Different encoding of legal and illegal speech sequences: beyond phonetic planning?

Transforming linguistic codes into articulated speech is thought to rely on different phonetic (motor speech) encoding/planning processes for practiced sequences and for unpracticed/uncommon speech sequences. However, transforming phonological codes into articulation likely involves processes beyond phonetic planning, going on even during articulation. Here we sought behavioural, acoustic and brain dynamics differences in the preparation of matched common/legal and uncommon/illegal speech sequences in 20 participants. Illegal syllables were initialised faster - contrary to what is generally expected -, had longer acoustic duration and differed from legal syllables in ERP waveform amplitudes and microstates in a time-window preceding and following the vocal onset. The pattern of results suggests that speech plans are of different size for legal and illegal syllables, and impact on the parametrisation of the corresponding motor programmes, allowing fast execution of the segmentalised illegal sequences for which incremental speech programming continues during articulation.