How Pupil Size Can Reveal What Is—Or Isn’t—Happening in Your Mind

A new study looked to the eyes for a potential test for aphantasia, or the inability to conjure up imagery in the mind.
A close up of a woman's eye.
Jena Ardell for Getty. 

One evening in 1960, the psychologist Eckhard Hess was at home, flipping through a book of “strikingly beautiful animal photographs.” His wife commented that the light in the room must be dim, because she could see that Hess’ pupils had grown to a large size, as eyes do in low light or darkness. 

“It seemed to me that there was plenty of light coming from the bedside lamp and I said so, but she insisted that my pupils were dilated,” Hess recounted in Scientific American in 1965.


He had a hunch that there was another reason his pupil size was changing. The next day at his lab at the University of Chicago, he showed his assistant a series of photos of neutral landscapes, with one “seminude pinup” slipped in at the end. 

“When I displayed the seventh picture, I noted a distinct increase in the size of his pupils; I checked the picture, and of course it was the pinup he had been looking at,” Hess wrote. 

Hess then embarked on decades of research studying the connection between pupil size and mental activity. His work—and that of many others since—in the field of “pupillometry” has showed that our pupils don’t only change size as a reflexive response to light conditions, but also in response to an assortment of other cognitive processes: emotional activation, a high amount of mental effort, and even simply mentally imagining objects that are bright or dark. 

This intriguing feature of our biology recently found a new application, as a potential test for aphantasia, or the inability to conjure up imagery in the mind’s eye. In a new study, published in eLife, researchers measured pupil responses in people with and without aphantasia who were asked to imagine bright and dark shapes. 

The pupils of those without aphantasia dilated or constricted according to whether they were imagining dark or bright objects, while the aphantasic’s pupils did not. These findings could lead to the first physiological test for aphantasia—an experience that’s only had an official name since 2015, and is currently measured through subjective questionnaires. 


Further probing the connection between pupil size and mental imagery could also provide a tool to better understand how we all visualize in our minds. “It is called the window to the soul,” said Joel Pearson, senior author on the new paper, and a psychologist and neuroscientist at the University of New South Wales. “But the retina is literally part of your brain, like a little bit of your brain is pulled out. It’s part of your brain that is exposed to the world.” 

There is a wide range in how vividly people experience imagery in their mind. On the far end of the spectrum are people who can’t imagine visuals in their minds at all. This was noted at least as far back as 1880, when Francis Galton, the polymath, eugenicist, and cousin of Charles Darwin, asked 100 men to describe the table where they ate breakfast every morning, and found that some couldn’t do so very well. 

“To my astonishment, I found that the great majority of the men of science to whom I first applied protested that mental imagery was unknown to them, and they, looked on me as fanciful and fantastic in supposing that the words ‘mental imagery’ really expressed what I believed everybody supposed them to mean,” Galton wrote. 

In 2003, a neurologist from the University of Exeter, Adam Zeman, met a 65-year-old man who lost the ability to mentally visualize people and places that were familiar to him after a surgery, though he still performed normally on other tests of perception, visual imagery and visual memory. Zeman published a case study on the man, known as MX, which the science journalist Carl Zimmer wrote about in Discover in 2010.


After publication, readers began to reach out, saying that they too couldn’t mentally imagine people or places. These people, though, hadn’t undergone a surgery or brain injury—they had never been able to do so. Zimmer forwarded these messages to Zeman, who was also being contacted by people with similar experiences.  

In 2015, Zeman and his colleagues published another paper on 21 people who had never had mental imagery, which they dubbed congenital “aphantasia.” The Greek word phantasia means "imagination." Now, thousands of people have answered the Vividness of Visual Imagery Questionnaire (VVIQ), and it’s estimated that around 2% of the population might have aphantasia. 

But as aphantasia research has continued, those who study it have tried to find new ways of measuring it. Questionnaires can be subjective. The VVIQ can assess for the intensity of a mental image, but could also inadvertently measure metacognition, or how aware a person is of their own thoughts. For instance, if two people are asked to imagine an apple, and rate how intensely they’re visualizing it, they could be imagining the same thing but give different ratings depending on how they interpret their visualization. 

This is where pupil size might help. ‪Sebastiaan Mathôt‬, an experimental psychologist at the University of Groningen in the Netherlands who studies pupillometry, said it’s widely accepted now that our pupils change sizes in response to many things, not just light. 


Seeing an image of something that we know is bright—like a picture of the sun—leads to more pupil constriction than a picture of the moon, even if both images have the same actual level of brightness. One of Mathôt’s studies found that pupils were larger when people were presented just with words associated with darkness, like night, and pupils became smaller with words associated with brightness, like day.

Pupil changes have been seen when people look at emotionally arousing pictures, as Hess discovered, and pupil size has also been associated with how much mental effort something takes; pupils are larger when people are asked to solve more difficult math problems. The economist Daniel Kahneman has found that in people asked to memorize numbers, pupil size reflected how long of a number they were tasked with remembering. 

“The general finding is clear,” Mathôt has written. “Whatever activates the mind causes the pupil to dilate.” 

In the new study, pupil size might offer a clue to something that isn’t happening in the mind: mental imagery. 

