If the eye is the window into the soul, the pupil is—quite literally—an opening into the eye. The pupil acts like the aperture on a camera, dilating or contracting to regulate the amount of light coming into the eye. We all know that our pupils get smaller in the light and bigger in the dark. This is the pupillary light response.
They claim that the size of the pupils tells us a lot about the emotions and intentions of their owners.
According to the researchers, the pupillary light response isn’t just a mechanical reaction to ambient light. Rather, as we shift our gaze from one spot to another, our pupils adjust their size in advance to the amount of light we expect to encounter at the new location.
Consider working at a computer: Most of the time, our gaze is fixed on the bright screen, so our pupils are contracted. But every now and then, we glance down at the keyboard, as when we need to reposition our fingers. The authors of the article claim that the pupils begin to dilate even before the downward eye movement begins. Because the pupillary light response is relatively slow—about a quarter of a second—anticipating the amount of light at the new location improves vision once our gaze gets there. (All of this, of course, operates below the level of consciousness.)
The pupillary light response is only one reason why the pupils change size. They also dilate when we’re aroused. The body has an alarm network called the autonomic nervous system that prepares us to take action whenever we detect a threat—or an opportunity—in our environment.
Encounter a bear while walking through the woods, and your autonomic nervous system goes on alert. Your heart and breath rates increase, you begin to sweat as your muscles tense up, and, among other bodily reactions, your pupils dilate. The autonomic nervous system prepares your body to take action against the threat—perhaps scampering up the nearest tree.