The other day I was researching for new material on verbal & non-verbal communication for a leadership programme I’m launching soon and stumbled across this fascinating illusion.
The McGurk Effect, first described by psychologist Harry McGurk, tricks your brain into hearing things that haven’t actually been said. Effectively, non-verbal cues and the way things are said determine what is communicated.
In other words, when our eyes tell us one thing and our ears another, the eyes have it! We literally hear what our eyes tell us. 

It works because people think they hear most words.
In fact, we rely on our eyes to hear words, as much as our ears!
So, when you add something such as text, suggesting to the eye (and then to the ears) what should be “heard” then it wins the day. Given ambiguous inputs, the eye will overrule the ear and substitute its judgment.

On screen, if you are given a line of text telling you what the speaker is saying you will hear the speaker saying it, even if he is not in fact saying it at all. 
This was recently demonstrated in the run up to the US presidential election, when MSNBC made a story out of a crowd chanting “Ryan” instead of “Romney”, questioning whether Mitt Romney is generating sufficient support.
It turns out they were providing misleading visual cues, by adding the word “Ryan” to the screen and using the McGurk Effect!
It’s an interesting demonstration of suggestibility and has set me wondering where else this plays out…