目が語る「心の中の映像」――瞳孔と心的イメージの科学

The “Mental Images” Your Eyes Reveal: The Science of Pupils and Mental Imagery

Have you ever noticed that when you step from a bright place into a dark room, the black part in the center of your eye—the pupil—dilates? Conversely, when you step out of a pitch-black movie theater, the brightness causes you to unconsciously squint. This is an automatic reaction by which the pupil adjusts to the amount of light. However, 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. For example, when recalling yesterday’s dinner or imagining a place you’ve never been, we are “seeing” something with our “mind’s eye” even without closing our eyes. This ability varies widely among people; some can conjure vivid, movie-like images, while others can only see them vaguely. And there are those who cannot form any visual images at all. This latter condition is called “aphantasia.”

To measure these differences, researchers use a technique called “pupil measurement.” This method involves recording pupil size using a special camera. Since people cannot consciously control their pupils, these changes objectively reflect the brain’s internal state.

In the experiment, participants were first shown bright and dark images on a screen. When they viewed the bright images, their pupils contracted, and when they viewed the dark images, their pupils dilated. Next, with the images removed from the screen, they were asked to “visualize” the images. People with vivid mental imagery showed the same pupil reactions as when they had actually seen the images. In contrast, people with aphantasia showed almost no change in their pupils.

There is another interesting experiment. Participants were asked to visualize a map of France and tell which side of Paris a certain city was located on. People with vivid mental imagery moved their eyes as if they were following an invisible map. However, people with aphantasia moved their eyes very little and answered by using knowledge such as “Bordeaux is west of Paris.” At that moment, their pupils dilated significantly. This indicates that solving problems by relying on memory requires more brain energy.

What this study shows is that there is no single “correct answer” when it comes to how the brain works. Whether people think in vivid images or through words and logic, they can arrive at the same answer using their own methods. Everyone thinks differently, and that’s okay. These differences make teams stronger. By collaborating with different approaches, we can solve problems that no one could accomplish alone.