A fascinating auditory illusion proves that the visual cue of moving lips plays an important role in accurately hearing what people say.

So this week I’m taking it back to a study published in Nature in 1976 to tell you about a freaky auditory illusion called the McGurk effect. However, it also requires some visual input, so I’ll have to send you to a video at http://snipurl.com/sciam-illusion.

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If we watch a video of a person mouthing the word “ga,” but have a synced voice-over of that person saying “ba,” what we end up hearing, is a third variation that’s never been said! That word is “da”.
 
And even though you now know it’s an illusion—you will still, when you see the video, think you are hearing “da”. But if you close your eyes, and do not see the person’s lips forming the word “ga,” you’ll hear what they are actually saying, which is “ba”.
 
We think of speech as dependent on auditory perception. But this study eerily shows just how important visual input is.
 
From this, it’s clear that our senses did not develop in isolation, but rather, they work in tandem to form an accurate perception of our world. Here we learn that the position of the lips is key in accurately hearing what someone is saying.

To experience the McGurk effect, please go to spinurl.

Source: Scientific American, May 12 2008