The mind's eye: Pupil size may be indicator of aphantasia

• Researchers investigated whether people with aphantasia—an inability for visual imagination—have different pupillary responses to people without the condition.
• They found that the pupils of those with aphantasia do not respond to visual imagery in the same way as those without.
• The researchers concluded that changes in pupil size could be used to measure the strength of visual imagination.

Pupils change size to let in the correct amount of light to optimize vision, for example, by contracting in bright conditions and dilating in dark conditions. Emerging research suggests that higher-order perceptual and cognitive processes may also drive changes in pupil size.

One study found that mental imagery may drive pupillary responses to light. Another study found that subjective interpretation of visual stimuli also influences pupil size, i.e. whether grayscale images are interpreted as the sun or the moon.

While some people report the images in their mind feeling almost as vivid as what they perceive in real life, a small percentage of otherwise healthy people cannot produce images in their minds (mental imagery). This condition is known as aphantasia and can begin from birth or develop later on.

Researchers hypothesized that by investigating pupillary responses of people with aphantasia, they might be able to better understand how visual areas of the brain and ocular reflexes work.

A recent study compared these pupillary responses in people with and without aphantasia to gain further insight.

“The fascinating finding reported here is that people imagining light or dark objects show the same type of pupillary action as we would normally find when the eyes are exposed to light and dark objects in the real world,” said Prof. Julia Simmer, who leads the MULTISENSE lab at the University of Sussex, and was not involved in the study.

“This suggests that their imagination is treated [as] a simulation of the real world. [Meanwhile,] people with aphantasia —who imagine in a more abstract way because they cannot build a picture in their mind’s eye—don’t show the same effect,” she told Medical News Today.

The study was published in eLife.

Pupil size and imagining shapes

The researchers recruited 42 psychology students with a mean age of 19.8 years who did not have aphantasia. They also recruited 18 individuals with aphantasia at an average of 35.8 years old.

For the study, all of the participants perceived and were later asked to imagine 32 white or gray shapes. The researchers measured the size of their pupils throughout.

The participants also engaged in other experiments and filled in a questionnaire to ensure a lack of decisional bias and inattention in their answers, as well as whether or not they had aphantasia.

In the end, the researchers found that in people without aphantasia, the vividness and luminosity of images— whether perceived or imagined—significantly influenced their pupillary light response.

Meanwhile, they found that the luminance and vividness of images—either perceived or imagined—had no significant effect on the pupillary size of those with aphantasia, reflecting their lack of subjective visual imagery.

The researchers further discovered that people with aphantasia exerted more cognitive effort when trying to imagine four shapes as opposed to one.

Underlying mechanisms

When asked what may explain the findings, Dr. Sebastiaan Mathot, assistant professor in experimental psychology at the University of Groningen in the Netherlands, who was not involved in the study, told MNT:

“One plausible explanation is that the pupil light response reflects, among other things, activity in visual brain areas. For most people, visual brain areas are also involved in mental imagery, thus resulting in pupil responses, especially when contrasting imagery of bright and dark things.”

“However, for people with aphantasia, these areas may not be involved in imagery, or less so, resulting in an absent or reduced effect of mentally imagining bright and dark things on the pupil light response,” he added.

The researchers also noted that pupil diameter may be encoded along with original visual information for bright objects and is thus replayed during memory decoding.

The researchers concluded that changes in pupil size could be used to measure the strength of visual imagination.

When asked about the findings’ limitations, Dr. Nicholas Davidenko, associate professor of psychology at the University of California, Santa Cruz, who was not involved in the study, told MNT that the participants were not matched for age. The non-aphantasic participants were college psychology students who had been offered extra credit for participation.

This means that the results may not account for age-related declines in pupillary response. However, he said that the study still holds even if age is unaccounted for.

Dr. Wilma A. Bainbridge, assistant professor of psychology at the University of Chicago, who was also not involved in the study, told MNT that the unmatched ages between groups are unlikely to have caused a meaningful difference.

Dr. Bainbridge said she would like to see research on whether age could affect short-term memory. Previous research, she pointed out, shows that aphantasic people usually perform normally in working memory tasks, as they’re likely able to perform them without imagery.

Findings ‘not so clear-cut’

Dr. Reshanne Reeder, a senior lecturer of psychology at Edge Hill University, the United Kingdom, told MNT that the results might not be so clear-cut.

She explained that the results demonstrate that the pupils of people with aphantasia constrict in both light and dark conditions, whereas the same was only true for the bright condition among controls.

“Pupil size differences were not statistically compared to baseline pupil size, so I can only speculate, but looking at the figures, it is not so clear-cut what is going on during imagery for either group” she noted.

“Here, a significant group difference is reported, but just over 60% of people with aphantasia show a pathological response to the pupillary test,” Dr. Sergio Della Sala, professor of human cognitive neuroscience at the University of Edinburgh, who was not involved in the study, told MNT.

“Hence, further studies should evaluate the clinical sensitivity and specificity of the test not by grouping the participants as with or without aphantasia, but as those performing normally or abnormally on the test, and then check whether they have or not aphantasia,” he explained.

Real-life applications?

Dr. Joseph Johnson, professor of psychology at Miami University, Ohio, who was not involved in the study, told MNT that if these findings are reproduced in future studies, researchers and clinicians may be able to use the pupillary response to more objectively measure visual imagery than current self-reports.

Adam Zeman, professor of cognitive and behavioral neurology at the University of Exeter, added that physiological reports from pupillary response could create a “triangulation” effect between first-person reports and measures of behavior.

While it is currently unknown whether aphantasia could interfere with the diagnosis and treatment of disorders, Dr. Mathot and Dr. Reeder hope that it may have health applications in the future.

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