Niko Troje says that a reported preference for seeing the silhouette spinning clockwise rather than counter-clockwise is dependent upon the angle at which the viewer is seeing the image.
"Our visual system, if it has a choice, seems to prefer the view from above," says Dr. Troje. "It's a perceptual bias. It makes sense to assume that we are looking down onto objects that are located on the ground below us rather than floating in the air above us."
In the Silhouette Illusion, a silhouetted woman is seen spinning on one foot, her leg extended. The appeal of the illusion is in the way the woman is spinning -- she can be perceived as spinning clockwise or counter-clockwise.
Dr. Troje and his team found that a view-from-above bias (VFA) is what makes the viewer prone to seeing the silhouette in a certain way, not one's personality or whether the viewer is left- or right-brained. When shown the silhouette illusion, the study's 24 participants most often reported that the woman was spinning counter-clockwise if viewed from above, and clockwise if viewed from below. Thus, the viewing angle causes the difference in perception.
The theory can also be applied to other popular illusions, including Neckar Cubes, that are often used in online personality tests.
The study was published recently in i-Perception, the new open-access sister journal of the established British journalPerception.