Categorical speech perception during active discrimination of consonants and vowels

Categorical perception of phonemes describes the phenomenon that, when phonemes are classified they are often perceived to fall into distinct categories even though physically they follow a continuum along a feature dimension. While consonants such as plosives have been proposed to be perceived categorically, the representation of vowels has been described to be more continuous. We aimed at testing this difference in representation at a behavioral and neurophysiological level using human magnetoencephalography (MEG). To this end, we designed stimuli based on natural speech by morphing along a phonological continuum entailing changes of the voiced stop-consonant or the steady-state vowel of a consonant-vowel (CV) syllable. Then, while recording MEG, we presented participants with consecutive pairs of either same or different CV syllables. The differences were such that either both CV syllables were from within the same category or belonged to different categories. During the MEG experiment, the participants actively discriminated the stimulus pairs. Behaviorally, we found that discrimination was easier for the between-compared to the within-category contrast for both consonants and vowels. However, this categorical effect was significantly stronger for the consonants compared to vowels, in line with a more continuous representation of vowels. At the neural level, we observed significant repetition suppression of MEG evoked fields, i.e. lower amplitudes for physically same compared to different stimulus pairs, at around 430 to 500ms after the onset of the second stimulus. Source reconstruction revealed generating sources of this repetition suppression effect within left superior temporal sulcus and gyrus, posterior to Heschl׳s gyrus. A region-of-interest analysis within this region showed a clear categorical effect for consonants, but not for vowels, providing further evidence for the important role of left superior temporal areas in categorical representation during active phoneme discrimination.