Extraction of Inter-Aural Time Differences Using a Spiking Neuron Network Model of the Medial Superior Olive.

Jörg Encke,Werner Hemmert

doi:10.3389/fnins.2018.00140

Jörg Encke, Werner Hemmert

Open Access

https://doi.org/10.3389/fnins.2018.00140

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Abstract

The mammalian auditory system is able to extract temporal and spectral features from sound signals at the two ears. One important cue for localization of low-frequency sound sources in the horizontal plane are inter-aural time differences (ITDs) which are first analyzed in the medial superior olive (MSO) in the brainstem. Neural recordings of ITD tuning curves at various stages along the auditory pathway suggest that ITDs in the mammalian brainstem are not represented in form of a Jeffress-type place code. An alternative is the hemispheric opponent-channel code, according to which ITDs are encoded as the difference in the responses of the MSO nuclei in the two hemispheres. In this study, we present a physiologically-plausible, spiking neuron network model of the mammalian MSO circuit and apply two different methods of extracting ITDs from arbitrary sound signals. The network model is driven by a functional model of the auditory periphery and physiological models of the cochlear nucleus and the MSO. Using a linear opponent-channel decoder, we show that the network is able to detect changes in ITD with a precision down to 10 μs and that the sensitivity of the decoder depends on the slope of the ITD-rate functions. A second approach uses an artificial neuronal network to predict ITDs directly from the spiking output of the MSO and ANF model. Using this predictor, we show that the MSO-network is able to reliably encode static and time-dependent ITDs over a large frequency range, also for complex signals like speech.

Highlights

Our remarkable sound localization acuity relies on the ability of the auditory system to decode the arrival time and intensity difference between the ear canal signals into information about the direction of sound sources
We presented a novel binaural model and used it to detect interaural time differences (ITDs) in arbitrary sound signals
In contrast to previous binaural models that used a phenomenological modeling approach (Pulkki and Hirvonen, 2009; Dietz et al, 2011), this study used biophysical neuron models based on the current knowledge about the function of the mammalian medial superior olive (MSO)

Summary

Introduction

Our remarkable sound localization acuity relies on the ability of the auditory system to decode the arrival time and intensity difference between the ear canal signals into information about the direction of sound sources. The existence of such neurons was already hypothesized by Jeffress (1948), who proposed an array of coincident detectors to be arranged along a neural delay line In this hypothesis, each neuron would respond maximally to a specific ITD (best-ITD)—generating a ITD Extraction Using an MSO Model topographical mapping of time differences within the nucleus. Lesion studies in cats showed that unilateral lesions at the level of the central auditory system (Jenkins and Masterton, 1982) as well as in cortical regions (Malhotra et al, 2004) mainly resulted in deficits localizing sounds from locations contralateral of the lesion These results lead Jenkins and Masterton (1982) to conclude that each auditory-hemifield is represented solely in the respective contralateral hemisphere, which would contradict the opponent coding mechanism. Results from psychoacoustic lateralization experiments using pure-tone adapter stimuli with fixed ITDs showed, that adaptation influences lateralization at ITDs close to that of the adapter but within the whole hemisphere (Phillips et al, 2006), which is more in line with an opponentchannel code

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