Speech perception is audiovisual as evidenced by bimodal integration in the McGurk effect. This integration effect may be specific to speech or be applied to all stimuli in general. To investigate this, Tuomainen et al. (2005) used sine-wave speech, which naïve observers may perceive as non-speech, but hear as speech once informed of the linguistic origin of the signal. Combinations of sine-wave speech and incongruent video of the talker elicited a McGurk effect only for informed observers. This indicates that the audiovisual integration effect is specific to speech perception. However, observers might only have been motivated to look at the face when informed and audio and video thus seemed related. Since Tuomainen et al. did not control for this, the influence of motivation is unknown. The current experiment repeated the original methods while controlling eye movements. 4 observers participated in the experiment, which consisted of 3 conditions. In the non-speech condition, observers were trained and tested in their ability to categorize sine wave speech tokens in arbitrary categories. The natural speech condition was similar but used natural speech signals and observers categorized phonetic content. The speech-mode condition again used sine-wave speech stimuli but observers were informed of the speech-like nature of the stimuli and classified them according to the phonetic content. In all conditions, a white square which in some trials dimmed briefly was overlaid on the nose of the speaker. Observers were required to report this after primary target categorization. We found a significant McGurk effect only in the natural speech and speech mode conditions supporting the finding of Tuomainen et al. Performance in the secondary task was similar in all conditions indicating that observers did look near the mouth. We conclude that eye-movements did not influence the results of Tuomainen et al. and that their results thus can be taken as evidence of a speech specific mode of audiovisual integration underlying the McGurk illusion.

Reference: Tuomainen, J., Andersen, T., Tiippana, K., & Sams, M. (2005). Audio-visual speech perception is special. Cognition, 96(1), B13-B22.