Discrepant visual speech facilitates covert selective listening in "cocktail party" conditions

The presence of congruent visual speech information facilitates the identification of auditory speech, while the addition of incongruent visual speech information often impairs accuracy. This latter arrangement occurs naturally when one is being directly addressed in conversation but listens to a different speaker. Under these conditions, performance may diminish since: (a) one is bereft of the facilitative effects of the corresponding lip motion and (b) one becomes subject to visual distortion by incongruent visual speech; by contrast, speech intelligibility may be improved due to (c) bimodal localization of the central unattended stimulus. Participants were exposed to centrally presented visual and auditory speech while attending to a peripheral speech stream. In some trials, the lip movements of the central visual stimulus matched the unattended speech stream; in others, the lip movements matched the attended peripheral speech. Accuracy for the peripheral stimulus was nearly one standard deviation greater with incongruent visual information, compared to the congruent condition which provided bimodal pattern recognition cues. Likely, the bimodal localization of the central stimulus further differentiated the stimuli and thus facilitated intelligibility. Results are discussed with regard to similar findings in an investigation of the ventriloquist effect, and the relative strength of localization and speech cues in covert listening.