Audition Flashcards
Ear canal selectively increases sound pressure by how much around what frequency?
10dB, 3000Hz.
Pinna selectively increases sound pressure by how much around what frequency?
10dB, 5000Hz.
Overall the external ear amplified up to 20dB from … Speech sound range.
2000-6000 Hz.
Name of three ossicles:
Malleus, incus, stapes.
Footplate of stapes contacts oval window of cochlea:
a coiled structure of progressively diminishing diameter wound around a conical bony core, filled with fluid.
Why does basilar membrane vibrate as stapes moves in and out of oval window?
Fluid incompressible, so compensatory movement in opposite direction.
What is the lateral lemniscus?
Tract of axons in the brainstem that carry sound information from the cochlear nucleus to various brainstem nuclei and ultimately the contralateral inferior colliculus in the midbrain.
What is the trapezoid body?
Bundle of decussating fibres within lateral lemniscus that carry information used for binaural computations. Travel to superior olivary complex then inferior colliculus.
Inferior colliculus receives descending input from where?
Auditory cortex and medial geniculate nucleus.
Why, without impedance matching, would only 0.1% of sound energy reaching eardrum reach inner ear.
Fluid of inner ear denser than air. Would be reflected.
Two factors that allow middle ear to match impedance:
Hydraulic amplification. Ear drum larger than stapes footplate (60mmsq vs 3mmsq). Because p=f/a, decreased area means increased pressure for a given force. 22x more pressure.
Mechanical amplification. Arm of malleus longer than incus, amplification by lever action. Factor of 1.3dB.
Clinical example of importance of middle ear:
Ostosclerosis. Stapes fixed from abnormal growth of bone. Conductive deafness.
How many mechanosensitive hair cells does the Organ of Corti contain?
16,000.
Mechanosensitive hair cells each possess…
hair like projections, arranged in increasing length, connected by tip-links.
Tip links primarily composed of what substances? They both require what to maintain their structure integrity? How has this been used to demonstrate tip links’ importance?
Cadherin-23 and protocadherin-15. Both require Ca2+. Ca2+ chelators bind to free calcium ions, transduction vanishes when they are applied.
How does the postmortem cochlea highlight that it must have active mechanical properties?
Sound pressure linearly increases basilar membrane motion. But this cannot explain sensitivity of mammalian hearing. Suggests active mechanism.
What about the healthy cochlea suggests that active amplification mechanisms are present?
Otoacoustic emissions.
Explain the action of the cochlear amplifier:
Force is generated in synchrony with vibrations to increase them. Force generated by OHCs, electromotility. Mediated by prestin. Depolarisation, shortens. Lowers tectorial membrane, greater deflection of inner hair cells, amplification.
What is electromotility?
Length change in response to electrical stimulation.
How are hair cells protected from loud sounds?
Olivocochlear neurons form a feedback loop from SOC to HCs in cochlea. Chlolinergic, act on hair cells through specialised nAChRs. Influx of Ca2+. opening K+ channels, hyperpol.
What gives cochlear tonotopic organisation?
Morphological gradients.
Membrane thicker at base, apex 5x broader.
Both gradients contribute to a base-apex decrease in basilar membrane stiffness.
Means high frequency sounds reach maximum displacement near base of the cochlea.
What evidence shows that tonotopy is a fundamental organising principle of the auditory system?
Preserved in higher areas. Formisano et al 2003. Tonotopy in PAC; high to low frequencies represented moving from medial to lateral, with mirror reversals along HG.
What does phase locking allow coding of?
The frequency and temporal pattern of sound waves.
What is phase locking?
AF fire APs at same rate as auditory input, at a specific point in the inputs phase. Timing between neural spikes provides direct representation of frequency.
Limitation on phase locking:
Refractory periods, 1ms. Greatest sustainable rate 500Hz.
Why can frequencies up to 3000Hz be coded temporally? But also why is this the maximum?
Volley principle. Several neurons take turns representing specific phase of input, responses compared.
