The eye is a jelly-filled chamber with a lens in front of it. This lens focuses light onto the retina, in the back of the eye, from where nerve impulses are sent to the brain. But the eye’s lens is an imperfect device. For example, different colors have different focal lengths. This means that if you focus on the blue stripe of Newman’s Who’s Afraid of Red, Yellow and Blue, the yellow and red stripes will be ever so slightly out of focus. This and other types of distortion are most pronounced for large lenses, so it is best to keep the surface of the lens as small as possible. For this reason, most of the eye’s lens is covered by the colorful iris, which serves more than just an aesthetic purpose. And the part that is not covered by the iris is your pupil.

Who’s afraid of red, yellow, and blue?

But there is also downside to having small pupils: They don’t let a lot of light through. This doesn’t matter when you are in a bright environment where even tiny pupils let through sufficient light. But in darkness small pupils simply won’t do: Optical distortions or no, in darkness pupils must increase their size in order to let through the bare minimum of light that is required for vision.

So there are two opposing forces that together determine the size of your pupil. On the one hand, small pupils suffer less from optical distortions. On the other hand, large pupils let through more light. The optimal balance depends mostly on the amount of light that is available, which is why pupils constrict in brightness and dilate in darkness, a phenomenon called the pupillary light response.

The pupillary light response is about as big and obvious as physiological responses get and has therefore been known since, well … probably since eye contact became fashionable, somewhere in the evolutionary dawn of mankind. But the light response has been the victim of a terrible stigma: It was believed to be a reflex.

Pupils come in all shapes and sizes. From left to right: man, cat, lizard, eagle, and octopus.

And in a way the light response is a reflex, because your pupils faithfully constrict when you step out into the sun. There is nothing you can do about that, short of taking medication or putting on sunglasses. But a number of recent papers show that the light response is actually much more subtle than people used to think. Among these papers I would like to modestly highlight our own, which just appeared in the open-access journal PLoS ONE. (With due credit to Binda and colleagues (2013), who did something quite similar.)

The basic experiment was very simple. Participants looked at the center of a display that was divided into a bright and a dark half, separated by a central gray band. Their task was to identify a target stimulus that could appear on either side of the screen, on the bright or the dark background. Just before the target stimulus appeared, a cue, which was a voice saying ‘left’ or ‘right’ or a pointing arrow (in different experiments), indicated the probable location of the target. Participants used this information to anticipate the target and shifted their attention to the cued side of the screen, which was sometimes bright and sometimes dark. Their eyes remained fixated on the center of the screen, of course: This was a purely covert shift of attention.

So to sum up the paradigm: We manipulated whether participants attended to something bright or something dark, while no eye movements were made and visual stimulation was kept constant.

Figure adapted from (Mathôt, van der Linden, Grainger, & Vitu, 2013).

In the figure above you can see what came out of our experiment. Indeed, when participants covertly attended to the bright side of the screen (the orange line), their pupils constricted, relative to when they attended to the dark side of the screen (the blue line). This difference arose from about 600 ms after the cue was presented. (You also see that overall the size of the pupil increases over time. This is related to the effort that participants invest in the task, which affects the size of the pupil in a way that is more-or-less independent of the light response.)

So attending to a bright object causes a pupillary light response, just like looking at a bright object directly. This is very cool, because, as I mentioned, until very recently it was assumed that the light response was nothing more than a simple reflex to light that was not related to ‘cognitive’ things such as attention. This finding shows that the size of your pupil is not only optimized for the brightness of your environment, but specifically for the objects that you are attending to.

There is more to the pupil than meets the eye.