First biological test for aphantasia uses pupil response, UNSW researchers say

A simple test might reveal whether a hidden disability called aphantasia affects an estimated several million people worldwide. Aphantasia is the brain’s inability to form mental images—faces, scenes, or objects—despite normal vision. People with the condition often realize they are different only after hearing others describe vivid inner pictures or dreams they cannot reproduce. Experts say the condition is under-recognized because it does not affect intelligence, language or external functioning, and because reliable testing has been lacking.

Researchers at the University of New South Wales in Australia have developed what they say is the first objective biological test for aphantasia, using pupillary response as a readout of mental imagery. The approach hinges on how the pupil expands and contracts in response to light and darkness, both when eyes are open and when the mind imagines the shapes.

In the UNSW study, reported in findings published in 2022 in the journal eLife, researchers monitored pupil size in participants with and without aphantasia while they were shown bright and dark shapes against a gray background. Both groups exhibited normal pupil responses while viewing the images, indicating intact eyes and early visual pathways. But when asked to visualize the same light and dark shapes, a distinction emerged: participants without aphantasia showed pupil changes consistent with the imagined brightness or darkness, whereas the aphantasic group’s pupils did not change.

“Our results show an exciting new objective method to measure visual imagery, and the first physiological evidence of aphantasia,” senior author Dr Joel Pearson, a neuroscientist, wrote in a university blog. He described it as the first biological, objective test for imagery vividness. The researchers noted that the lack of a pupil response does not mean people with aphantasia are not trying. In fact, when asked to imagine four objects rather than one, both groups showed pupil dilation, a sign of increased mental effort, even if the imagery was not visual in nature.

“Our pupils are known to get larger when we are doing a more difficult task,” added Lachlan Kay, a PhD candidate in the Future Minds Lab at the university. “Imagining four shapes 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.”

Pearson described the finding as very exciting, saying it marked the first time researchers had shown that people with aphantasia actively attempt mental imagery. He said it puts to rest claims that they may simply not be attempting to create a mental image. It is estimated that around two to five per cent of the population have aphantasia — equivalent to millions of people worldwide — according to the British Psychological Society. Rather than thinking in pictures, people with the condition tend to process information through facts, concepts and abstract knowledge, according to the Aphantasia Network, a global online community for those affected. The term derives from ancient Greece, where the philosopher Aristotle discussed ‘phantasia’ as the mind’s ability to form images.

There is no single presentation of the condition. Some people are born with it, known as congenital aphantasia, while others acquire it later in life due to brain injury, trauma or medical conditions. Aphantasia can be complete or partial. Some people experience no mental imagery at all, while others report dim, fleeting or fragmented images. While visual aphantasia is the most common form, researchers say the condition can also affect the ability to imagine sounds, touch, smells, tastes and movement.