Post on 10-Feb-2022
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Lecture 7 – Hearing 2
Raghav RajanBio 354 – Neurobiology 2
February 04th 2015
All lecture material from the following links unless otherwise mentioned:1. http://wws.weizmann.ac.il/neurobiology/labs/ulanovsky/sites/neurobiology.labs.ulanovsky/files/uploads/purves_ch12_ch13_hearing_balance.pdf2. http://www.ib.cnea.gov.ar/~redneu/2013/BOOKS/Principles%20of%20Neural%20Science%20-%20Kandel/gateway.ut.ovid.com/gw2/ovidweb.cgisidnjhkoalgmeho00dbookimagebookdb_7c_2fc~36.htm
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General announcements
● Extra class time – Friday 5:30pm? (or earlier?)
● 19th class - Thursday – 10:30-11:30am
– CHM 321 (Organic Synthesis I), Math 429 (Differential Geometry), Phy 324 (Quantum Mechanics II)
● Quiz tomorrow – 5th February
● Course presentation – Wednesday 18th February (5:30pm)
– Groups and topics for presentation
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From earlier classes ...!!
● What happens to percepts in split-brain patients? (Sahana)
● Aperture control (Vishnu)
● Pin-wheel centers – orientation tuning (Ruchi)
● Responses to natural scenes (Radhika)
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Auditory system and hearing
● Structure and anatomy of auditory system
● How is sound energy converted into electrical and chemical signals?
● Coding in the auditory system
– Frequency
– Intensity
– Source localization
● Higher order functions
– Identifying auditory objects (sounds, voices)
– Speech
– Music
– Echolocation
– Avoiding echolocation – jamming by moths
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Vibrations of basilar membrane – frequency
specificity● Sound transferred from tympanum to round
window through middle ear bones
● Make fluid move over the basilar membrane
● Basilar membrane uniform only in some birds and reptiles
● Apex of basilar membrane is 5 time broader than base
● Thin and floppy at apex
● Thicker and more taut at base
● Georg von Bekesy, Helmholtz
● Tonotopic map
● Distance from apex of cochlea and frequency response – logarithmic relationship (not linear)
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But the frequency relationship does not come out only
passive basilar membrane properties
● Fluids in inner ear would damp vibrations of basilar membrane – this needs to be overcome
● Based on passive basilar membrane properties – models are not able to account for exquisite sensitivity of the auditory system
● Models mostly account for responses at high intensity
● But responses at low intensity – cannot be fully explained
● Therefore, the possibility of active amplification especially at low intensities
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The ear also emits sounds!!
● Sounds picked up by sensitive microphone in the external ear after a sound stimulus (latency different for different frequencies – ~5-20ms)
● Spontaneous sound emissions can also be recorded with very sensitive microphones
● Possibility of the cochlea acting as a mechanical amplifier
● Tinnitus
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Outer hair cells may be one source of these sounds
● 95% of auditory nerve fibers come from inner hair cells
● Outer hair cells receive mostly input from cells in the superior olivary complex
● Electrical stimulation of outer hair cells makes their cell bodies contract or relax
● Inactivating outer hair cells changes tuning curve of auditory nerve fibers
● Not the only source, though
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Cochlear output – high temporal precision, high frequency resolution
● Auditory nerve output
– Frequency- labelled line code, phase locking to positive phase of stimulus
– Output with high temporal resolution
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● How is this processed in higher auditory areas?
● What information is extracted from this auditory nerve output?
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One-to-one innervation of inner hair cells by auditory nerve fibers
● Each auditory nerve fiber connects with 1 inner hair cell
● 1 inner hair cell contacts many (~10) auditory nerve fibers
● Some feedback input too on inner hair cells
● Most feedback onto outer hair cells
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Auditory nerves contacting the two sides of inner hair cells have different properties
http://www.jneurosci.org/content/31/3/801/F1.large.jpg
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Different spontaneous rates depending on which
part they contact
http://www.jstor.org/stable/1688751?seq=1#page_scan_tab_contents
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Different fibers also have different sensitivities – different thresholds
● High spontaneous rate fibers have low threshold (high sensitivity)
– Get saturated very quickly, therefore small dynamic range
● Low spontaneous rate fibers have high threshold (low sensitivity)
– Have a larger dynamic range
● Mechanisms unclear – maybe postsynaptic differences
● Therefore, each inner hair cell provides multiple channels to the brain providing non-redundant information about that frequency
http://web.mit.edu/hst.723/www/ThemePapers/Implants/MooreReview2003.pdf
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Auditory nerve
responses are also phase
locked to frequency of stimulus
● Frequency information
– Place code
– Frequency code
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Pathway to brain
● VIIIth cranial nerve – auditory nerve projects to cochlear nucleus in medulla
● Ipsilateral projection only for auditory nerve
● Projection copied to 3 different parts of cochlear nucleus
● First binaural interactions in the Superior Olivary nuclei
● Tonotopy maintained in higher order areas too
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Auditory system and hearing
● Coding in the auditory system
– Frequency
– Intensity
– Source localization
● Higher order functions
– Identifying auditory objects (sounds, voices)
– Speech
– Music
– Echolocation
– Avoiding echolocation – jamming by moths
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Source of sound can be from left/right and from up/down
● Azimuth – horizontal plane – left/right
● Elevation – vertical plane – up/down
http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html
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Inter-aural level (intensity) differences (ILD) and inter-aural time differences (ITD)
ILD ITD
http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html
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Inter-aural level (intensity) differences (ILD) are present at high frequencies
ILD ITD
http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html
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Inter-aural level (intensity) differences (ILD) are dependent both location (azimuth) and frequency of
sound
ILD ITD
http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html
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ILDs are represented in the lateral superior olive (LSO) and medial nucleus of the trapezoid body (MNTB)
● Excitation from one side
● Inhibition from other side
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ITDs > 10μs can be localized by human subjects!
● 10μs much less than the time scale of an action potential - 1ms
http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html
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How are such small ITDs processed in the brain – Lloyd Jeffress model (1948)
Coincidence detection and delay lines
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Neurons in the MSO are maximally responsive at specific ITDs
http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html
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Anatomy of neurons in the MSO (nucleus laminaris) in the barn owl support this model
http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html
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Map of space is computed by neurons in the midbrain of the barn owl
● Not a feature that is mapped onto the auditory system directly (unlike visual system)
● Computed using ILD, ITD and other cues
http://jacknife.med.yale.edu/nsci590-2005/pdfs/knudsen1978.pdf
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Auditory system and hearing
● Structure and anatomy of auditory system
● How is sound energy converted into electrical and chemical signals?
● Cochlear output – high temporal precision, high frequency resolution
● Coding in the auditory system
– Frequency
– Intensity
– Source localization
● Higher order functions
– Identifying auditory objects (sounds, voices)
– Speech
– Music
– Echolocation
– Avoiding echolocation – jamming by moths