A Case Study Uncovered The Only Known Human To Change Their Pupil Size On Command

Here's a party trick you've probably never seen before. According to a 2021 case study by researchers at Utrecht University in the Netherlands, there's a man in Germany who can control more than just the muscles in his arms, legs, and hands — he can also control the muscles of his pupils (via Live Science).

The pupils are the black bits in the middle of your eyes (via Verywell Health). Though they might look like flat discs, pupils are actually holes that let light into your eye from the outside world. It's this light that lets you see, but if there's too much or too little, your eyes won't work properly. Just think about being in a pitch-black room, or trying to look at someone who's got a flashlight pointed your way. There's not much your eyes can do in these sorts of extreme situations, but in a relatively normal setting, your eyes can usually adjust to accommodate changes in lighting patterns. They do this by changing the pupil size. When you're in a dim room, your pupils will expand, becoming larger so more light can reach your retina (via All About Vision). On the other hand, in sunnier environments, your pupils will get smaller, shutting out light so that you aren't so blinded by the sun.

We have no real control over how our pupils behave — at least not consciously. The shrinking and enlargement of your pupils is an automatic process that happens more or less immediately when the amount of light coming into your eyes changes, according to Live Science. In fact, even if you want to deliberately change your pupil size, you can't directly choose to do so. There are thought techniques you can apply that might help control your pupil size — like thinking about the sun to make your pupils shrink — but these are considered "indirect methods," per Live Science.

In this sense, the muscles in your eyes are like the muscles in your heart: They're something you can affect, but not something you can simply decide to move. However, researchers now think there may be a group of people to whom these usual rules don't apply. Like us, they have automatic pupillary responses to sunlight and shade. But they may also have the additional ability to regulate their own pupil size. The first of these people was identified in a case study done in Europe after he came to researchers to show them his strange ability: being able to change his pupil size at will by whole millimeters, per Live Science.

But how is it that someone even notices they can control their pupils? A German man called D.W. identified the unique talent when he was a teenager who loved video games. Per Live Science, he describes the feeling of changing his pupil size with the following: "Constricting the pupil feels like gripping, tensing something; making it larger feels like fully releasing, relaxing the eye." When they brought him into the lab, researchers were stunned to find his claims corroborated: He could shrink his pupils to less than a millimeter in diameter or expand them to nearly 2.5 millimeters, entirely at will.

Technically, the researchers weren't able to totally discount the possibility that D.W. might be using indirect methods — like thinking of the sun — to get his eyes to change. But extensive research, including sticking D.W. in an MRI machine, indicated that this was not the case. In fact, all indicators pointed to D.W. changing his pupil size totally voluntarily, according to Live Science. Now, researchers are getting word from other people who say they have the same strange talent. They plan to expand their research on these individuals to try to pinpoint what, exactly, might be going on that allows these people to control the minuscule muscles in their eyes.