The McGurk effect is a perceptual phenomenon which indicates an interaction between hearing and vision in speech perception, and therefore suggests that speech perception is multimodal - that it involves information from more than one sensory modality.

The McGurk effect may be experienced when a video of one phoneme's production is dubbed with a sound-recording of a different phoneme: often, the perceived phoneme is a third, intermediate phoneme. For example, a visual /ga/ combined with a heard /ba/ is often heard as /da/.

The McGurk effect is robust: that is, knowledge about it seems to have little effect on one's perception of it (unlike, say, certain optical illusions, which break down once one 'sees through' them).

The McGurk effect is named after its discoverer, Harry McGurk, and was first described in McGurk & MacDonald (1976).

References

McGurk, H & MacDonald, J (1976) "Hearing lips and seeing voices", in Nature, Vol.264, No.5588, 746-748