Hearing

1. Hearing

3. middle ear inner ear outer ear hammer eardrum anvil stirrup oval window tectorial membrane hairs basilar membrane round window

4. middle ear outer ear inner ear eardrum • Outer ear: • Mechanical protection of the middle ear • Diffracts and focuses sound waves (pinna) • The ear canal acts as a resonator (3-5 kHz enhancement) • The end of the canal has an eardrum which vibrates with sound

5. Characteristic acoustic impedance of a tube filled with gas or fluid Z0= rc/A, r-the density of the medium c-the velocity of sound A-cross-sectional area of the tube air outside salty liquid (cochlear fluid) inside middle ear inner ear outer ear stirrup hammer eardrum anvil • Middle ear: • Converts impedance of the air to the impedance of the cochlear liquid • ZAIR:ZLIQ = 1:4000 99.9% loss of energy if no impedance match • Protects inner ear • Reactions to intense sounds (but rather slow 60-120 ms reaction time) • Low-pass filter 15 dB/oct from 1 kHz

6. Cochlea middle ear outer ear inner ear oval window 0rgan of Corti basilar membrane tectorial membrane hairs round window Inner ear: Mechanical frequency analysis of the incoming sound Converts mechanical movements to electrical pulses Changes in acoustic pressure => movement of bones in middle ear => movement of membrane on oval window => vibrations in the cochlear liquid => vibrations of basilar membrane

7. Basilar membrane as a mechanical frequency analyzer pliable apical end stiff basal end 0.05 mm 0.5 mm 500 Hz 100 Hz

8. Cochlea as frequency analyzer

9. How selective is the basilar membrane ? Frequency response • Movement of the basilar membrane in dead animal observed by a microscope • von Bekesy 1960 input output system Ratio of output to input output/input frequency

10. Selectivity very different after the death of the animal! Cochlea is most likely an active system with a positive feedback loop that accounts for the high cochlear sensitivity. dead animal (von Bekesy) “tired” animal “fresh” animal

11. small piece of radioactive material glued on basilar membrane • Doppler shift in emitted g-rays indicates amplitude of the membrane vibrations Nonlinear system! (curves vary with intensity)

12. Code for the brain Sensory neurons produce spikes Spike rate increases with an increase in the stimulus intensity (here it was a weight on a muscle) Adaptation: after a while, the firing rate decreases even when the stimulus intensity stays the same

13. Action potential in a brain cell of a fly exposed to visual scenes Shapes of five individual action potential (spikes) 0 150 time [ms]

14. Stimulus at t=0 (sudden change of the scene that fly sees)

15. From movements to electrical pulses middle ear outer ear inner ear • The basilar membrane contains ~15,000-20,000 hair cells (sensory cells) • Inner hair cells transduce vibration into electrical signal and send them to the brain • Outer hair cells receive signals from the brain, which could change mechanical properties of the organ of Corti organ of Corti

16. basilar membrane movements => bending of hair cells => electrical pulses ~ 40 hairs/cell ~ 140 hairs/cell tectorial membrane tunnel of corti basilar membrane inner hair cells outer hair cells auditory nerve fiber auditory nerve fiber inner hair cells – information outer hair cells – govern cochlear mechanics ?

17. Intracellular voltage as a function of stimulus pressure (600 Hz sinusoid) one-way rectifier inner hair cell outer hair cell electrode out 0 in

18. Intracellular voltage changes in an inner hair cell for different frequencies of stimulation electrode electrode ?

19. Spikes on the auditory nerve are in phase with the signal • Only in one half of the cycle • One-way rectification • Period histogram • where the spike appears with respect to the waveform

20. Coding of the stimulus intensity threshold of firing sound level [dB]

21. Tuning curves

22. Reverse correlation technique

23. Bandwidths of tuning curves increase with frequency (frequency resolution decreases with frequency)

24. Place Theory of Hearing • Tones of certain frequencies excite certain areas of the cochlea that are connected to certain auditory fibres. • the fibres are distributed tonotopically (by their best frequencies) in the auditory nerve • this tonotopical organization is preserved throughout the higher areas of hearing all the way to the brain

25. Place theory of peripheral auditory processing firing rate depends on sound intensity BP1 BP2 Firing of the auditory nerve BRAIN signal BPn bank of cochlear band-pass filters Bandwidths of tuning curves increase with frequency (frequency resolution decreases with frequency) bandwidth sound level [dB] characteristic frequency

26. 5 frequency [kHz] 0 1.2 0 time [s]

27. Response in brain of fly to a change of the scene Response of hearing periphery to a change in acoustic scene (switching on and off a tone)

28. Response of horseshoe crab’s visual neuron to change in light

29. Two-tone suppression(lateral inhibition) tone elicits certain response (firing rate) intensity second tone in the + area increases the firing rate second tone in the – area decreases the firing rate frequency

30. Sensitivity of visual neuron (retinal ganglion cell) of a frog to changing size of a dot “on center” (“off surround”) bright dot responds to increase in light intensity “off center” (“on surround”) dark dot responds to decrease in light intensity

31. 2-dimensional “receptive field” in vision

32. Receptive field on your skin