Science Watch

Rotating tires, spinning tops and revolving ferris wheels are part of modern life. But there's something about their movement, round and round, that most people find fascinating as well as confusing.

In fact, a group of psychologists has documented over the years that people view and process rotation very differently from other types of motions. We rarely gape at objects moving from place to place, but become mesmerized by spinning tops. We have natural intuition about the properties of most moving objects, but we're mostly dumbfounded by the properties of rotating objects. And in physics classes, we tend to panic when faced with lessons on torque and angular momentum more so than any other aspect of the discipline.

Researchers are now piecing together an understanding of why it is that rotations tend to produce in people more wonderment than understanding.

This month, in the Journal of Experimental Psychology: Human Perception and Performance (Vol. 26, No. 1, p. 18-30), psychologist David Gilden, PhD, of the University of Texas at Austin, and his former graduate student Christy Price, add another piece to the increasingly interesting puzzle: They have discovered that people easily remember the direction of objects moving toward or away from them, and past them from left or right, but that people have virtually no memory for the direction in which an object rotates.

"We're finding a hierarchy of incompetence when it comes to rotations," says Gilden, who also holds a PhD in astronomy. "People are not extracting any information from rotating objects; they don't remember what they see, so they don't understand what they're seeing."

The research, which was funded by the National Institute of Mental Health, has practical implications for educators trying to teach concepts such as angular momentum and torque, says Gilden. Since seeing rotations does little to educate, teachers are doomed to teach rotations from a formal point of view.

They also have implications for psychology: Memory, it seems, may be immutably linked to occurrences that have consequences for our goals and actions. When it comes to motion, our goals and actions are most concerned with where things are going, not their orientation, says Gilden. Because rotations are generally inconsequential, we didn't evolve to pay any particular attention to them.

"This series of experiments is quite unique," says University of Virginia psychologist Dennis Proffitt, PhD, who also studies motion perception. "As far as I know, no one has looked at the idea that some motions are easy to notice and remember, while others we don't notice and remember at all. The most interesting thing is that this finding is part of a larger program of study finding that rotations are difficult to see, imagine and understand."

Captivating, yet unmemorable

When Gilden first started examining how the brain processes motion, he hypothesized that high-level cognitive understanding of motion would be constrained by the way the image of a moving object was projected onto the retina. The more complex the image, the harder it would be to understand the motion, he predicted.

Different types of motions project different images on the retina. For example, even though an object moving toward a person and an object moving past a person are both considered the same type of motion--what psychologists call translations where an object moves from one place to another--they project very different visual images. In fact, objects moving toward a person are called "looming" because they project a pattern of growth on the retina.

Looming motions, it turns out, are as complicated visually as rotations and seem to be processed in the same area of the brain. So if the complexity of the image on the retina predicts cognitive function, people's memory for looming motions should be about the same as their memory for rotations.

But it's not. In several experiments Gilden and Price found that after viewing groups of moving objects on a computer screen, people accurately recalled five minutes later the direction of motion of both looming and translating objects. In contrast, they performed at chance when trying to remember the direction rotating objects moved--clockwise or counterclockwise.

Based on these findings, Gilden concludes that "the visual system isn't organized in terms of the complexity of what is on the retina, but rather in terms of what the practical consequences are for behavior."

In fact, looming and translating motions--which are really the same type of motion, moving objects from one location to another--have had consequences for people throughout our evolution, says Gilden. It was critical, for example, to remember toward which direction an animal you were hunting passed, or to notice whether a wild beast was approaching or retreating. In contrast, rotating objects in our evolutionary past didn't require any special notice--it makes little difference which way an object rotates because it can get to any given orientation by spinning in either direction.

Interestingly, people remember the direction of rolling objects--which both rotate and translate--less well than they remember the direction of translating objects, but better than they remember the direction of rotating objects, Gilden and Price report in their article. People, they add, accurately remember the direction of the translation, but not the direction of the rotation.

"Rotations simply don't draw our attention," says Gilden. "If we do attend [to the direction of a rotation], it requires so much concentration that it excludes analysis of the rest of the environment. Rotations capture your attention to the exclusion of everything else."

Far-reaching implications

The consequence is that we have a hard time understanding the properties and rules of rotating objects, says Proffitt. Sure, some people gain a working understanding of certain types of rotations, but this understanding is never intuitive and often doesn't generalize to all rotations.

Even physicists and engineers who learn to work with angular momentum and torque get muddled when they aren't allowed to use the equations they've developed to analyze problems, finds Proffitt in another series of experiments.

"Engineers, bicyclists, divers, ice skaters all understand rotations, but only within the domain in which they encounter rotation," says Gilden. "Take them outside that domain and ask them to evaluate another rotational system and they don't understand. Despite my training in physics and my ability to solve problems involving tops, I still wonder why they don't fall over."

The implication for the average person is that it will take personal experience and practice to truly understand something like pulling a car out of a spin, says Gilden. Our intuition won't work. In addition, it means that physics instructors may need to come up with new techniques for teaching rotation if they truly want their students to understand.

In the meantime, Gilden is trying to find the point in the visual system where rotating motions are sloughed off and not passed on to higher cognitive and memory areas of the brain. He believes that because rotations held no evolutionary significance for us, the brain simply developed to ignore them at a highly cognitive level. The result is a strange fascination.

"When a top spins and doesn't fall, people gape at it," says Gilden. "That's a strange response. We don't gape in wonderment at our environment for the most part. But that's what we do with rotations."

This article is part of the Monitor's "Science Watch" series, which reports news from APA's journals.