Hearing by eye: how much spatial degradation can be tolerated?

In the McGurk effect (McGurk and MacDonald, 1976 Nature 264 746-748), illus ory auditory perception is produced if the visual information from lip move ments is discrepant from the auditory information from the voice. A study i s reported of the tolerance of the effect to varying levels of spatial degr adation (videotaped images of a speaker's face were quantised by a mosaic t ransform). The illusory effect systematically decreased with an increase in the coarseness of the spatial quantisation. However, even with the coarses t level (11.2 pixels/face) the illusion did not completely disappear. In ad dition, those participants who did not experience the illusion nevertheless showed the effects of auditory-visual interaction in their clarity ratings of the auditory stimulus. It is concluded that auditory-visual interaction in visible speech perception is based on relatively coarse-spatial-scale i nformation.