Maximum because hair bundles cannot bend backwards and forwards faster, glutamate cannot be released in a phasic manner, DC response.
Loudness of a stimulus encoded by… This means parallel channels.
Varying response patterns of different afferent fibres with different sensitivities and dynamic ranges.
High sensitivity fibres exhibit saturating responses for stimulation at moderate intensities such as…
30dB.
Low sensitivity fibres respond in a graded fashion to levels even in excess of…
100dB.
In the cochlear nerve, information distributed to specific cells that…
Form parallel ascending pathways through the brainstem to the inferior colliculus, either directly or through relay regions.
Bushy cells are located in what portion of the cochlear nerve? What do they do?
AVCN anteroventral.
Preserve precise timing of sound, firing in phase with input.
Project to SOC. Binaural computations.
Octopus cells are located in what portion of the cochlear nerve? What do they do?
PVCN posteroventral. Detect rapid sound onsets. Some project directly to IC, some to SOC.
Fusiform cells are located in what portion of the cochlear nerve? What do they do?
DCN dorsal. Analyse complex spectral cues, aiding in sound differentiation. Project to SOC.
What evidence shows that summation of input from core to belt regions occurs?
Single-cell recordings in primates show that the neurons in the core region respond to narrowly defined frequencies (e.g., responding maximally to a pure tone of 200 Hz), whereas cells in the belt region respond to a broader band of frequencies (e.g., responding to noise between 200 Hz and 300 Hz). Consistent with belt neurons summing input from core neurons.
What is the anatomy of the ventral stream? What is its role?
Projects from anterior belt region to temporal lobes. Sound recognition, “what”.
What is the anatomy of the dorsal stream? What is its role?
Projects from posterior bely region to parietal lobes. Sound localisation, “where”(though may be lateralised, LH dorsal stream may be responsible for speech repetition and articulation “how”).
Evidence for dorsal/ventral split:
Rauscheker and Tian (2000), neural responses in anterior belt region showed high degree of specialisation for monkey calls, posterior spatial selectivity.
Hill and Miller (2010), cocktail party effect, parietal activity selective for location, temporal for pitch.
How is map of interaural phase created?
Comparing AP timing in responses to sound from the two ears.
Where are ITDs calculated?
MSO.
According to the Jeffress neural co-incidence model, how are ITDs mapped?
Why is this model wrong?
MSO neurons respresent specific ITDs.
May be the case in barn owls with very narrow ITD tuning curves but humans have wide ones.
Thus more likely population coding whereby the ratio of active neurons that respond best to input from contralateral vs ipsilateral ear are compared.
Where are ILDs calculated?
LSO.
Why is intensity at the near ear greater than the far ear?
Because sounds with wavelengths similar to or smaller than the head are deflected by the head.
Interaural intensities can differ in sounds that have frequencies greater than about…
2000 Hz.
How are ILDs calculated?
Neurons in LSO balance ipsilateral excitation with contralateral inhibition. When sound arrives from ipsilateral side excitation relatively stronger. Transition between the dominance of excitation and inhibition reflects the location of the sound source in the horizontal plane.
How does the structure of the external ear contribute to sound localisation in the horizontal plane?
Bumps and ridges of pinna produce reflections of entering sound. Delays between direct path and reflected path change as a sound source moves vertically. Combined sound (direct and reflected) subtly different when comes from above or below.
What evidence shows the importance of the pinna in localisation? And what evidence shows that the brain develops an internal model of how sound gets distorted by own head/ear?
Batteau (1967) casts of pinnae, microphone into ear canal, played sounds. Could only localise if pinna attached.
Wenzel et al (1993). Performance improved if participants’ own ear shapes. HRTF.
Although owls don’t have a pinnae, they can use the same techniques as humans do for horizontal localisation for vertical. Why?
Ears are at different heights on their head.
What region of the brain may be responsible for integrating input with HRTF? What evidence shows?
Planum temporale (posterior to PAC). Hunter et al (2003), region responds more to sounds subjectively located outside the head than inside (headphones).
