Effects of invisible lip movements on phonetic perception.

We investigated whether 'invisible' visual information, i.e., visual information that is not consciously perceived, could affect auditory speech perception. Repeated exposure to McGurk stimuli (auditory /ba/ with visual [ga]) temporarily changes the perception of the auditory /ba/ into a 'da' or 'ga'. This altered auditory percept persists even after the presentation of the McGurk stimuli when the auditory stimulus is presented alone (McGurk aftereffect). We used this and presented the auditory /ba/ either with or without (No Face) a masked face articulating a visual [ba] (Congruent Invisible) or a visual [ga] (Incongruent Invisible). Thus, we measured the extent to which the invisible faces could undo or prolong the McGurk aftereffects. In a further control condition, the incongruent faces remained unmasked and thus visible, resulting in four conditions in total. Visibility was defined by the participants' subjective dichotomous reports ('visible' or 'invisible'). The results showed that the Congruent Invisible condition reduced the McGurk aftereffects compared with the other conditions, while the Incongruent Invisible condition showed no difference with the No Face condition. These results suggest that 'invisible' visual information that is not consciously perceived can affect phonetic perception, but only when visual information is congruent with auditory information.