During a recent trip to the eye doctor, I had to have my eyes dilated. Usually the nurse gives me a pair of Ray Charles' sunglasses before I leave, but it was a busy day and I left in a hurry without any extra protection against the sunlight. Once outside, I had to squint so tightly that I could hardly see, and on my haphazard walk back to the office, I nearly bumped into the nurse, who had just finished her lunch break. "You know, we usually give you a pair of sunglasses."
For two fluid-filled orbs recessed in the middle of our faces, our eyes serve a pretty important function. They are our window to the outside world, but for the outside world, our eyes offer a window into our brains, providing clues as to what's going on inside our heads.
But how do they work? The eyes are organs uniquely designed to give us sight. Similar to a camera, light passes through our lenses and is flipped upside down onto the backside of our eye, the retina. Once light hits the retina, it is converted into an electrical signal that is transmitted into the nervous system, where our brain interprets what we're seeing. Because the retina processes some information, it is considered part of the brain, the only part that can be observed without undergoing surgery or a brain scan.
In order to control the amount of light which hits the retina, our iris, the colored part, acts as a shutter, expanding or contracting to determine how much light gets through the pupil.
Our irises primarily operate to help us see, but they respond to more than just light; our pupils also dilate when we see someone we're attracted to. A few centuries ago, women in Italy used to artificially dilate their eyes using an extract from the bella donna plant in an effort to woo suitors. Humans judge potential mates with larger pupils as more attractive because it indicates their mutual interest in us. Bella donna, Italian for ‘beautiful lady,' fell out of fashion because it also distorted vision, an unpleasant price to pay for beauty. Ophthalmologists still use the active ingredient, atropine, to dilate our eyes.
Pupils also respond to emotions. When happy, our pupils dilate, but when sad they contract. When we see a sad face, we mirror the emotion by contracting our own pupils. This indicates our degree of compassion: the more our pupils shrink in response to another’s sadness, the more our brains are wired for empathy.
Further insight into our brain's activity can be gleaned from our pupils: when we learn new information, our pupils dilate as we encode the memory. When we recall what we've learned, our pupils dilate again. The size of dilation is proportional to mental effort—if you're trying to remember a seven-digit phone number, your pupils dilate a little, but if you have to remember the area code on top of it they grow even more. This holds true only as long as we're still able to process the information. Once we get any longer than ten-digits, dilation plateaus because we've reached the capacity of our short-term memory.
Studying pupils gives us a window into some of our intricate mental functions.
Evolutionarily, the eyes are rudimentary. Almost all animals have them, and they don't perform calculus or rocket science. But that doesn't mean they aren't a useful telltale of some of the inner-workings of the brain. For one thing, they are easy to observe. Measuring a pupil is much simpler and cheaper than a brain scan. The more we understand about their connections to the rest of the brain, the more we can infer by simply measuring pupils.
Pupil size can actually forecast complex decisions we make in our lives. Is it better to work on a current project that will lead to an expected reward or to brain-storm new ideas that may pay off bigger in the long-run? If your pupils are large, you're more likely to explore new ideas, whereas small pupils predict you'll toil on the project you've already got going.
This may seem far-fetched. How can the size of the pupil—whose primary purpose is to let light into our eye—predict which task we'll choose? There are a few parts to this answer.
In monkey studies, where we can directly monitor brain activity by inserting electrodes into the area we want to study, we've found that pupils dilate in response to norepinephrine activity in the brain. Norepinephrine is one of the primary neurotransmitters responsible for arousal and alertness. There are two ways norepinephrine can be released, in a steady stream or in fluctuating pulses. When it's released in a steady stream, the pupils remain consistently large, but when it comes in pulses the pupils are small, but grow with each pulse of norepinephrine.
So what do steady stream and pulsatile release indicate in terms of brain function? A steady stream of norepinephrine primes the brain to explore and act on impulse. If norepinephrine comes in pulses, it facilitates focusing on a task and keeping the brain alert for an extended period of time. When given an easy task with a high reward, our pupils shrink and then pulse in size, indicating we've shifted to work mode. However, if a task is difficult and the reward is low, our pupils get consistently larger and we begin to explore new options.
It's remarkable how much of our brain's functions we can see by observing our eyes. But are you likely to remember this post? If your pupils just got larger, chances are that you will.
Hess et al., 1965 E.H. Hess, A.L. Seltzer and J.M. Shlien, Pupil response of heterosexual and homosexual males to pictures of men and women, J. Abnorm. Psychology 70 (1965), pp. 165-168
Harrison NA, Wilson CE, Critchley HD. (2007). Processing of observed pupil size modulates perception of sadness and predicts empathy. Emotion. 7(4):724-9
Cabestrero R, Crespo A, Quirós P. Pupillary dilation as an index of task demands. Percept Mot Skills. 2009 Dec;109(3):664-78.
Gilzenrat MS, Nieuwenhuis S, Jepma M, Cohen JD. Pupil diameter tracks changes in control state predicted by the adaptive gain theory of locus coeruleus function. Cogn Affect Behav Neurosci. 2010 May;10(2):252-69.
Many thanks to Audrey Nath for insights and suggestions—and helping me streamline this entry.