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Here are the simple instructions given by a Harvard University assistant professor to people participating in a recent science study: “Imagine the following scene. Visualize it in your mind’s eye, as vividly as you can: a person walks into a room and knocks a ball off a table.”

The assistant professor, Tomer Ullman, then asked those in the study about their mental images: “Did you see how big the ball was? How about the person’s hair color?” Most participants visualized the former but not the latter. Ullman and his colleagues term this absence of details “noncommitment” to mental imagery (意象).

Brain imaging studies show that mental imagery engages the same neurons (744770) in similar ways as perception (EXXI). Visualizing things seems to have much in common with actually seeing them. But if mental images are indeed pictures, why do they lack such simple details?

Ullman and his colleagues conducted a series of experiments in which participants visualized the ball and table scene and were then presented with the questions selected by the researchers. The findings show 78% of the participants did not visualize at least two details. People are often unaware of how little detail their mental images contain until asked. They don’t notice how much they don’t notice. It has nothing to do with a person forgetting the contents of a mental image, and it also is found in people with vivid imaginations.

“Nearly everyone can tell you the size of the ball but not the person’s hair color,” Ullman says. “It’s like there’s one hierarchy when we construct images, and spatial properties are high up. Then things like colors are further down.” This fits with Kosslyn’s “skeletal image” theory, in which overall shape is generated first, and other details are added as needed.

“There are imagery-based systems for interviewing people who witnessed a crime to guide them through trying to visualize it as accurately as possible,” Kosslyn says. Imagination is an issue, but understanding noncommitment better could help develop ways of getting more accurate eyewitness evidence, he says. “That’s worth a lot.”