The McGurk Effect is an example of 'sensory dominance'; in this case sight ruling over sound in the hierarchy of the senses.

The discovery came by accident in 1976, while researchers Harry McGurk and John MacDonald were conducting a study about language development in infants. They were overdubbing a film of someone talking to camera, playing a different soundtrack to what the person on screen was saying, when they suddenly noticed this amazing phenomenon.

In the video below, you will see a person on the left repeatedly mouthing ‘ba, ba, ba’; and on the right of the screen, the same person mouthing ‘fa, fa, fa’. The sound that is playing is always ‘ba, ba, ba’ - you are always hearing the person on the left. So, when you look at the one on the right, you’re seeing ‘fa’, but actually hearing ‘ba’. And that's where the McGurk Effect kicks in - you don't hear 'ba', you hear 'fa'.

With two incongruent pieces of sensory information coming at you, the brain has to decide which to go with; and sight wins out. When looking at the person on the right, your brain will make you hear ‘fa, fa, fa’, even though you are actually hearing ‘ba, ba, ba’. Close your eyes and you hear ‘ba’. Look at the guy on the right and you hear ‘fa’. Look at the guy on the left, you hear ‘ba’. Look at the guy on the right again, you hear ‘fa’.

It’s crazy and you cannot stop it, even though you know what’s happening. When you see someone mouthing an ‘f’, but you are actually hearing a ‘b’, you cannot help but hear an ‘f’ because sight is achieving sensorial dominance over hearing. Give it a go.