It is well known that a powerful perceptual experience can change the way a person sees things later. If you are startled to discover a mouse in your kitchen, you may suddenly you see mice in every dark corner - or at least think you do. Is it possible that imagining something, just once, might also change how you perceive things?

To test how imagery affects perception, the researchers had subjects imagine simple patterns of vertical or horizontal stripes, which are strongly represented in the primary visual areas of the brain. They then presented a green horizontal grated pattern to one eye and a red vertical grated pattern to the other to induce what is called binocular rivalry.

During binocular rivalry, an individual will often alternately perceive each stimulus, with the images appearing to switch back and forth before their eyes. The subjects generally reported they had seen the image they had been imagining, proving the researcher's hypothesis that imagery would influence the binocular rivalry battle.

Additional experiments found that the effect of imagery on perception was approximately the same as showing the research subject a faint representation of one of the patterns between trials. Stronger shifts in perception were found if subjects either viewed or imagined a particular pattern for longer periods of time. They found that both imagery and perception can lead to a build-up of a "perceptual trace" that influences subsequent perception.

"We found that imagery leads to a short-term memory trace that can bias future perception," Joel Pearson, research associate in the Vanderbilt Department of Psychology. and lead author of the study, said. "This is the first research to definitively show that imagining something changes vision both while you are imagining it and later on."

They also discovered that changing the orientation of the image from what had been imagined greatly reduced the impact of imagery on perception. Because orientation is processed in early visual areas, this suggests that imagery's interaction with perception may occur at early stages of visual processing.

The new findings offer an objective tool to assess the often-slippery concept of imagination.

"It has been very hard to pin down in the laboratory what exactly someone is experiencing when it comes to imagery, because it is so subjective," Frank Tong, associate professor of psychology and co-author of the study, said. "We found that the imagery effect, while found in all of our subjects, could differ a lot in strength across subjects. So this might give us a metric to measure the strength of mental imagery in individuals and how that imagery may influence perception."

The findings may also help settle a longstanding debate in the research community over whether mental imagery is visual—that one imagines something just as one sees it—or more abstract.

"More recently, with advances in human brain imaging, we now know that when you imagine something parts of the visual brain do light up and you see activity there," Pearson said. "So there's more and more evidence suggesting that there is a huge overlap between mental imagery and seeing the same thing. Our work shows that not only are imagery and vision related, but imagery directly influences what we see."

The research was funded by the National Institutes of Health, an Australian Research Council Discovery Grant and an Australia National Health and Mental Research Council Martin Fellowship. Pearson is a member of the Vanderbilt Vision Research Center. Tong is a member of the Vanderbilt Vision Research Center and the Vanderbilt Center for Integrative and Cognitive Neuroscience.