Entrained theta oscillations guide perception of subsequent speech: behavioural evidence from rate normalisation

Hans Rutger Bosker,Oded Ghitza

doi:10.1080/23273798.2018.1439179

Abstract

ABSTRACTThis psychoacoustic study provides behavioural evidence that neural entrainment in the theta range (3–9 Hz) causally shapes speech perception. Adopting the “rate normalization” paradigm (presenting compressed carrier sentences followed by uncompressed target words), we show that uniform compression of a speech carrier to syllable rates inside the theta range influences perception of subsequent uncompressed targets, but compression outside theta range does not. However, the influence of carriers – compressed outside theta range – on target perception is salvaged when carriers are “repackaged” to have a packet rate inside theta. This suggests that the brain can only successfully entrain to syllable/packet rates within theta range, with a causal influence on the perception of subsequent speech, in line with recent neuroimaging data. Thus, this study points to a central role for sustained theta entrainment in rate normalisation and contributes to our understanding of the functional role of brain oscillations in speech perception.

Highlights

Speech is a communicative signal with inherent slow amplitude modulations
The functional role of this neural entrainment in speech perception remains a topic of debate: is entrainment causally involved in shaping successful speech perception (Riecke, Formisano, Sorger, Başkent, & Gaudrain, 2018; Zoefel, Archer-Boyd, & Davis, 2018) or is it merely a response-driven epiphenomenon of speech processing (Obleser, Herrmann, & Henry, 2012)? The present study will put forward psychoacoustic findings suggesting that, does neural entrainment to a particular syllable rate shape the decoding of concurrent speech (Experiment 1), and that neural entrainment might persist when the entraining rhythm has ceased, influencing the perception of subsequently presented words (Experiments 2 and 3)
The extent to which these theta oscillations persisted into the target window correlated with the observed behavioural biases: the more evidence for sustained entrainment in the target window, the greater the behavioural rate normalisation effect (Kösem et al, 2017). These findings suggest that neural oscillations actively shape speech perception (Bosker, 2017a; Bosker & Kösem, 2017; Peelle & Davis, 2012)

Summary

Introduction

Speech is a communicative signal with inherent slow amplitude modulations. These amplitude modulations fluctuate at a rate around 3–9 Hz, in various speech types and in different languages (Bosker & Cooke, in press; Ding et al, 2017; Krause & Braida, 2004; Varnet, Ortiz-Barajas, Erra, Gervain, & Lorenzi, 2017), driven primarily by the syllabic rate of speech. The extent to which these theta oscillations persisted into the target window correlated with the observed behavioural biases: the more evidence for sustained entrainment in the target window, the greater the behavioural rate normalisation effect (Kösem et al, 2017) These findings suggest that neural oscillations actively shape speech perception (Bosker, 2017a; Bosker & Kösem, 2017; Peelle & Davis, 2012). Compressed speech with a syllable rate of 15 Hz would not be predicted to bias the perception of subsequent Dutch target vowels towards /a:/, because the syllable rate is outside theta If this compressed speech signal would be repackaged such that the packet delivery rate would fall below 9 Hz, rate normalisation effects should be restored. If Experiment 1, targeting overall intelligibility, succeeds in replicating the findings from Ghitza (2014), this allows us to use the Dutch speech materials for the subsequent experiments, targeting rate normalisation

Method

Results