The Pupils Tell All: How Our Eyes Reveal How Well We Can Imagine

What is Mental Imagery?

Have you ever stumbled out of a dark movie theater into bright sunlight and had to squeeze your eyes shut? That was your eyes protecting themselves from the sudden flood of light. Not only do your eyelids react, but the black circles in the center of your eyes, called pupils, shrink to block some of the brightness. When the situation is reversed and you walk into darkness, your pupils grow larger to let in more light. This automatic change helps your eyes control light. But here is something surprising: your pupils do not only respond to the light around you, they also respond to the light you imagine! Scientists have found that when people picture something bright, like the sun or a white sheet of paper, their pupils get smaller. And when people picture something dark, like a cave or the night sky, their pupils grow larger. In other words, your eyes can reveal what your mind’s eye is seeing, even if nothing has changed in the real world.

This ability to “see without seeing” is called mental imagery. Everyone’s mental imagery is a little different: some people see vivid, almost movie-like images in their heads, while others have fainter or less detailed images. A small group of people have no mental imagery at all, a condition called aphantasia (you can read more about aphantasia in this Frontiers for Young Minds article). So how can our eyes reveal what our mind is “seeing”? One method scientists use is called pupillometry, which refers to measuring the size of the pupils with a special camera. Because you cannot control your pupils directly (you can move your eyes, of course, but you cannot voluntarily control pupil size), changes in their size give scientists a window into how the brain works when people imagine images.

How do the Pupils Work?

Your eyes work a bit like a camera. The colored part of your eye, called the iris, is made of tiny circular muscles that open and close the pupil (Figure 1). The pupil is not a structure itself, but rather an opening whose size determines how much light enters the eye. When you step into a bright environment, the muscles of the iris automatically tighten, making the pupil smaller so that less light floods in. In the dark, the muscles automatically relax, letting the pupil widen to capture as much light as possible.

Figure 1 - Light enters the eye through the pupil, the black opening in the center of the eye. The iris, the colored muscles around the pupil, makes the pupil bigger or smaller depending on how bright the environment is. The light then travels to the back of the eye and hits the retina. The retina changes the light into nerve signals that are sent to the brain, which turns them into the images we see (Image created with Copilot).

Scientists call this automatic adjustment the pupillary light response, and it ensures that just the right amount of light reaches the back of your eyes, where the retina turns light into signals your brain can understand (Figure 1). Without this response, things would look blurry or seeing would be uncomfortable, especially when moving quickly between dark and bright places. The cells in your retina also adapt too, becoming more or less sensitive in particularly bright or dark environments.

For a long time, scientists thought the pupils only reacted to the light around us. But recent research has shown that pupils also respond to many things happening in the brain, like paying attention (like when focusing on a game), working hard to solve a tricky problem, or feeling strong emotions such as excitement. One especially fascinating example is mental imagery: just picturing something bright or dark can make the pupils shrink or grow, even when the light in the room has not changed. This means pupillometry can reveal what the mind is doing.

How do Scientists Measure Mental Imagery Through the Eyes?

Studying mental imagery is tricky, because scientists cannot see inside someone’s mind. They usually ask people to describe their mental images, but that is not always reliable. Sometimes people have trouble finding the right words, and sometimes they are not even sure what they see in their mind’s eye. This is where pupillometry helps (you can read more about pupillometry in this Frontiers for Young Minds article). During an experiment, a participant might first look at a bright or dark picture on a screen, which makes their pupils shrink or grow while scientists use pupillometry to measure their size. Then the picture disappears, and they are asked to imagine the picture again. If their pupils change in the same direction as before, that is strong evidence that the picture really came back in their mind’s eye (Figure 2).

Figure 2 - Your pupils automatically get smaller in bright light and larger in the dark. In the graph, people with vivid mental imagery and people with aphantasia both show shrinking pupils when they look at a bright picture (the yellow portion of the plot). Then, they are asked to imagine the picture for a short time, and the researcher tells them when to stop (the blue portion). People with vivid imagery show the same shrinking again, as if their eyes are responding to the imagined light. People with aphantasia, who cannot form mental pictures, show no change in pupil size (Graph created with ChatGPT).

What makes pupillometry so useful is that it offers scientists a more objective way to study imagery. When researchers ask people to rate their own images, they might say things like “my image is clear” or “it is fuzzy”. But self-reports can be tricky because some people may underestimate or overestimate their imagery, and others may struggle to describe it at all. Pupillometry provides an outside signal that is not under a person’s control. If someone’s pupils shrink more when imagining a bright scene than a dark one, that tells scientists that their imagery is strong. If the pupils barely change, the person’s imagery may be weaker, even if the person says otherwise.

