As many as , the inability to picture images in one’s head. But formally diagnosing the condition is difficult. A simple physiological test involving a webcam could one day offer a solution.
at Macquarie University in Australia and her colleagues studied the effectiveness of their test on 56 people without aphantasia and 18 people who said they have the condition.
The test is based on changes to pupil size. Looking at a bright object causes a person’s pupils to constrict in order to reduce the amount of light hitting the retina. Dim objects, on the other hand, cause the pupils to dilate to boost the amount of light reaching the retina. The researchers speculated that a similar effect could be observed if someone was told to imagine a bright or dark object.
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“My favourite theory is that when you imagine a mental image, you recruit brain areas involved in perception… and these areas are connected to parts of the brain that are controlling the size of the pupil,” says at the Paris Brain Institute, who worked on the study.
In their tests, the researchers tracked each participant’s pupil size using an infrared camera and showed them a bright image of an object on a screen for 5 seconds, which they were told to memorise. After the image disappeared and the participant’s pupils returned to their original size, they were asked to imagine the object in their heads. This task was repeated with 16 bright images and 16 dark images.
The pupils of all participants changed in response to seeing bright and dark images on the screen. About 90 per cent of those without aphantasia also showed pupil size changes when told to imagine those images. However, the same was true of just 39 per cent – 7 out of 18 – of people who said they had aphantasia.
Andrillon suggests the test could one day be used to check if someone has aphantasia. The current method, called the binocular rivalry task, requires mirrors and can’t be easily done at home, he says.
But Keogh says the test still needs to be refining before it can be used widely.
“We cannot run this study without access to infrared glasses that can measure pupil size,” she says. This is because the pupil size changes seen in those without aphantasia are still very small – a change in diameter of about 0.2 to 0.4 millimetres.
The team wants to gather more data with larger sample sizes and hopes to develop a test that can be done at home via a laptop webcam.
Andrillon says such a test would be important because many people only discover they have aphantasia later in life. “We’re not always necessarily experiencing things the same way other people are experiencing things,” he says.
“It is a promising step for the field to have a more objective way to measure visual imagery ability, especially for researchers who are interested in visual imagery extremes like aphantasia,” says at the University of Oxford. But she says the study needs to be replicated with larger sample sizes.
eLife
