Researchers uncover surprising limit on human imagination

Tomer D. Ullman.

Niles Singer/Harvard Staff Photographer

Science & Tech
Researchers uncover surprising limit on human imagination

Humans can track a handful of objects visually, but their imaginations can only handle one

Christy DeSmith

Harvard Staff Writer

August 13, 2025 4 min read

Human beings can juggle up to 10 balls at once. But how many can they move through the air with their imaginations?

The answer, published last month in Nature Communications, astonished even the researchers pursuing the question. The cognitive psychologists found people could easily imagine the trajectory of a single ball after it disappeared. But the imagination couldn’t simultaneously keep tabs on two moving balls that fell from view.

“We set out to test the capacity limits of the imagination, and we found that it was one,” said co-author Tomer D. Ullman, associate professor in the Department of Psychology. “I found this surprising, so I can understand if others do, too.”

Ullman, who heads Harvard’s Computation, Cognition, and Development lab, has a long-time interest in what is known as intuitive physics. Think of the brain conjuring a ball as it rolls downhill, or sounding the alarm over two objects on a sure-fire collision course.

“How do we interact with the physical world around us?” wondered Ullman, who is also affiliated with the Kempner Institute for the Study of Natural and Artificial Intelligence. “I subscribe to the theory that the brain may be running mental simulations, kind of like a video game.”

These couldn’t be perfect simulations of physical environments, right down to the level of atoms and molecules. So Ullman’s lab has worked to understand what kinds of hacks and workarounds make mental simulations possible.

“The human imagination is just really cool, and we find a lot of people are quite interested in how it works,” he offered.

A sizable body of research has explored the capacity limits of human perception, or how many objects the brain can track in a visual scene. “Maybe you’re a parent watching multiple kids, or maybe you’re a lifeguard on duty,” Ullman said. “Obviously you can’t keep track of everything.”

Neuroscientists, psychologists, and computational modelers have found visual tracking is limited to just a handful of moving objects. But few have explored the imagination’s capacity limits.

In the new study, online participants were shown an animation of a bouncing ball, as if on a racquetball court, before it vanished. Others saw two balls ricocheting at completely different cadences before both disappeared. Designing the experiments with Ullman was lead author Halely Balaban, an assistant professor of cognitive psychology at the Open University of Israel.

Also devised were two computational models to explain how the imagination might follow these invisible balls to their moment of impact. The first model posited that multiple objects would be moved in parallel, while the second envisioned independently moving each ball in more of a serial fashion.

Ullman and Balaban found their online recruits were pretty good at predicting when a single invisible ball would have hit the ground. But people fumbled at tracking two.

“It was harder than any of us expected,” said Ullman, noting how reliably the exercise produced laughs.

Based on past findings, the co-authors originally thought the imagination could probably track about three or four objects. There were also intuitive reasons to think the mind’s eye could move multiple objects in parallel.

“If I close my eyes right now, I can see a tower of blocks falling down,” Ullman noted. “It doesn’t feel limited. People feel like they should be able to move more than one.”

In fact, a follow-up experiment found people were slightly better at tracking two balls that moved in tandem before disappearing. But performances still paled next to a yet another follow-up, in which study participants tracked two balls that remained visible until impact.

When it comes to tracking objects that have disappeared, the researchers found, the human imagination relies largely on a serial model, moving each piece one after the other.

A separate follow-up tested whether people might be conserving mental energy by employing a serial model. After all, running a simulation via the parallel model would require more effort. Imagine a computer running multiple simulations at once.

“We offered participants a bunch of money if they could get this right,” Ullman explained. “But that didn’t seem to matter.”

For Ullman, the findings open an exciting frontier. “There has been decades and decades of work on how the mind uses clever tricks to keep track of what’s in front of you,” he said. “But there’s been so little on the tricks and limitations of the mind’s eye. I could imagine a lot more work to do here.”