An invisible speaker can facilitate auditory speech perception

Abstract

Watching moving lips facilitates auditory speech perception when the mouth is attended. However, recent evidence suggests that visual attention and awareness are mediated by separate mechanisms. We investigated whether lip movements suppressed from visual awareness can facilitate speech perception. We used a word categorization task in which participants listened to spoken words and determined as quickly and accurately as possible whether or not each word named a tool. While participants listened to the words they watched a visual display that presented a video clip of the speaker synchronously speaking the auditorily presented words, or the same speaker articulating different words. Critically, the speaker’s face was either visible (the aware trials), or suppressed from awareness using continuous flash suppression. Aware and suppressed trials were randomly intermixed. A secondary probe-detection task ensured that participants attended to the mouth region regardless of whether the face was visible or suppressed. On the aware trials responses to the tool targets were no faster with the synchronous than asynchronous lip movements, perhaps because the visual information was inconsistent with the auditory information on 50% of the trials. However, on the suppressed trials responses to the tool targets were significantly faster with the synchronous than asynchronous lip movements. These results demonstrate that even when a random dynamic mask renders a face invisible, lip movements are processed by the visual system with sufficiently high temporal resolution to facilitate speech perception.