The video below demonstrates something called the McGurk effect, a famous perceptual illusion that psychologists Harry McGurk and John MacDonald discovered in the 1970s while studying how children develop language. The two actually discovered the effect by accident when a sound technician synced a video they were studying to the wrong sound track.
First watch and listen to the video, and see what you hear. Does the sound change for you over the course of the video?
Although the sound is actually exactly the same throughout the video (it's always "ba"), most people will perceive it as changing from "ba" to "va" as the image changes.
What is happening here? Basically, your brain is receiving conflicting signals from your eyes and your ears. While your ears are hearing "ba," your eyes are seeing a mouth pronounce the syllable "va." When that happens, your brain lets the visual information override the auditory information, and the sound appears to change.
The effect works so well that you don't necessarily have to be looking right at the person's mouth. You might perceive the McGurk effect even if you are looking at another part of your screen. If you have a lot of extra time and energy, you can even replicate the McGurk effect yourself while looking in a mirror and mouthing the sound.
However, some people will not perceive the effect as strongly. Studies have shown that women perceive the effect more strongly than men do. And those with damage to the brain, dyslexia, autism and certain language disabilities may not hear the sound change at all. Fascinatingly, research has shown that the McGurk effect doesn't work well for Japanese listeners, possibly because of the language's different sounds, and a cultural tendency in Japan to avoid looking at a speaker's face.
You can also learn to override the McGurk effect with practice. People who often watch dubbed TV or movies -- which are more common in countries that have their own language but don't have a large film industry, like Vietnam or Russia -- can learn to ignore what shape the actors' mouths are making on television.
This is just one example of the many fascinating techniques your brain uses to manage and simplify the huge amount of sensory information it is receiving at all times. Sometimes when your brain tries to deal with uncertain situations, it jumps to conclusions that just aren't quite right.
To read about more fascinating examples, check out our article on the topic -- "Your reaction to this confusing headline reveals more about you than you know."