Specialization of left auditory cortex for speech perception in man depends on temporal coding

Speech perception requires cortical mechanisms capable of analysing and enc oding successive spectral (frequency) changes in the acoustic signal. To st udy temporal speech processing in the human auditory cortex, we recorded in tracerebral evoked potentials to syllables in right and left human auditory cortices including Heschl's gyrus (HG), planum temporale (Pi) and the post erior part of superior temporal gyrus (area 22). Natural voiced (/ba/ /da/ /ga/ and voiceless (/pa/, /ta/, /ka/) syllables, spoken by a native French speaker, were used to study the processing of a specific temporally based a coustico-phonetic feature, the voice onset time (VOT). This acoustic featur e is present in nearly all languages, and it is the VOT that provides the b asis for the perceptual distinction between voiced and voiceless consonants . The present results show a lateralized processing of acoustic elements of syllables. First, processing of voiced and voiceless syllables is distinct in the left, but not in the right HG and PT. Second, only the evoked poten tials in the left HG, and to a lesser extent in Pi, reflect a sequential pr ocessing of the different components of the syllables. Third, we show that this acoustic temporal processing is not limited to speech sounds hut appli es also to non verbal sounds mimicking the temporal structure of the syllab le. Fourth, there was no difference between responses to voiced and voicele ss syllables in either left or right areas 22. Our data suggest that a sing le mechanism in the auditory cortex, involved in general (not only speech-s pecific) temporal processing, may underlie the further processing of verbal land non-verbal) stimuli. This coding, bilaterally localized in auditory c ortex in animals, takes place specifically in the left HG in man. A defect of this mechanism could account for hearing discrimination impairments asso ciated with language disorders.