Can you "see" images in your mind? Some people can't - Adam Zeman

When reading Lewis Carroll’s “Alice’s Adventures in Wonderland,” most readers visualize the Queen's croquet game play out in their heads. “. .

. it was all ridges and furrows; the balls were live hedgehogs, the mallets live flamingos, and the soldiers had to double themselves up and to stand on their hands and feet, to make the arches. ” A few might see vivid details, such as the pattern of the hedgehog's quills or the flush of the queen's face.

However, a small fraction of readers have a drastically different experience. As the scene plays out on the page, within their heads, they “see” absolutely nothing. This inability to clearly visualize images in the mind’s eye is known as aphantasia, and it applies to around 4% of the world’s population.

It was first characterized by a psychologist in the 19th century, who asked study participants to visualize their breakfast table and then rate the vividness and color of this mental picture. He determined that mental imagery exists on a spectrum. At one end are people with aphantasia, and at the other are people with hyperphantasia, with imagery so real, it rivals seeing.

And most people land somewhere in between these two extremes. But how do we know if someone with aphantasia isn’t describing the same experience as someone with mental imagery, just in different terms? In other words, how does one objectively measure what’s going on in someone else’s mind?

In one study, scientists looked for clues in people's eyes. They investigated differences in pupillary light reflexes, or how the pupil automatically constricts in response to light. Even just imagining that you are looking into a light will cause the pupil to narrow— or at least it will if you have mental imagery.

They found that the eyes of people with aphantasia don’t constrict when imagining light. Brain-imaging studies may help decipher another perplexing phenomenon: people with aphantasia can see mental imagery when they dream. How this happens is likely explained by the contrasting ways the brain generates deliberate visualization versus dream imagery.

Typically, conjuring a mental image involves multiple brain regions, and is sometimes referred to as a top-down process. First, actively trying to visualize an object activates cognitive control regions of our brain. This then drives activity in regions of the brain associated with memory and vision, creating a mental picture.

People with hyperphantasia tend to have stronger connections between these regions compared to those with aphantasia. As for dreaming, many scientists believe this imagery is produced by a different bottom-up pathway, through activity deep in the brain spontaneously activating visual and memory systems. So, what causes the spectrum of mental imagery to develop?

Aphantasia often runs in families, suggesting that the vividness of your mental canvas may be influenced by your genes. While most people with aphantasia have it their entire lives, some people can develop it later in life— often due to brain injury or psychological conditions. But in most cases, imagery extremes aren’t considered disorders in need of treatment, but rather intriguing variations in human experience.

For example, for those with a mind's eye, the excitement of a thrilling story will cause them to sweat a little, even if they don't notice it. People with aphantasia, however, lack this sweat response— presumably because it depends on the emotional effect of imagining the the storyline. At the same time, scientists have speculated that aphantasia may be protective against certain mental health disorders, specifically those related to negative imagery, like PTSD— though more research is needed.

Recalling details, like the food served at last year’s holiday party, may not be as difficult for people with hyperphantasia, as they tend to have a richer memory of past events and can “relive” these experiences in greater detail. Differences in mental imagery may even influence your career choice. A survey of over 2,000 people found that those with aphantasia are more likely to work in STEM professions, while people with hyperphantasia tend to gravitate towards jobs in the arts, media, and design.

We may never be able to fully understand what’s happening in another person's mind. The inner worlds of those around you might be quite different from your own.