Visual speech improves the intelligibility of time-expanded auditory speech.

This study investigated the effects of intermodal timing differences and speed differences on word intelligibility of auditory-visual speech. Words were presented under visual-only, auditory-only, and auditory-visual conditions. Two types of auditory-visual conditions were used: asynchronous and expansion conditions. In the asynchronous conditions, the audio lag was 0-400 ms. In the expansion conditions, the auditory signal was time expanded (0-400 ms), whereas the visual signal was kept at the original speed. Results showed that word intelligibility was higher in the auditory-visual conditions than in the auditory-only condition. The results of auditory-visual benefit revealed that the benefit at the end of words declined as the amount of time expansion increased, although it did not decline in the asynchronous conditions.