Your eyes decide what you hear
When your eyes see a mouth move one way but your ears hear a different sound, your brain invents a third sound to fix the conflict.
The McGurk Effect proves that vision is so dominant it can override your physical hearing. If a video shows a person saying 'ga-ga' while the actual audio plays 'ba-ba,' you will likely hear 'da-da.'
Your brain refuses to accept the contradiction between what it sees and what it hears. Instead of choosing one over the other, it merges the two signals into a hallucinated middle ground.
This happens because humans are hardwired to lip-read even when they don't realize they are doing it. We use the shape of the mouth to categorize the muffled or complex frequencies of speech.
Even after you learn how the trick works, you cannot stop it. Your brain will continue to feed you the fake sound as long as your eyes are open.
Closing your eyes is the only way to hear the truth. The moment you break visual contact with the mouth, the 'da-da' sound instantly reverts to the original 'ba-ba.'
This explains why talking over a bad cell phone connection is easier than watching a dubbed movie with poor lip-syncing. When the visual and audio signals don't line up, your brain works overtime to bridge a gap that shouldn't exist.
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