1

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

2

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

3

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)

4

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

5

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)

6

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

7

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

8

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

9

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

●

● How is this processed in higher auditory areas?

● What information is extracted from this auditory nerve output?

10

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

11

Inner hair cells are polarised – towards pillar cell and towards other side (modiolus)

12

Auditory nerves contacting the two sides of inner hair cells have different properties

http://www.jneurosci.org/content/31/3/801/F1.large.jpg

13

Different spontaneous rates depending on which

part they contact

http://www.jstor.org/stable/1688751?seq=1#page_scan_tab_contents

14

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

15

Auditory nerve

responses are also phase

locked to frequency of stimulus

● Frequency information

– Place code

– Frequency code

16

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

17

Tonotopy maintained in cochlear nucleus

18

Tonotopy present in primary auditory cortex too

19

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

20

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

21

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

22

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

23

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

24

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

25

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

26

How are such small ITDs processed in the brain – Lloyd Jeffress model (1948)

Coincidence detection and delay lines

27

Neurons in the MSO are maximally responsive at specific ITDs

http://www.cns.nyu.edu/~david/courses/perception/lecturenotes/localization/localization.html

28

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

29

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

30

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