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More than any other cue, the sight of advancing feet alerts humans to the presence of moving creatures, according to researchers from Queen's University in Ontario and Ruhr University in Bochum, Germany. The investigators rendered walking human and animal figures as constellations of white dots on a computer screen (right). Volunteers were shown a random sequence of these dot clusters—some in correct anatomical orientation, some upside down, some scrambled—and were asked to determine which direction the “creatures” were walking. Participants tended to become confused with upside-down figures. The reason? The feet. Subjects responded accurately if an upside-down figure had right-side-up feet but did worse if the feet alone were upside down. “It's only a few dots that convey the information—the dots that connect to the feet,” says study co-author Nikolaus F. Troje, a specialist in biological cybernetics.
To Troje, the result suggests that the visual system may contain a “life detector” attuned to the pattern of feet moving against gravity. “I think it's a very old system that we probably share with lots of other animals,” he says. In another recent report, newly hatched chicks responded to right-side-up or scrambled dot clusters with correct foot motions but not to inverted figures. Troje speculates that a life detector could explain why cats stalking prey place their feet so deliberately, adding that a foiled detector may underlie phobias of creatures that move without clear footfalls, such as snakes, insects and birds.