目が語る「心の中の映像」――瞳孔と心のイメージの科学

The “Mental Images” Revealed by Our Eyes: The Science of Pupils and Mental Imagery

When you step out of a dark movie theater, the bright light makes you squint unconsciously. This is a reaction of the pupil—the black part at the center of the eye—as it adjusts to the amount of light. Recent research has shown that the pupil reacts not only to actual light but also to “light imagined in the mind.”

“Mental imagery” refers to the ability to visualize things in your mind that you are not actually seeing. When we recall yesterday’s dinner, we are watching a visual scene through our “mind’s eye.” This ability varies from person to person; some people see vivid images as clear as a movie, while others see only a blurry outline. There are also people who cannot visualize anything at all, a condition known as “aphantasia.”

Researchers are investigating these differences by measuring pupil size. They use a special camera to record pupil size. Since people cannot consciously control their pupils, these changes provide an honest reflection of the brain’s state.

In the experiment, participants were first shown bright and dark images. When viewing bright images, their pupils contracted, and when viewing dark images, they dilated. Next, when asked to “visualize” those images in their minds, people with vivid mental imagery showed the same pupillary response as when they actually saw the images. People with aphantasia showed almost no change.

In another experiment, participants were asked to visualize a map of France in their minds and answer whether a certain city was located to the left or right of Paris. People who could vividly visualize the map thought of “seeing” the map while moving their eyes. People with aphantasia barely moved their eyes and answered by using their knowledge—for example, “Bordeaux is west of Paris.” At this point, their pupils dilated significantly. This is because thinking based on knowledge requires more energy.

This research shows us that there is no single “correct” way to use the brain. Everyone has their own way of thinking, and they can arrive at the same answer using their own methods. And it is these differences that make a team stronger.