In Brief

People aren't as likely to notice unanticipated or unusual visual changes as they are probable ones, and they overestimate their ability to detect both types of changes, reports a study in APA's Journal of Experimental Psychology: Human Perception and Performance (Vol. 30, No. 4).

Psychologist Melissa Beck, PhD, of George Mason University, and colleagues researched how susceptible people are to change blindness--the phenomenon where people fail to notice changes occurring to objects within their visual fields, such as cars shifting position in surrounding lanes.

The researchers hypothesized that people more often notice changes they can logically anticipate (such as a red traffic light in one scene turning green in the next) than improbable ones (such as a red light turning into a stop sign) because their understanding of the visual world leads them to more closely monitor properties that are likely to change.

To test that theory, the researchers photographed 10 everyday places, such as a living room, or classroom, digitally altered each image twice and grouped the photos into 10 sets of three--the original image plus two similar, manipulated images. One altered image contained a probable change (a curtain over what was originally an uncovered window); the other contained an improbable change (the window disappeared).

Then, Beck assigned 68 undergraduates to either a performance or a prediction condition. Participants viewed 20 images on a computer monitor: the original 10 plus 10 corresponding altered images--five improbable and five probable. While viewing the original images, participants thought about a story they could write using the scene. That way, Beck says, participants didn't focus on seeking changes and processed the scene for meaning--much like we do when looking around in the real world, she said. After six seconds, the altered photo suddenly replaced the original and participants attempted to determine what changed.

Researchers also interspersed 22 filler images of scenes that did not have an altered counterpart. Participants focused on potential stories for these images as well and did not know which scenes would change until the altered image appeared.

Researchers told participants in the prediction condition how each image would change beforehand. Then, after seeing the alteration, students rated if they would have detected the change if they didn't know it was coming.

The performance group noticed about 40 percent of probable changes and 20 percent of improbable changes. By comparison, the prediction group reported they would have identified 60 percent of improbable and 70 percent of probable changes.

The findings suggest people better detect probable changes yet overestimate their ability in either case. In fact, notes Beck, it appears they're far less likely to notice improbable changes even though they believe they're as likely to recognize them as probable changes.

"It makes more sense for an efficient visual strategy to observe things you expect to change," Beck explains. This efficiency is particularly important because of the extreme limits on the amount of information we are aware of, she suggests.

The findings also cast light on the phenomenon of change blindness by suggesting that people have less conscious control over what they see than they think they do, since they don't notice improbable changes, Beck says.

"This is counterintuitive," she says. "We think we choose aspects to process visually, but it seems this process is occurring at some level not under conscious control."

She believes we may overestimate our detection abilities "because we have such a rich visual experience. We believe we perceive everything, but the level of detail we retain is very coarse."

--M. GREER