Effects of vocoding and intelligibility on the cerebral response to speech
1 Université de Toulouse, Université Paul Sabatier, Centre de Recherche Cerveau et Cognition, 31062 Toulouse, France
2 CNRS, UMR 5549. Hôpital Purpan, Toulouse, France
3 CHU Purpan, Toulouse, France
4 Centre for Cognitive NeuroImaging (CCNi) & Institute of Neuroscience and Psychology, University of Glasgow, Glasgow, UK
5 Brain Innovation B.V., Maastricht, the Netherlands
6 International Laboratories for Brain, Music and Sound, Université de Montréal & McGill University, Montreal, Quebec, Canada
BMC Neuroscience 2011, 12:122 doi:10.1186/1471-2202-12-122Published: 30 November 2011
Degrading 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.
Vocoding 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.
Different 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.