The researchers tracked the eye movements and pupil sizes of 42 people who said they did have visual imagination. They were shown either bright or dark triangles for five seconds, watched a blank screen for eight seconds, and then were asked to imagine what they had seen and rate how vividly they could imagine it. People’s pupils changed in response to seeing the bright or dark triangles, and also when people imagined the bright or dark triangles. A larger pupillary response was associated with a person having stronger and more vivid imagery.


Yet in 18 people who had self-reported aphantasia, their pupils did not change when they were asked to imagine those same shapes. Interestingly, their pupils did change in response to seeing the actual images of dark and bright triangles, and their pupils dilated when they were asked to do a task that required a lot of cognition. It was only the mental imagery task that their pupils didn’t respond to. (These pupil changes probably aren’t ones that you can see on your own, however. Unlike pupil dilations or contractions caused by light, psychosensory pupil changes are much smaller, Mathôt said.)

Aphantasia is a “fascinating but elusive, essentially subjective, phenomenon,” Zeman, who wasn’t involved in the new study, told Motherboard. “Scientific understanding of aphantasia will be greatly enhanced by objective approaches which can measure its physiological correlates. The work of Kay and colleagues, which relies on the previous observation that imagined visual stimuli affect the size of the pupil, take a valuable step in this direction.” 

It doesn’t mean that the more subjective measurements will be tossed aside. Zoe Pounder, a postdoctoral researcher at the University of Oxford who studies aphantasia, said she thinks that there will still be a role for questionnaires alongside objective tests. 

“What this means is that researchers can be less reliant on these subjective methods, especially those that are used currently to identify people with aphantasia within research studies,” Pounder said, who wasn’t involved in the new paper. “Although further research is required to replicate these findings in larger samples and to determine how such a method can be adapted to use within online research settings.” 


Pearson said that they’re working on more studies now to replicate their findings, this time using photographs of real world objects, rather than just abstract shapes. They’re also starting trials to see if they can do these pupil measurements through a webcam—which would open up the possibility for much larger sample sizes. 

The other end of the visualizing spectrum is called hyperphantasia, and is an extreme ability to mentally visualize. Zeman told Scientific American that people with hyperphantasia have said they “easily lose themselves in daydreams about the past or the future.” Pearson said wants to do a future study of pupil response for this experience too. 

Overall, Pearson said he hopes this finding will bring more attention to the study of mental imagery and visualization, now that there might be objective tools to measure these phenomena. It’s a classic technique in neurology to study the processes of the brain by studying where it’s gone awry. Studying aphantasia could help us learn more about  how what we picture in our minds affects us all, Pearson said, and what role imagery might play in everyday behavior and cognition, from investing, moral dilemmas, decision making, to risk perception. 

One of Zeman’s aphantasic participants from the 2015 paper was Tom Ebeyer, who has since co-founded the Aphantasia Network. Ebeyer first realized that other people could mentally visualize at college, when he was around 20 years old. But many people who have aphantasia still don't know it. “They aren’t aware that their friends and family are visualizing,” Ebeyer said. 


Having an additional test, like a pupil test, would be helpful since it can be difficult to comprehend or explain what the experience of imagery in your mind is actually like. “The language that we use around imagination in general can be confusing if you’re unaware that aphantasia is even possible,” Ebeyer said. 

Ebeyer thinks it could also promote the legitimacy of aphantasia. There has still been deliberation of whether people with aphantasia do have the capacity for mental imagery, but lack the self awareness of it. One paper from 2016 questioned whether aphantasia was caused by a “refusal” to imagine in the mind, rather than an inability to do so, or posited it could be psychogenic, meaning having a more psychological origin rather than a purely biological one. 

Pupil dilation is an unconscious process, Pearson said. Since the people in their study with aphantasia did not show any pupil change when asked to visualize, but did show it for other tasks, the findings suggest that aphantasics are actually not able to visualize. 

Ebeyer said that knowing this quality about yourself, and your own mental imagery abilities is meaningful, even if aphantasia isn’t experienced negatively, which it rarely is. Having aphantasia may lead to some differences, which are still being studied, in visual working memory, visualizing the future, or potentially dreaming. In one study, Pearson and his colleagues measured the physical response of people who read scary stories in a darkened room—some who had aphantasia and others who didn’t. They found that those with aphantasia didn’t show a physiological response to imagining the scary scenarios compared to the others in the study. 

But aphantasia also reveals how little we know about the role of mental imagery. For instance, it seems like it’s not necessary to be able to conjure up mental images to have a creative profession, since many people who have come forward who have aphantasia have visually oriented jobs. 

As the fantasy novelist Mark Lawrence, who has aphantasia, wrote in the Guardian in 2020, “I don’t have a problem with imagination. I write books that are often praised for clear and evocative visual description.” He simply approaches visualization in another way, and doesn’t see aphantasia as a deficit. In some ways, he finds it a better path to creativity. 

“You see a horse if asked to imagine one,” he wrote. “I find this rather limiting. I imagine a web of horse-stuff that leads me down many paths. The idea of seeing one particular horse actually lacks appeal. What if it’s not the horse I want?” 

Aphantasia gives us this peek into what the internal experience of others is like. Like Zimmer wrote when Zeman’s 2015 study came out, it can prompt us to “think about ways to experience life that are radically different from my own, and it offers clues to how the mind works.”

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