Now You Hear It, Now You Don't: Transient Traces of Consonants and their Nonspeech Analogues in the Human Brain

Jonas Obleser,Sophie K Scott,Carsten Eulitz

doi:10.1093/cercor/bhj047

Abstract

The apparently effortless identification of speech is one of the human auditory cortex' finest and least understood functions. This is partly due to difficulties to tease apart effects of acoustic and phonetic attributes of speech sounds. Here we present evidence from magnetic source imaging that the auditory cortex represents speech sounds (such as [g] and [t]) in a topographically orderly fashion that is based on phonetic features. Moreover, this mapping is dependent on intelligibility. Only when consonants are identifiable as members of a native speech sound category is topographical spreading out in the auditory cortex observed. Feature separation in the cortex also varies with a listener's ability to tell these easy-to-confuse consonants from one another. This is the first demonstration that speech-specific maps of features can be identified in human auditory cortex, and it will further help us to delineate speech processing pathways based on models from functional neuroimaging and non-human primates.

Highlights

Whenever our ear is hit by speech, a cascade of automatic processing steps takes place, leading to a surprisingly robust mapping of the heard sound stream onto meaning
This study set out to scrutinize the temporal and topographical mapping of consonantal speech sounds in human auditory cortex, and to determine the influence of intelligibility on this mapping, as opposed to purely acoustic variation
Would acoustic manipulations that preserve the spectro-temporal complexity while rendering the consonants unintelligible evoke the same NIOOm mapping? Ifso, the NIOOm could hardly be interpreted as reflecting a cognitive processing step, such as feature integration and categorical perception

Summary

Introduction

Whenever our ear is hit by speech, a cascade of automatic processing steps takes place, leading to a surprisingly robust mapping of the heard sound stream onto meaning. This is possible only because of efficient, yet largely unrevealed decoding of the speech signal throughout structures of the auditory pathway. Within recent years and mainly due to the emergence of powerful neuroimaging techniques, the neuroanatomical structures subserving speech perception have been unravelled It has been shown repeatedly and is widely accepted that structures surrounding the primary auditory areas (located on medial parts of HeschI's gyrus in the supratemporal plane) are crucially involved in speech processing. A major advantage for speech processing is the potential to identify the signature of an initial processing step in the speech 'sound decoding cascade, e.g. as reflected in the N100/ N100m brain wave deflection elicited - 100 ms after stirnulus onset by a vast array of auditory events in virtually every healthy subject (Naatanen and Picton, 1987; Naatanen and Winkler, 1999)

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