Cerebral hemispheric involvement in the acquisition of new phonetic categories

This study was concerned with the involvement of the cerebral hemispheres in the acquisition of perceptual skill with novel speech sounds. Two groups of eight subjects rated on three occasions the dissimilarity of pairs of Mandarin Chinese speech sounds varying on voicing and aspiration, presented to the left or right ear with contralateral noise. The experimental group received listening experience with long passages of Mandarin containing the target sounds. Multidimensional scaling analysis of dissimilarity ratings indicated that listening experience leads to increased perceptual differentiation of phonetic categories drawn from a language unfamiliar to the listener. This improvement occurred sooner with presentation of the target sounds to the right hemisphere than to the left if the phonological contrast was along the voicing dimension. Improvement in the perception of stimuli varying on aspiration occurred in right ear presentations only. This finding supports the position that speech perception mechanisms at the feature level may be distributed asymmetrically across the hemispheres.