Lecture 7 Hearing 2. Raghav Rajan Bio 354 Neurobiology 2 February 04th All lecture material from the following links unless otherwise mentioned:

Transcription

1 Lecture 7 Hearing 2 All lecture material from the following links unless otherwise mentioned: 1. _balance.pdf 2. Raghav Rajan Bio 354 Neurobiology 2 February 04th

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 2

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) 3

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 4

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) 5

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 6

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 7

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 8

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? 9

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 10

11 Inner hair cells are polarised towards pillar cell and towards other side (modiolus) 11

12 Auditory nerves contacting the two sides of inner hair cells have different properties 12

13 Different spontaneous rates depending on which part they contact 13

14 Different fibers also have different sensitivities different thresholds High spontaneous rate fibers have low threshold (high sensitivity) Low spontaneous rate fibers have high threshold (low sensitivity) Get saturated very quickly, therefore small dynamic range 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 14

15 Auditory nerve responses are also phase locked to frequency of stimulus Frequency information Place code Frequency code 15

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 16

17 Tonotopy maintained in cochlear nucleus 17

18 Tonotopy present in primary auditory cortex too 18

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 19

20 Source of sound can be from left/right and from up/down Azimuth horizontal plane left/right Elevation vertical plane up/down 20

21 Inter-aural level (intensity) differences (ILD) and interaural time differences (ITD) ILD ITD 21

22 Inter-aural level (intensity) differences (ILD) are present at high frequencies ILD ITD 22

23 Inter-aural level (intensity) differences (ILD) are dependent both location (azimuth) and frequency of sound ILD ITD 23

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 24

25 ITDs > 10μs can be localized by human subjects! 10μs much less than the time scale of an action potential - 1ms 25

26 How are such small ITDs processed in the brain Lloyd Jeffress model (1948) Coincidence detection and delay lines 26

27 Neurons in the MSO are maximally responsive at specific ITDs 27

28 Anatomy of neurons in the MSO (nucleus laminaris) in the barn owl support this model 28

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 29

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 30