in OHCs amplification
Ruggero and Rich (1991)- Furosemide, a drug that reduces the flexing of the basilar membrane in resposne to sound, is thought to act by deactivating OHC motor proteins like prestin
What is the result of amplification of basilar membrane motion by the OHCs
Increased transduction process, meaning higher sensitivity and sharper frequency tuning in the inner ear
How can descending input from the brain to the cochlea regulate auditory sensitivity
Stimulation of efferent fibres projecting to the OHCs causes them to release ACh, changing the shape of OHCs and affecting the responses of IHCs
Clinical evidence for the importance of OHCs in amplification
Damage to OHCs by drugs toxic to them eg quinine can cause nerve deafness, associated with loss of hair cells in the cochlea, and is treated with a hearing aid that amplifies the sound in the place of missing hair cells
Evidence for the amplificayion by OHCS favouring weaker sounds
Ruggero et al (1997)- plotted the mechanical gain of the basilar membrane in response to pure tones, fuond OHCS amplify weaker sounds more strongly with a gain of up to 60dB
What form of nonlinearity does the cochlear amplifier follow
Compressive linearity- allows a wide range of sound pressure amplitude inputs to be mapped onto a more limited range of basilar membrane vibration amplitudes
Evidence for the differential amplification of sound across the basilar membrane
Two-tone suppresion- OHCs lie side by side so the amplification they create can’t operate entirely independently on each small patch of the basilar membrane.. if two pure tones are close in frequency, the response to one may appear disproportionately small if both are simultaneous (Cooper, 1996)
What is the auditory nerve also called
The auditory vestibular nerve
Evidence for the existence of the dorsal and ventral cochlear nuclei
Brain stem damage can only produce deafness in one ear if a cochlear nucleus on one side is destroyed (all other brain stem nuclei are binuaral)
What is phase locking
The consistent firing of a cell at the same phase of a sound wave eg at the peaks of a wave
What is the volley principle
The pooled activity of a population of neurons that fire in a phase-locked manner (but each on a different cycle of the input signal) can sum a response to every cycle, so represent frequency
How is frequency encoded over 3kHz (upper limit of phaselocking)
By tonotopy alone
Evidence for specialisation of auditory neurons for timing
Oertel et al- some ANs have very low membrane resistance and fast time constants, helping them convey precise timing info
Evidence for specilaisation of cochlear nuclei cells
Golding Bal and Ferragamo- octopus CN cells have exceptionally large mutually opposing types of voltage-sensitive ion channels that give the cells fast time constants, allowing them to coincidence detect in the submillisecond time range
What is the difference between the requirements for horizontal vs vertical sound localisation
Good horizontal localisation requires a comparison of the soudns reaching the 2 ears, but good vertical localisation does not
How do we detect ITD with continuous tones that are always present at both ears
The time at which the SAME PHASE of the sound wave reaches each ear is compared instead eg peak
In what circumstances can we not use interaural delay of the same phase of a sound to determine sound location
Continuous tones at HIGH FREQUENCIES (>2000Hz) as one cycle of the sound is smaller than the distance between your ears, so many peaks will fit in between your ears and there is not longer a simple relation between sound direction/peak arrival time
How does the brain localise continuous tones at high frequencies
Uses the interaural intensity difference between the ears, due to the sound shadow cast by your head- neurons sensitive to intensity differences can use this info to locate the sound
What is the duplex theory of sound localisation
20-2000Hz- ITD is used
2000-20,000Hz- ILD is used for localisation
Evidence for possible place coding of ITD
Recordings from LSO show each neuron gives its greatest response to a particular interaural delay, meaning each may encode a particular position in the horizontal plane
Study sggesting an alternate method for sound localisation by ITD in mammals
Studies on gerbils suggest synaptic inhibition rather than axonal delay lines generates the sensitivity of LSO to ITD
How can vertical sound localisation be impaired
By placing a tube in the auditory canal to bypass the pinna, or simply covering the pinna convolutions
How do we assess vertical localisation
The bumps and ridges of the pinna reflect the sound, meaning the delays between the direct path and reflected path change as sound moves vertically, so the combined sound is subtly different when it comes from above or below
How do axons from the MGN project to A1
Via the internal capsule in an array called acoustic radiation
How is the structure of A1 similar to the layers of V1
Layer I contain few cell bodies, layers II/III contain mostly small pyramidal cells, layer IV where MGN axons terminate, layers V and VI contain mostly large pyramidal
Evidence suggesting columnar organsiation in A1
In electrode penetrations in monkeys, the cells tend to have similar characteristic frequencies, suggesting a frequency columnar organisation
In which way is the tonotopic map on A1
Low frequencies- rostrally and laterally
High frequencies- caudally and medially
Isofrequency bands running mediolaterally across A1
How do cortical neurons differ
Different temporal response patters, some intensity tuned, some respond to clicks/frequency-modulated sounds/animal vocalisation
Example of a higher level audio processing region
Wernicke’s area, impairs ability to interpret language
What are the 2 types of deafness
Conduction deafness, nerve deafness
What can cause conduction deafness
Rupture of tympanic membrane, pathology of ossicles eg many diseases impair transfer of suond by binding the ossiclds to the bone of the middle ear
What is conductino deafness
Deafness caused by a disturbance in the conduction of sound from the outer ear to the cochlea
What is nerve deafness
Associated with loss of neurons in the auditory nerve or hair cells in the cochlea
What can cause nerve deafness
Tumours affecting the inner ear, drugs toxic to hair cells eg quinine, exposure to loud noises
How do cochlear implants work
Take advantage of the tonotopic arrangment of auditory nerve fibres- electrical stimulation near the base of the cochlea evokes perception of high frequency sounds and vv
What supports the idea of isofrequency bands in the cortex
Animal studies indicate small lesions in A1 can produce specific localisation defecits for sounds within a limited frequency range
Why do high frequency sounds not travel far along the basilar membrane
At high frequencies, the fluid filled column pathway is a source of mechanical resistance due to inertia, as it takes a lot of force to push and pull the cochlear fluid at such a high frequency, so choosing to travel along the basilar membrane path offers less resistance
What determines whether a wave will pass far or not far along the basilar membrane
Its frequency- it will travel along a ‘compromise path’ that is long enough for stiffness to somewhat decrease but not too long inertial resistance is super high
How is the basilar membrane a biological Fourier analyser
The membrane can be seen as a set of logarithmically spaced mechanical filters each with their own resonance frequency, like a gamma-tone filter bank (Schnupp et al, 2011)
What is a gamma tone filter bank
Gamma filters suppress frequencies other than those that match their resonance characteristics, so a set each tuned to a characteristic frequency can filter incoming sounds via passing them through the filters in parallel, allowing for frequency analysis and production of a ‘neurogram’
What forms the filters that make up the gamma tone filter bank on the basilar membrane
Each small piece of the basilar membrane that responds to a characterstic frequency
What is basilar membrane filtering later sharpened by
Zwislocki and Kletsky (1979)- further neural processes that form a psycholgically vulnerable second filter
Evidence of ‘dancing’ hair cell
Video by Ashmore (2008), recorded from an OHC in a guinea pig cochlea injected with an electrical current waveform of a song, cell appears to stretch and contract rhythmically to te song
