Effects of vocoding and intelligibility on the cerebral response to speech

Kuzma Strelnikov,Julien Rouger,Pascal Barone,Zoé Massida,Pascal Belin

doi:10.1186/1471-2202-12-122

Abstract

BackgroundDegrading speech through an electronic synthesis technique called vocoding has been shown to affect cerebral processing of speech in several cortical areas. However, it is not clear whether the effects of speech degradation by vocoding are related to acoustical degradation or by the associated loss in intelligibility. Using vocoding and a parametric variation of the number of frequency bands used for the encoding, we investigated the effects of the degradation of auditory spectral content on cerebral processing of intelligible speech (words), unintelligible speech (words in a foreign language), and complex environmental sounds.ResultsVocoding was found to decrease activity to a comparable degree for intelligible and unintelligible speech in most of the temporal lobe. Only the bilateral posterior temporal areas showed a significant interaction between vocoding and intelligibility, with a stronger vocoding-induced decrease in activity for intelligible speech. Comparisons to responses elicited by environmental sounds showed that portions of the temporal voice areas (TVA) retained their greater responses to voice even under adverse listening conditions. The recruitment of specific networks in temporal regions during exposure to degraded speech follows a radial and anterior-posterior topography compared to the networks recruited by exposure to speech that is not degraded.ConclusionsDifferent brain networks are involved in vocoded sound processing of intelligible speech, unintelligible speech, and non-vocal sounds. The greatest differences are between speech and environmental sounds, which could be related to the distinctive temporal structure of speech sounds.

Highlights

Degrading speech through an electronic synthesis technique called vocoding has been shown to affect cerebral processing of speech in several cortical areas
Behavioural results In both the Intelligibility and Voice Discrimination tasks, correct responses decreased with a decreasing number of vocoder channels (Figure 1)
The results of the present study suggest that the correlations with vocoder degradation reported in previous studies with only linguistic stimuli [3-5] cannot be attributed to linguistic processes, and areas in the temporal cortex and in the cerebellum are involved in speech intelligibility

Summary

Introduction

Degrading speech through an electronic synthesis technique called vocoding has been shown to affect cerebral processing of speech in several cortical areas. There has been an increasing number of studies that focused on the cerebral mechanisms of distorted speech processing [1-5] These studies are warranted because distorted speech may constitute a model of complex speech comprehension, which is analogous to the complexity of imperfect real-life conditions. These studies should have clinical implications, because they serve to further elucidate the cerebral mechanisms of sound perception after cochlear implantations. High levels of speech comprehension can be achieved over the first year of using the neuroprostheses [13,14]

Methods

Results

Discussion

Conclusion