In the movie The Kings Speech, Geoffrey Rush plays a language therapist to King George VI played by Colin Firth, who has a fixed mental set or belief that he cannot speak without stammering.
“I stammer, and no-one can fix it,” he says.
Rush gets him to read a speech out loud from Shakespeare’s Hamlet (“To be or not to be”) and Firth can’t read it without stammering.
Rush then says he’s going to record him and gets him to read the speech again with a slight variation. He confuses Firth by having him read the speech with headphones on and listening to loud classical music.
As a result, the Firth can’t hear himself speaking (and neither can we, which is part of the dramatic effect).
Firth leaves without listening to the recording because he is so locked into his mental set. However, later on when he does happen to listen to the recording, he is amazed to discover that he had read the speech without stammering at all.
When faced with a fixed mental set or belief, trying to change it using the things we know or perceptions that we would normally use, often fails because these are the very things that reinforce the belief.
In this case, Firth knows he stammers because he can hear himself. This creates a self-perpetuating cycle: the more he hears himself stammer, the more he reinforces the fixed mental set that he stammers which causes him to stammer more.
Once he is deprived of the sense of hearing or it gets confused in some way, it breaks the cycle.