![]() “The pupils reflect the extent of mental effort in an incredibly precise way,” Kahneman told the German news magazine Der Spiegel, adding, “I have never done any work in which the measurement is so precise.” When he instructed subjects to remember and recite a series of seven digits, their pupils grew steadily as the numbers were presented one-by-one and shrunk steadily as they unloaded the digits from memory. As Kahneman says in his recent book, Thinking Fast and Slow, he could divine when someone gave up on a multiplication problem simply by watching for pupil contraction during the experiment. Try 29 times 13, and they will widen further and remain dilated until you reach the answer or stop trying. Calculate 9 times 13, and you pupils will dilate slightly. Princeton psychologist Daniel Kahneman showed several decades ago that pupil size increases in proportion to the difficulty of the task at hand. ![]() By recording subjects’ eyes with infrared cameras and controlling for other factors that might affect pupil size, like brightness, color, and distance, scientists can use pupil movements as a proxy for other processes, like mental strain. The iris is made of two types of muscle: in a brightly lit environment, a ring of sphincter muscles that encircle and constrict the pupil down to as little as a couple of millimeters across in the dark, a set of dilator muscles laid out like bicycle spokes, which can expand the pupil up to 8 millimeters-approximately the diameter of a chickpea.Ĭognitive and emotional events can also dictate pupil constriction and expansion, though such events occur on a smaller scale than the light reflex, causing changes generally less than half a millimeter.
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