KENSINGTON, Australia — Can you remember the shape and color of an object and picture it in your mind? It turns out millions of people around the world can’t. Scientists call this condition aphantasia and a new study has found an interesting way of finding people who have it — using the pupils of your eyes.

A team at the University of New South Wales discovered that an aphantasia patient’s pupils did not respond when researchers asked them to imagine either light or dark objects. Conversely, people without aphantasia saw their pupils either constrict or dilate as they pictured different objects in their mind.

What’s it like living with aphantasia?

One of our StudyFinds colleagues, Sam, has aphantasia and we asked him: what happens when someone asks you to “imagine” something?

“All the traits of what is described go through my head in words. ‘Imagining’ for me is really about telling a little story,” he explains. “On the positive side, I think I have a particularly good memory for a sequence of events and words that were said in a given situation. Additionally, as others with aphantasia have reported, I can usually move past any negative life events relatively quickly since I do not recall them visually.

“That said, now that I understand that others can visualize things in a robust way, I assume there may be a negative affect on my ability to diagram or think creatively without actively putting pen to paper,” he adds. “I also wish I could imagine something happening in my mind as that sounds like a lot of fun. Why don’t people who visualize normally sit around and daydream all day?”

Your eyes are the window to your imagination

Researchers started their experiment with 42 people who reported having very visual imaginations and no signs of aphantasia. The team fitted each person with special glasses which tracked their eye movements and pupil sizes during the study.

Study authors then showed each person a set of dark and light shapes set against a grey background. As expected, the participants’ pupils constricted (shrank) while seeing bright shapes. This mimicked the effect of what the eyes do when seeing a bright light or looking up into a sunny sky.

On the other hand, the group’s pupils dilated (grew larger) when seeing dark objects. This is what happens when your eyes enter a dark room and try to bring in more light to see.

In the second part of the test, researchers asked each person to simply imagine those same light and dark shapes and then report on the “vividness” of the image in their mind. Results show that people without aphantasia still saw their pupils constrict or dilate depending on the object they tried to picture with their mind.

“The pupillary reflex is an adaption that optimizes the amount of light hitting the retina,” says Professor Joel Pearson, senior author of the paper, in a university release. “And while it was already known that imagined objects can evoke so-called ‘endogenous’ changes in pupil size, we were surprised to see more dramatic changes in those reporting more vivid imagery. This really is the first biological, objective test for imagery vividness.”

400 million people may have mind blindness

After testing this control group, the team moved on to examine 18 people with a self-reported case of aphantasia. Researchers found that aphantasia patients see their pupils react just like anyone else’s when seeing light and dark objects.

However, in the second test where they had to imagine those objects without seeing them, aphantasic individuals had no change in their pupil sizes.

“One of the problems with many existing methods to measure imagery is that they are subjective, that is to say they rely on people being able to accurately assess their own imagery. Our results show an exciting new objective method to measure visual imagery,” says Prof. Pearson, “and the first physiological evidence of aphantasia. With over 1.3 million Australians thought to have aphantasia, and 400 million more internationally, we are now close to an objective physiological test, like a blood test, to see if someone truly has it.”

The team did note that aphantasic participants did see their pupil sizes change when they imagined a different number of objects — such as trying to picture four circles instead of just one. Study authors believe it takes more mental effort to imagine multiple objects at once, causing the pupils to change in size.

“Our pupils are known to get larger when we are doing a more difficult task,” explains Lachlan Kay, a PhD candidate in the Future Minds Lab at UNSW. “Imagining four objects simultaneously is more difficult than imagining just one. The pupils of those with aphantasia dilated when they imagined four shapes compared to one, but did not change based on whether the shapes were bright or dark. This indicated that the participants with aphantasia were indeed trying to imagine in this experiment, just not in a visual way.”

An online test for aphantasia?

Previous studies have shown that “mind blindness” can keep people from performing tasks such as counting sheep to fall asleep. Interestingly, it can also make someone harder to scare because their mind cannot conjure up frightening images while they hear a scary story.

“Our previous work has shown that aphantasic individuals are able to perform visual working memory tasks, remembering many images for a short period of time, without using visual imagery,” says Dr. Rebecca Keogh, a postdoctoral research fellow based at Macquarie University.

“These findings further highlight the wide variability of the human mind that can often remain hidden until we ask someone about their internal experiences or invent new ways to measure the mind. It reminds us that just because I remember or visualize something one way, doesn’t mean everyone does.”

“This really is an exciting time. We are very close to having objective, reliable tests for extreme imagery, aphantasia and hyperphantasia (extremely strong visual imagery) that could be scaled up to run online for millions of people everywhere,” Prof. Pearson concludes.

“We know that thinking in pictures or not affects the number of details in lifelong memories, how emotional we get when reading, and how we hold things in short term memory. This new method will allow us to understand the brain mechanisms of extreme imagery and the global implications for how we think, make decisions and feel.”

The study appears in the journal eLife.