The impact of when, what and how predictions on auditory speech perception.

An impressive number of theoretical proposals and neurobiological studies argue that perceptual processing is not strictly feedforward but rather operates through an interplay between bottom-up sensory and top-down predictive mechanisms. The present EEG study aimed to further determine how prior knowledge on auditory syllables may impact speech perception. Prior knowledge was manipulated by presenting the participants with visual information indicative of the syllable onset (when), its phonetic content (what) and/or its articulatory features (how). While when and what predictions consisted of unnatural visual cues (i.e., a visual timeline and a visuo-orthographic cue), how prediction consisted of the visual movements of a speaker. During auditory speech perception, when and what predictions both attenuated the amplitude of N1/P2 auditory evoked potentials. Regarding how prediction, not only an amplitude decrease but also a latency facilitation of N1/P2 auditory evoked potentials were observed during audiovisual compared to unimodal speech perception. However, when and what predictability effects were then reduced or abolished, with only what prediction reducing P2 amplitude but increasing latency. Altogether, these results demonstrate the influence of when, what and how visually induced predictions at an early stage on cortical auditory speech processing. Crucially, they indicate a preponderant predictive role of the speaker's articulatory gestures during audiovisual speech perception, likely driven by attentional load and focus.