What Scientists Have Learned

Scientists have conducted a few experiments to see how the pupils reveal what is going on in the mind’s eye. In one of the earliest studies, people were shown simple bright or dark shapes on a screen, like white or black triangles [1]. Later, when asked to imagine those shapes while looking at a blank background, their pupils responded just as if they were really seeing the shapes: shrinking for bright images and growing for dark ones. This gave the first evidence that mental imagery can affect the eyes in the same way as real light.

More recently, scientists asked whether pupil changes could show how vivid a person’s mental imagery is [2]. They found that people who reported clearer, more detailed images had bigger pupil responses, while those with weaker images had smaller ones. A group of people with aphantasia, who cannot form visual images at all, showed no pupil change when asked to imagine light and dark scenes. This showed that aphantasia can be detected by studying the eyes.

Other researchers have shown that mental imagery can affect the eyes in other ways. In one study, participants were asked to picture a map of France and judge whether different cities were to the left or right of Paris, the capital [3]. People with typical imagery often moved their eyes across the blank screen as if scanning an invisible map, and their pupils adjusted along with the task. People with aphantasia, however, solved the problem differently. Instead of “looking” at a map in their mind’s eye, many relied on facts they had learned, such as remembering that a certain city is west of Paris. Their pupils got larger during the task, which showed that this way of working required extra mental effort. This finding shows that people with aphantasia can solve imagery tasks, they simply use alternative strategies to do so (Figure 3).

Figure 3 - People solve imagery tasks in different ways. Some see a clear picture in their mind (like the person on the left), others see only a faint or blurry one (like the person in the middle). People with aphantasia tend to use facts instead of mental pictures (like the person on the right). Even though some strategies may take more effort, everyone can still solve the imagery problem (Image created with ChatGPT).

Why Studying Pupils Matters

Studying pupils may seem like a strange thing to do, but it opens a big window into the mind. Pupillometry helps scientists measure something that used to be invisible: the images people create in their heads. By looking at the eyes, researchers can discover how vivid someone’s mental pictures are, how much effort they are using, and even which strategies they rely on to solve problems.

This research shows that there is no single “right” way for the brain to work. Some people picture things very clearly, while others think more in words, facts, or logical steps. Both approaches can lead to the same correct answer but using different strategies. Pupillometry reveals these hidden differences, showing that people’s brains operate in very diverse ways.

That is a powerful lesson for all of us. People’s minds work differently, and that is okay. In fact, those differences are what make classrooms and teams stronger. One person might imagine a scene in detail, another might remember key facts, and another might spot a clever shortcut; together, these unique people with different approaches can solve problems in ways that no single mind could do alone.

Glossary

Pupils: ↑ The black circles in the center of the eyes.

Mental Imagery: ↑ The ability to picture things in your mind without actually seeing them, like recalling a memory or imagining something new.

Aphantasia: ↑ The inability to form visual mental images, while still being able to think about and remember these images in other ways.

Pupillometry: ↑ A technique that records pupil size with a camera to study how the eyes react to light, pictures, and mental activity.

Iris: ↑ The colored part of the eye.

Pupillary Light Response: ↑ The eye’s reflex that makes pupils smaller in light and larger in darkness, helping us see clearly in different environments.

Retina: ↑ The rear part of the eye that has light-sensitive cells, called rods and cones, which turn light into neural signals the brain uses to see.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

AI Tool Statement

The author(s) declared that generative AI was used in the creation of this manuscript. Generative AI (ChatGPT and Microsoft Copilot) was solely used to generate the three figures present in this manuscript.

Any alternative text (alt text) provided alongside figures in this article has been generated by Frontiers with the support of artificial intelligence and reasonable efforts have been made to ensure accuracy, including review by the authors wherever possible. If you identify any issues, please contact us.

[1] ↑ Laeng, B., and Sulutvedt, U. 2014. The eye pupil adjusts to imaginary light. Psychol. Sci. 25:188–97. doi: 10.1177/0956797613503556

[2] ↑ Kay, L., Keogh, R., Andrillon, T., and Pearson, J. 2022. The pupillary light response as a physiological index of aphantasia, sensory and phenomenological imagery strength. Elife 11:e72484. doi: 10.7554/eLife.72484

[3] ↑ Takamura, Y., Razzak, E., Coustaty, S., Liu, J., Pouget, P., Spagna, A., et al. n.d. Oculomotor and pupillary markers of visuospatial mental imagery in aphantasia and typical imagery. [Preprint]. Retrieved from: https://osf.io/76waj/download (Accessed September 6, 2025).