Motor activation from visible speech: Evidence from stimulus response compatibility

In speech perception, phonetic information can be acquired optically as wel l as acoustically. The motor theory of speech perception holds that motor c ontrol structures are involved in the processing of visible speech, whereas perceptual accounts do not make this assumption. Motor involvement in spee ch perception was examined by showing participants response-irrelevant movi es of a mouth articulating /b Lambda/ or /d Lambda/ and asking them to verb ally respond with either the same or a different syllable. The letters "Ba" and "Da" appeared on the speaker's mouth to indicate which response was to be performed. A reliable interference effect was observed. In subsequent e xperiments, perceptual interference was ruled out by using response-unrelat ed imperative stimuli and by preexposing the relevant stimulus information. Further, it was demonstrated that simple directional features (opening and closing) do not account for the effect. Rather, the present study provides evidence for the view that visible speech is processed up to a late, respo nse-related processing stage, as predicted by the motor theory of speech pe rception.