Auditory speech detection in noise enhanced by lipreading

Audiovisual speech stimuli have been shown to produce a variety of perceptual phenomena. Enhanced detectability of acoustic speech in noise, when the talker can also be seen, is one of those phenomena. This study investigated whether this enhancement effect is specific to visual speech stimuli or can rely on more generic non-speech visual stimulus properties. Speech detection thresholds for an auditory /ba/ stimulus were obtained in a white noise masker. The auditory /ba/ was presented adaptively to obtain its 79.4% detection threshold under five conditions. In Experiment 1, the syllable was presented (1) auditory-only (AO) and (2) as audiovisual speech (AVS), using the original video recording. Three types of synthetic visual stimuli were also paired synchronously with the audio token: (3) A dynamic Lissajous (AVL) figure whose vertical extent was correlated with the acoustic speech envelope; (4) a dynamic rectangle (AVR) whose horizontal extent was correlated with the speech envelope; and (5) a static rectangle (AVSR) whose onset and offset were synchronous with the acoustic speech onset and offset. Ten adults with normal hearing and vision participated. The results, in terms of dB signal-to-noise ratio (SNR), were AVS < (AVL approximate to AVR approximate to ASR) < AO. That is, AVS was significantly easiest to detect, there was no difference among the synthesized visual stimuli, and all audiovisual conditions resulted in significantly lower thresholds than AO. To determine the advantage of the AVS stimulus, in Experiment 2, a preliminary mouth gesture was edited from the video speech token. This manipulation defeated the advantage for both the original and the edited AVS stimulus, while the audiovisual detection enhancement persisted. Overall, the results showed enhanced auditory speech detection with visual stimuli but no advantage for a fine-grained correlation between acoustic and optical speech signals. (C) 2004 Elsevier B.V. All rights reserved.