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The auditory system Romain Brette ( @ens.fr) Romain Brette Ecole Normale Supérieure

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Text of The auditory system Romain Brette ( @ens.fr) Romain Brette Ecole Normale Supérieure

Neurons

The auditory systemRomain Brette ([email protected])Romain BretteEcole Normale Suprieure

What is sound?Hearing vs. seeingHearingSeeingAcoustical waves, 20 20,000 Hz= 1.7 cm 17 mElectromagnetic waves, 380-740 nmSpeed of sound: 343 m/s at 20C.3Hearing vs. seeingHearingSeeingAcoustical waves, 20 20,000 Hz= 1.7 cm 17 mElectromagnetic waves, 380-740 nmSpeed of sound: 343 m/s at 20C.4Hearing vs. seeingHearingSeeingAcoustical waves, 20 20,000 Hz= 1.7 cm 17 mElectromagnetic waves, 380-740 nmInformation about volumesInformation about surfacesSpeed of sound: 343 m/s at 20C.5Hearing vs. seeingHearingSeeingAcoustical waves, 20 20,000 Hz= 1.7 cm 17 mElectromagnetic waves, 380-740 nmInformation about volumesInformation about surfacesSounds are produced by sourcesLight is reflected by sourcesSpeed of sound: 343 m/s at 20C.6Hearing vs. seeingHearingSeeingAcoustical waves, 20 20,000 Hz= 1.7 cm 17 mElectromagnetic waves, 380-740 nmInformation about volumesInformation about surfacesSounds are produced by sourcesLight is reflected by sourcesThe source is transient, sounds are eventsThe source is persistent, one can look around a visual objectSpeed of sound: 343 m/s at 20C.7Hearing vs. seeingHearingSeeingSounds from different locations are mixed at the earLight rays from different locations are separated in the eye

Speed of sound: 343 m/s at 20C.8The information in soundSpatial location

Vision: Direction of an object is mapped to place on the retina.Place on the retina varies systematically with self-generated movements.Hearing:Direction is mapped to relationships between binaural signals, among other cuesRelationships vary systematically with self-generated movements,but only if sounds are repeated

More about this: http://briansimulator.org/category/romains-blog/what-is-sound/9The information in soundShape

Vision: the way the visual field changes with viewpoint determines the visual shapeHearing: the sound does not change with viewpoint.But: there is information about shape in the spectrum.Larger object => smaller frequencies (= change of space units).

M. Kac (1966) Can one hear the shape of a drum? Am. Math. Monthly 73 (4)W.W. Gaver (1993) What in the world do we hear? Ecological Psychology 5(1)In speech: shape of the vocal tract is linguistic informationThe sound does not change with viewpoint: there is no intrinsic information about shape (i.e., the information cannot be related to spatial shape without an external label given e.g. by vision or touch).10The information in soundPitch

In voiced vowels, the glottis opens and closes at a fast rate, producing a periodic sound (typically about 100 Hz for men, 200 Hz for women).

Vowel oRepetition rate contains information about intonation and speaker (used for grouping)The information in soundSummary: what the auditory system needs to process

Precise temporal and intensity relationships between binaural signals

Frequency spectrum

Temporal information

More generally: spectro-temporal information at different scalest*f>1/2(Gabor)The time-frequency trade-off:Anatomy and physiology of the auditory systemThe ear

cochleainner earvestibular system (head movements)cochlea (hearing)outer earmiddle earinner earThe basilar membrane

The basilar membrane

Hair cells

outer hair cellsinner hair cellsauditory nervetectorial membranebasilar membrane

Section of the organ of CortiOuter hair cells: involved in frequency selectivity and sensitivity (amplification)Inner hair cells are the main transducers. Most AN fibers (type I) contact inner hair cellsHair cells have stereociliaElectron microscopy image17Hair cells

K+ channels open when the stereocilia is deflectedCreates a current18Auditory nerve fibers

Tuning curves(threshold)

Response curves(Response curves: Rose et al. 1971, probably cat)(tuning curves: from Liberman & Kiang 1978, 6 different units in the cat)Hair cells are contacted by AN fibers.Also: adaptation19Phase locking

Response to a tone (multiple trials):Time (ms)Phase locking: neurons fire at preferred phases of the input tonePhase

(Phase locking: from Philip Joris)(right: Rose et al. 1967, 1s tone, 80 dB SPL)20Phase locking

(barn owl)

Response to a tone (multiple trials):Time (ms)Phase locking: neurons fire at preferred phases of the input tonePhaseVector strength(Phase locking: from Philip Joris)Phase locking to high frequencies in the auditory nerve and cochlear nucleus magnocellularis of the barn owl, Tyto alba.Kppl C.J Neurosci. 1997 May 1;17(9):3312-21.21Reverse correlation

At different levels. Cat AN fibers. Carney & Yin (1988).Explain what it is.22A simple model of auditory nerve fibers

bank of filterssoundNB: does not capture nonlinear effects

half-wave rectification(+ possibly low-pass filtering for decrease of phase-locking)

+ random spikes (Poisson)MNTBICCICCDNLLINLLVNLLDNLLINLLVNLLLNTBLNTBLSOMSOSPNSPNMNTBLSOMSODCNPVCNAVCNDCNAVCNPVCNDCDCSCSCLNLNMMGBDMGBVMGBSGNPFInsCAIIAIPFInsCAIIAIMNTBNCATNCATN.VIIIMMGBDMGBVMGBSGNThe rest of the auditory systemA large part is subcortical and even before thalamus (brainstem)24Sound localization: acoustical cues3D localization

= azimuthd = elevation(azimuth)Acoustical cues for sound localization

or head related impulse responses (time domain; HRIRs)Other cues for distance: level is distance-dependent high frequencies are more filtered with distance reverberation correlates with distanceOther cues for elevation: pinna filters out specific frequencies depending on elevation(convolution)27HRTFs and HRIRs in the rabbitKim et al., JARO (2010)

HRIRHRTF28Interaural time differences (ITDs)

distant sound source = plane wavePath length difference with spherical head:r(sin + )This is valid when wavelength