Categorization of speech in early auditory evoked responses

Categorical perception is a pervasive phenomenon in phonemic identification and has been demonstrated in the last decades through behavioral experiments and, more recently, in neurophysiological studies. However, the precise neural mechanisms underlying the transformation of continuous physical properties into discrete phonemic units is still largely unknown. This paper presents a study that aims at investigating the neurological correlates of phonemic categorization at a very early stage of auditory processing. We recorded the auditory evoke potentials (AEP) of twelve subjects using scalp electrodes placed between the mastoid bone and the vertex of the head. The subjects passively heard four stimuli in the continuum /ba/-/pa/ with varying voicing onset times (VOT). Those stimuli were chosen individually for each subject according to her psychometric response to the identification task run previously. The individual AEPs were represented using the coefficients of the discrete wavelet transform. Using linear discriminant analysis, we were able to show an early categorization of the responses in terms of VOT, which mimics the behavioral responses of the subjects. The category signatures in the AEPs arose during the vocalic part of the syllable, which suggests that phonemic categorization may take place at the lower level stages of the auditory pathway.