Spatial hearing and sound localization mechanisms in the brain

Henri Pöntynen

February 9, 2016

Outline

• Auditory periphery: from acoustics to neural signals

- Basilar membrane

- Organ of Corti

• Spatial cues and neural processing

- ILD- ITD- Monaural cues

• References

Auditory periphery

Auditory periphery: organs of hearing

● Outer ear: protection, amplification, localization cues

● Middle ear: amplification (impedance matching), protection

● Inner ear: transduction of acoustic energy into neural signals

Cochlear anatomy● Coiled structure with three liquid filled chambers

● Transduction by the basilar membrane and the organ of Corti

Basilar membrane

● Sound pressure variations coupled to the inner ear via the tympanic membrane and the ossicular chain

● The mechanical properties of the basilar membrane vary along its length

→ Wide and flexible apex→ Stiff and narrow base

● Resonance position dependson stimulus frequency

→ Tonotopy

● Complex stimuli decomposed into frequency bands by ”mechanical frequency analysis”

Cochlear transduction in the Organ of Corti

Localization cues

Localization cues● Direction-dependent features in acoustic signals

● Horizontal localization via binaural cues

→ Signal sampled at 2 spatial locations, i.e., at the 2 ears

→ Interaural level difference (ILD)

→ Interaural time difference (ITD)

● Elevation via monaural cues

→ Spectral features of the sound pressure signal at the eardrum

Brain circuit for spatial cues

Binaural cues: ITD

● Difference in signal arrival times at the two ears is determined by horizontal coordinate of the source.

● Delays < 1ms

● Effective at low frequncies

Medial superior olive (MSO)

Coincidence detector model of MSO

● ITD processed in the MSO by coincidence detection principle

● Tuned detector neurons show highest activity when left/right signals are present simultaneously.

● Axon length compensates for ITD to align the signals at some detector neuron

● ITDs represented as activation of corresponding coincidence detectors.

Binaural cues: ILD● Diffraction when wavelength > dimensions of the head

→ Sound waves are unaffected by the acoustic obstacle presented by the head→ Sound pressure level is the same at both ears.

● At high frequencies, the head forms an acoustic shadow

→ Lower sound pressure level at the contralateral ear→ The degree of shadowing depends on the horizontal coordinate of the source→ Maximum ILD at ±90°, minimum ILD at 0° & 180°

Lateral superior olive (LSO)

Neural processing of ILD● LSO in the superior olive

● MNTB = medial nucleus of the trapezoidal body

● IE neurons in the LSO perform ”binaural substraction” to estimate the level difference.

→ Excitatory input from ipsilateral cochelea,

→ Inhibitory input from contralateral cochlea

→ Spike rate of LSOs in both hemispheres determined by ILD

Monaural cues

● Spectral features formed by constructive and destructive interference of direct sound and delayed reflections

→ Shoulder reflections→ Reflections within pinna cavities

● All source locations produce a unique reflection pattern

→ Head-related transfer function (HRTF)→ Unique to each subject

Monaural cues

Neural processing of monaural cues

● Neural processing of monaural cues is poorly understood

● A1: Neuron circuit in dorsal cochlear nucleus

→ Interaction between type II, IV and ”wideband inhibitor” neurons.

→ Inhibited response to tuned spectral notches

● Type IV output projected to type O neurons in the IC

● A2: Neuron circuit in inferior colliculus

→ Interaction between type O, IV and inhibitory and ”wideband excitatory” neurons

→ Excitatory response to tuned spectral notches

Summary

Summary

● Stimuli divided into frequency bands by the basilar membrane

● Low-level localization mechanisms in the midbrain

● Horizontal coordinate resolved mainly with binaural cues

→ interaural time difference at low frequencies (MSO)→ Interaural level difference at higher frequencies (LSO)

● Monaural cues: horizontal and vertical coordinates

→ Spectral features formed by acoustic reflection patterns→ Processed by neurons sensitive to spectral notches

Related literatureBlauert, Jens. Spatial hearing: the psychophysics of human sound localization. MIT press, 1997.

Schnupp, Jan, Israel Nelken, and Andrew King. Auditory neuroscience: Making sense of sound. MIT Press, 2011.

Plack, Christopher J. The sense of hearing. Psychology Press, 2013.

William M. Hartmann. Signals, sound, and sensation. Springer Science & Business Media, 1997.

Pulkki, Ville, and Matti Karjalainen. Communication Acoustics: An Introduction to Speech, Audio and Psychoacoustics. John Wiley & Sons, 2015.

Bear, M. F., Connors, B. W., Paradiso, M., Bear, M. F., Connors, B. W., & Neuroscience, M. A. (1996). Exploring the brain. Neuroscience: Williams & Wilkins.

Ahveninen, Jyrki, Norbert Kopčo, and Iiro P. Jääskeläinen. "Psychophysics and neuronal bases of sound localization in humans." Hearing research 307 (2014): 86-97.

Grothe, Benedikt, Michael Pecka, and David McAlpine. "Mechanisms of sound localization in mammals." Physiological Reviews 90.3 (2010): 983-1012.

Takanen, Marko, et al. "Evaluation of sound field synthesis techniques with a binaural auditory model." Audio Engineering Society Conference: 55th International Conference: Spatial Audio. Audio Engineering Society, 2014.

Takanen, Marko, Olli Santala, and Ville Pulkki. "Visualization of functional count-comparison-based binaural auditory model output." Hearing research 309 (2014): 147-163.

Takanen, Marko O., Olli Santala, and Ville Pulkki. "Combining the outputs of functional models of organs responsible for binaural cue decoding." Proceedings of Meetings on Acoustics. Vol. 19. No. 1. Acoustical Society of America, 2013