What do an orgasm, a multiplication problem and a photo of a dead body have in common? Each induces a slight, irrepressible expansion of the pupils in our eyes.
For more than a century scientists have known that our eyes' pupils respond to more than changes in light. They also betray mental and emotional commotion. In fact, pupil dilation correlates with arousal so consistently that researchers use pupil size, or pupillometry, to investigate a wide range of psychological phenomena. And they do this without knowing exactly why our eyes behave this way.
"Nobody really knows for sure what these changes do," says Stuart Steinhauer, director of the Biometrics Research Lab at the University of Pittsburgh School of Medicine. He views the dilations as a by-product of the nervous system processing important information.
The visual cortex in the back of the brain assembles the actual images we see. But a different, older part of the nervous system—the autonomic—manages the continuous tuning of pupil size (along with other involuntary functions such as heart rate and perspiration). Specifically, it dictates the movement of the iris to regulate the amount of light that enters the eye, similar to a camera aperture. The iris is made of two types of muscle: a ring of sphincter muscles that encircle and constrict the pupil down to a couple of millimeters across to prevent too much light from entering; and a set of dilator muscles laid out like bicycle spokes that can expand the pupil up to eight millimeters—approximately the diameter of a chickpea—in low light.
Stimulation of the autonomic nervous system's sympathetic branch, known for triggering "fight or flight" responses when the body is under stress, induces pupil dilation. Whereas stimulation of the parasympathetic system, known for "rest and digest" functions, causes constriction. Inhibition of the latter system can therefore also cause dilation. The size of the pupils at any given time reflects the balance of these forces acting simultaneously.
The pupil response to cognitive and emotional events occurs on an even smaller scale than the light reflex, with changes generally less than half a millimeter. By recording subjects' eyes with infrared cameras and controlling factors that might affect pupil size, such as ambient brightness, color and distance, scientists can use pupil movements as a proxy for other processes, like mental strain.
Princeton University psychologist Daniel Kahneman showed several decades ago that pupil size increases in proportion to the difficulty of a task at hand. Calculate nine times 13 and your pupils will dilate slightly. Try 29 times 13 and they will widen further and remain dilated until you reach the answer or stop trying. Kahneman says in his book, Thinking Fast and Slow, that he could divine when someone gave up on a multiplication problem simply by watching for pupil contraction during the experiment.
"The pupils reflect the extent of mental effort in an incredibly precise way," Kahneman said in an interview with 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.