Noise is all over the place now, but that's okay. The brain can still maintain a conversation, even if it doesn't really hear the cues. He predicts what he hears and fills in the blanks.
“Our brains have evolved to overcome the constant interference that surrounds them in the real world,” says Matthew Leonard of the University of California.
Scientists have known since the 1970s that the brain fills in unheard passages of speech, but how it does it (a phenomenon called perceptual reconstruction), no one really knew. To investigate the phenomenon, Leonard's team played to volunteers words that were either partially drowned out by noise or simply not distinguishable.
The experiment involved people who had already had hundreds of electrodes implanted in their brains to monitor epilepsy. These electrodes record seizures, but can also record other types of brain activity.
The team played a recording of a word to the volunteers, which could be either "faster" or "factor", where the sound in the middle was replaced by noise. The data from the electrodes show that the brain reacted as if it actually heard the sound "s" or "k". It seems that one area of the brain called the inferior frontal gyrus predicts what a person hears and does so two tenths of a second before the superior temporal gyrus processes sound.
And while such a prediction may seem like a clever trick, scientists have found that its effectiveness is limited. The brain doesn't seem to use the context of the conversation to improve the accuracy of its guesses. Even when the volunteers were given an introductory phrase, for example, "I drive a car," they might well have heard the word "factor" rather than the more contextually appropriate "faster".