Auditory and auditory-visual perception of clear and conversational speech.

Research has shown that speaking in a deliberately clear manner can improve the accuracy of auditory speech recognition. Allowing listeners access to visual speech cues also enhances speech understanding. Whether the nature of information provided by speaking clearly and by using visual speech cues is redundant has not been determined. This study examined how speaking mode (clear vs. conversational) and presentation mode (auditory vs. auditory-visual) influenced the perception of words within nonsense sentences. In Experiment 1, 30 young listeners with normal hearing responded to videotaped stimuli presented audiovisually in the presence of background noise at one of three signal-to-noise ratios. In Experiment 2, 9 participants returned for an additional assessment using auditory-only presentation. Results of these experiments showed significant effects of speaking mode (clear speech was easier to understand than was conversational speech) and presentation mode (auditory-visual presentation led to better performance than did auditory-only presentation). The benefit of clear speech was greater for words occurring in the middle of sentences than for words at either the beginning or end of sentences for both auditory-only and auditory-visual presentation, whereas the greatest benefit from supplying visual cues was for words at the end of sentences spoken both clearly and conversationally. The total benefit from speaking clearly and supplying visual cues was equal to the sum of each of these effects. Overall, the results suggest that speaking clearly and providing visual speech information provide complementary (rather than redundant) information.