When a Lie Reveals a Truth: Insights from the McGurk Effect

A dramatic illusions reveals secrets about consciousness.

Posted Feb 04, 2016

This is the first of a three-part series of blogs concerning a lively, open-peer discussion —involving over 30 leading scientists—concerning consciousness and the brain.

The conscious field is composed of all the things — percepts, urges, and thoughts — that one is conscious of at one moment in time.  Everyday experience reveals that the field brings together things of very different kinds (for example, a smell versus a sound).  However, in the nervous system, information of various sources can be brought together unconsciously, as in the case of intersensory processing. For example, the dramatic McGurk effect (McGurk & MacDonald, 1976) involves interactions between visual and auditory processes: An observer views a speaker mouthing “ga” while presented with the sound “ba.” Surprisingly, the observer is unaware of any intersensory interaction, perceiving only “da.” (See neural evidence for this effect in Nath and Beauchamp, 2012.)  Click here to experience this dramatic illusion. 

Intersensory processes of this kind have allowed scientists to contrast the differences in the brain between conscious and unconscious integrations, revealing the primary, integrative role of conscious processing (click here and here for further discussion).  This is not the first time that an illusion, which is a untruth, has perhaps revealed an interesting truth.

Source: Wikipedia

In the illusion presented at left, square A is exactly the same shade as that of square B. 


McGurk, H. & MacDonald, J. (1976). Hearing lips and seeing voices. Nature, 264, 746–748.

Nath, A. R., & Beauchamp, M. S. (2012). A neural basis for interindividual differences in the McGurk effect, a multisensory speech illusion.  NeuroImage, 59, 781–787.