Here are some voice recordings that have been tweaked almost beyond recognition, placed side-by-side with the unaltered recordings. The tweaked recordings are almost impossible to make out (though if you listen to them over and over, you can figure out what they are saying) but after you listen to the unaltered recordings, the tweaked recordings are very easy to understand.
Most naive listeners hear this as a set of simultaneous whistles, or science fiction sounds. However, for listeners that have previously heard this sound: Listening to the sine-wave speech sound again produces a very different percept of a fully intelligible spoken sentence. This dramatic change in perception is an example of "perceptual insight" or pop-out. We have argued that this form of pop-out is an example of a top-down perceptual process produced by higher-level knowledge and expectations concerning sounds that can potentially be heard as speech.Here's one example: Sine Wave Speech | Clear Speech
It's almost, but not quite, as weird as the McCollough Effect optical illusion that lasts 24 hours or longer.
More here: Clear Speech
(Via Mind Hacks)