People who can't 'see with their mind's eye' have different wiring in the brain

People with aphantasia lack the ability to summon crisp images in their "mind's eye." But even though they can't visualize in this way, the blueprints for those imaginary images might still be nestled in their brains, a new study suggests.

The work, published in the journal Current Biology Jan. 10, provides early evidence that the brains of people with aphantasia can light up as if they were generating mental images in their primary visual cortex — the main part of the brain responsible for processing visual information. However, these signals may be getting lost in translation.

The new research suggests that the signal "warps or stretches" before it is perceived consciously by the person with aphantasia, study co-author Joel Pearson, a professor of psychology at the University of New South Wales in Australia, told Live Science.

"We don't know yet, from these data, how it's different, but we know that it's different enough," he said.

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These results add to mounting evidence that people with aphantasia "seem to engage their visual cortex differently when they try to imagine than people without aphantasia," Nadine Dijkstra, a senior research fellow at University College London's Wellcome Centre for Human Neuroimaging who was not involved in the study, told Live Science in an email.

For the research, Pearson and colleagues recruited 14 people with aphantasia and 18 people without aphantasia. The team used a trick called "binocular rivalry," which involved flashing two striped patterns of different colors in front of the participants' eyes.

The brain constantly merges visual information from the left and right eyes to construct one cohesive image, and thus it cannot fully process this binocular rivalry. Its attempt to process the flashing stripes typically results in a visual illusion in which the two patterns fluctuate, with one image dominating for a few seconds.

For participants who can see things in their mind's eye, asking them to think of one of the two patterns can bias which image they perceive first. People with aphantasia, however, are much less likely to be influenced by this bias. "The stronger the [mental] imagery, the more likely it is to bias how they see the binocular rivalry pattern," Pearson explained.

Pearson and colleagues introduced this technique as a way to test for aphantasia in a previous paper. The approach goes beyond simply asking people to fill out a questionnaire, and it's a strength of the new study, Dijkstra said.

To study the participants' brain activity, the team used functional MRI, which tracks the flow of oxygenated blood in the brain. Increased oxygenated blood flow to a specific region of the brain is an indirect measure that indicates that region is more active.

The scientists found that all of the participants, both those with and those without aphantasia, showed an uptick in activity in the primary visual cortex during the experiment. This brain activity was observed both when the participants were asked to look at the striped patterns — a state called "perception" — and when they were asked to imagine the patterns — called "imagery."

However, people with aphantasia showed slightly weaker brain activity during perception than those without the condition. This suggests there is a "different level of processing — or type of processing — in that group" when they're directly observing an image, Pearson said.

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And there was an even more surprising finding, he added. Typically, patterns spotted in a person's right field of view are processed on the left side of the brain, and vice versa. However, the opposite seemed more likely to be true in people with aphantasia, hinting that they may have totally "different wiring in the brain," Pearson said.

To delve further, the scientists trained computer algorithms to recognize the brain activity that appeared during these tests. Based on the brain activity alone, these algorithms accurately deduced the visual patterns the participants were either perceiving or attempting to imagine. This worked in both groups, suggesting "there's a reliable signal in that part of the brain, that primary visual cortex, which is pictorial," even among people with aphantasia, Pearson said.

However, then the researchers tested how well the algorithms could "cross-decode" these signals. In short, how closely did the brain activity triggered during perception match that triggered by mental imagery?

In people without aphantasia, the signals were very similar. "In fact, they're overlapping enough in the brain to let the algorithm confuse the two," Pearson said. But in people with aphantasia, "we saw no cross-decoding," he said, suggesting there may be a fundamentally different process occurring.

These findings don't explain why people with aphantasia aren't seeing images in their conscious mind, even though their brain cells are firing. Pearson is planning further experiments to investigate this question.

"It's like a murder mystery or something. I'm hooked," he said. "I've got to find out what is this representation — there, in the visual cortex — and why is it unconscious?" he said.

Dijkstra cautioned that the study is small and that its results are "slightly contradictory" to other work done in the field. Still, she said, "they all suggest that the involvement of the visual cortex is different in aphantasia, which could perhaps explain the lack of conscious imagery."

"This is a very new research field," she added, "which means that a lot of questions are still unanswered."