McGurk effect

What would you say if I tell you that squares A and B are exactly the same shade of grey?

I needed a while to be convinced of it. This is an optical illusion. An optical illusion is always characterized by visually perceived images that, at least in common sense terms, are deceptive or misleading. Therefore, the information gathered by the eye is processed by the brain to give, on the face of it, a percept that does not tally with a physical measurement of the stimulus source. A conventional assumption is that there are physiological illusions that occur naturally and cognitive illusions that can be demonstrated by specific visual tricks that say something more basic about how human perceptual systems work.
For more about this, read here.
Probably you are not convinced yet, you can see the proof here or here. Do it yourself and you'll be convinced!
What does all this have to do with language? Have you ever thought about an effect like this... but with voice?
Well... now I want you to turn on your speakers, play this short video and hear and see carefully. What is the syllable he is pronouncing? See video here.
If you don't have Quicktime to see it, you can use this file or go to this site.
Your conclusion? 98 % of adults will say they hear the sound da. Now, please play it again, but close your eyes. Don't keep reading. Just play it again and listen. What is he pronouncing? Remember to close your eyes!
Maybe you want to play it again. Are you sure he is pronouncing ba? Well, he is! And the image is of him pronouncing ga. This is called the McGurk effect (sometimes also called McGurk-MacDonald effect). It is a perceptual phenomenon which demonstrates an interaction between hearing and vision in speech perception. It suggests that speech perception is multimodal, that is, that it involves information from more than one sensory modality. You can read the abstract of the research paper published by Nature magazine in 1976.