Dream Pictures

In one of his 15,000 research participants, William Dement (1999) observed the moment the brain’s perceptual window to the outside world slammed shut. Dement asked this sleep-deprived young man, lying on his back with eyelids taped open, to press a button every time a strobe light flashed in his eyes (about every 6 seconds). After a few minutes the young man missed one. Asked why, he said, “Because there was no flash.” But there was a flash. He missed it because (as his brain activity revealed) he had fallen asleep for 2 seconds. Unaware that he had done so, he had missed not only the flash 6 inches from his nose but also the abrupt moment of his entry into sleep.


William Dement was able to isolate the change in brain activity that occurs when a person falls asleep by asking a sleep-deprived man to push a button every time a strobe light flashed before his eyes. Eventually he missed a flash when he fell asleep for two seconds, and Dement was able to determine the corresponding brain activity.

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