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Communication acoustics Ch 7: Physiology and Anatomy of Hearing Ville Pulkki and Matti Karjalainen Department of Signal Processing and Acoustics Aalto University, Finland September 14, 2016 This chapter Structure of ear Cochlea Functioning of the cochlea Cochlear non-linearities Auditory nerve Auditory nervous system Structure of the ear malleus incus stapes and oval window semicircular canals auditory nerve cochlea round window Eustachian tube pinna external middle inner ear concha eardrum ear canal Simplified diagram of the ear external ear middle ear eardrum inner ear air liquid auditory nerve ear canal ossicles pinna Eustachian tube traveling wave basilar membrane Acoustical effect of outer ear magnitude [dB] 20 10 0 -10 1 3 10 frequency [kHz] Magnitude response from frontal sound source to eardrum Middle ear: bone conduction Ossicles Malleus (hammer-shaped bone) Incus (anvil-shaped bone) Stapes (stirrup-shaped bone) Match partially the impedance difference from air to liquid (1:3000), malleus incus A1 oval window A2 P2 P1 eardrum oval window stapes eardrum a) b) Middle ear conduction and features Signal transfer function is a bandpass filter |SVFT| [dB (ref 1 mm/s/Pa)] −10 −15 −20 −25 −30 −35 2 10 3 10 Frequency [Hz] 4 10 Adapted from Aibara et al. (2001) Other middle ear features Acoustic reflex = stiffening of muscles attached to ossicles with loud sounds Eustachian tube, balancing air pressure between the middle ear and the environment Structure of the ear malleus incus stapes and oval window semicircular canals auditory nerve cochlea round window Eustachian tube pinna external middle inner ear concha eardrum ear canal Inner ear, the Cochlea Cochlea is a spiral-shaped, liquid-filled tube of about 2.7 turns and 35 mm long Stapes vibration enters the cochlea through oval window, and exits from round window Basilar membrane divides the cochlea into two parts helicotrema bony shelf oval window Reissner’s membrane base (basal end) apex (apical end) stapes basilar membrane round window Inner ear, the Cochlea Basilar membrane between bony shelves Division to scala vestibuli and scala tympani Reissner’s membrane separates scala media, where higher concentration of K+ Organ of Corti: hair cells (shown as shaded) Tectrorial membrane Re scala media iss ne tectorial membrane r's m scala vestibuli em br an e outer inner hair cells auditory nerve bony shelf basilar membrane scala tympani Hair cells S inner haircell S N outer haircell N S + + K K Hair cells Vibration of the basilar membrane causes bending of stereocilia and this opens ion channels which modulates potential within the cell Activation of the cell releases neurotransmitter to synaptic junctions between hair cell and neural fibers of the auditory nerve A neural spike is generated that propagates in the auditory nerve fiber Next spike possible only after at least 1 ms Passive frequency selectivity in cochlea Basilar membrane is nonhomogeneous transmission line Frequencies resonate at different positions oval window basilar membrane stapes helicotrema liquid inertia increase vibration membrane mass & width increase stiffness decrease round window high f med f low f Traveling wave in basilar membrane Traveling wave has maximum vibration amplitude depending on the frequency of wave (characteristic frequency = CF) High frequencies resonate close to the oval window and low frequencies close to helicotrema amplitude maximum vibration of membrane traveling wave direction apex base basilar membrane position direction of liquid vibration Active processing in cochlea Outer hair cells actively amplify vibration at their characteristic frequency Effect is highest at low levels 60 velocity / pressure gain [mm / s / Pa] velocity [dB] 20 gain 5 & 10 dB 30 40 40 20 50 60 0 70 80 -20 0 30 60 input SPL [dB] 90 2 3 4 5 6 8 10 stimulus frequency [kHz] Adopted from Ruggero et al. (1997) 20 Velocity of basilar membrane with different levels Higher level causes broader excitation in frequency Excitation spreads more towards higher frequencies characteristic frequency [Hz] 40 dB 500 Hz sine 90 dB 500 Hz sine 326 391 464 545 636 737 850 977 1118 1276 1452 1649 1869 2114 2389 0 5 10 time [ms] 15 20 0 5 10 time [ms] 15 20 Animations Link to cochlea anatomy animation Link to cochlea / organ of corti animation Auditory nerve / auditory cortex demo Auditory nerve fibers Several auditory nerves are connected to each inner hair cell Auditory nerves send a spike (binary output) when they receive enough neurotransmitter from hair cell Different nerves are differently sensitive to level Firing rate [spikes/s] 80 60 (h) 40 (m) (l) 20 0 0 20 40 60 80 Stimulus level [dB] 100 Auditory nerve fibers Firing rate overshoot and undershoot with onset and offset of excitation Firing rate [spikes/s] Excitation level vs time high > 40 20-30 < 10 0 0 150 300 Time [ms] 450 600 Auditory nerve fibers Response of nerves with different frequencies Firing rate Statistically, half-wave rectification appears 340 Hz one cycle Adapted from Joris et al. (1994) 670 Hz Time 1425 Hz 2830 Hz 7100 Hz Auditory nerve fibers Response of nerves in cat with vowel sounds /a/ and /I/ with different levels Average rate shows increasing saturation with level Frequency distribution of firing rate does not carry all information The instantaneous temporal pattern of activation seems to carry more information Normalized firing rate 1.0 0.5 0.0 1.0 0.5 /I/ 54dB 44dB 34dB 47dB 37dB 0.0 0.1 /a/ 27dB 0.5 1.0 Characteristic frequency [Hz] 5.0 10.0 Adapted from Sachs and Young (1979) Higher levels in processing Eye Ear Cochlea Cochlear nucleus SC spatial cues IC LSO Left hemisphere - Right hemisphere LSO + VCN DCN Transforming vibrations to neural impulses MGB, Cortex + MSO + Preamplification, preprocessing L NL spectral cues Head movement information IC MSO Processing of directional cues in left-right dimension Processing of spectral information Integration of visual and auditory cues Integration of auditory and movement [front-back-inside resolving] Spatial segregation of auditory events References These slides follow corresponding chapter in: Pulkki, V. and Karjalainen, M. Communication Acoustics: An Introduction to Speech, Audio and Psychoacoustics. John Wiley & Sons, 2015, where also a more complete list of references can be found. References used in figures: Aibara, R., Welsh, J.T., Puria, S., and Goode, R.L. (2001) Human middle-ear sound transfer function and cochlear input impedance. Hearing Res., 152(1), 100–109. Joris, P.X., Carney, L.H., Smith, P.H., and Yin, T. (1994) Enhancement of neural synchronization in the anteroventral cochlear nucleus. I. responses to tones at the characteristic frequency. J. Neurophys., 71(3), 1022–1036. Ruggero, M.A., Rich, N.C., Recio, A., Narayan, S.S., and Robles, L. (1997) Basilar-membrane responses to tones at the base of the chinchilla cochlea. J. Acoust. Soc. Am., 101(4), 2151–2163. Sachs, M.B. and Young, E.D. (1979) Encoding of steady-state vowels in the auditory nerve: Representation in terms of discharge rate. J. Acoust. Soc. Am., 66, 470–479. Physiology and anatomy of hearing Pulkki Dept Signal Processing and Acoustics 23/23 September 14, 2016