Psychologists have devised some ingenious ways to help unpack the human "now." Consider how we run those jerky movie frames together into a smooth and continuous stream. This is known as the "phi phenomenon." The essence of phi shows up in experiments in a darkened room where two small spots are briefly lit in quick succession, at slightly separated locations. What the subjects report seeing is not a succession of spots, but a single spot moving continuously back and forth. Typically, the spots are illuminated for 150 milliseconds separated by an interval of fifty milliseconds. Evidently the brain somehow "fills in" the fifty-millisecond gap. Presumably this "hallucination" or embellishment occurs after the event, because until the second light flashes the subject cannot know the light is "supposed" to move. This hints that the human now is not simultaneous with the visual stimulus, but a bit delayed, allowing time for the brain to reconstruct a plausible fiction of what has happened a few milliseconds before. In a fascinating refinement of the experiment, the first spot is colored red, the second green. This clearly presents the brain with a problem. How will it join together the two discontinuous experiences—red spot, green spot—smoothly? By blending the colors seamlessly into one another? Or something else? In fact, subjects report seeing the spot change color abruptly in the middle of the imagined trajectory, and are even able to indicate exactly where using a pointer. This result leaves us wondering how the subject can apparently experience the "correct" color sensation before the green spot lights up. Is it a type of precognition? Commenting on this eerie phenomenon, the philosopher Nelson Goodman wrote suggestively: "The intervening motion is produced retrospectively, built only after the second flash occurs and projected backwards in time." In his book Consciousness Explained , philosopher Daniel Dennett points out that the illusion of color switch cannot actually be created by the brain until after the green spot appears. "But if the second spot is already 'in conscious experience,' wouldn't it be too late to interpose the illusory content between the conscious experience of the red spot and the conscious experience of the green spot?

— Paul Davies

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