The function of consciousness in multisensory integration.

The function of consciousness was explored in two contexts of audio-visual speech, cross-modal visual attention guidance and McGurk cross-modal integration. Experiments 1, 2, and 3 utilized a novel cueing paradigm in which two different flash suppressed lip-streams cooccured with speech sounds matching one of these streams. A visual target was then presented at either the audio-visually congruent or incongruent location. Target recognition differed for the congruent versus incongruent trials, and the nature of this difference depended on the probabilities of a target appearing at these respective locations. Thus, even though the lip-streams were never consciously perceived, they were nevertheless meaningfully integrated with the consciously perceived sounds, and participants learned to guide their attention according to statistical regularities between targets and these unconsciously perceived cross-modal cues. In Experiment 4, McGurk stimuli were presented in which the lip-streams were either flash suppressed (4a) or unsuppressed (4b), and the McGurk effect was found to vanish under conditions of flash suppression. Overall, these results suggest a simple yet fundamental principle regarding the function of consciousness in multisensory integration - cross-modal effects can occur in the absence of consciousness, but the influencing modality must be consciously perceived for its information to cross modalities.