Your brain is an extremely complex pattern-recognition machine. Neuroscientist Jamshed Bharucha demonstrates to our audience how three indistinct synthesized sounds can be superimposed to create the illusion of human speech. Our brain is able make inferences from the most prominent energy peaks in a simple phrase and recognize the words being spoken even if most of the information is stripped away.
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There is more. A few years back I observed an interesting effect. When I stayed late at work into very late hours, the building would come "alive" with various sounds - heating, air circulation, hum of the transformers and various electrical equipment - that is, all the sounds that are present during the day but which we don't even notice as our environment is usually quite noisy. And being alone in a "whispering" building, I'd catch faint "conversations" or even music playing, barely audible. When I tried to listen, it would be just white noise, but when my mind was on some mental task, these phantom conversations and music would appear again. This is how our brain organizes white noise into familiar structures, presenting us with "ghosts". It was an interesting experience.