Our brains are virtuosos at linking the disjointed parts of an object that is heavily obscured, such as a figure seen through dense woods. A report in this week's Science reveals one of the brain's tricks for doing this: a great ability to notice things happening at the same time. The study is "perhaps the most interesting new work in [its field] to come out in the past 10 years," says neuroscientist William Newsome of Stanford University School of Medicine.
Neurobiologists had already shown that cues such as color, continuity, and texture help the brain assemble objects from their parts, a phenomenon called "binding." Like a well-drilled marching band that spells out the school's letters by having some members march in a different direction than the rest, a common direction of motion can also make shapes jump out of a pattern. Randolph Blake and graduate student Sang-Hun Lee of Vanderbilt University in Nashville, Tennessee, wanted to test whether the brain could see a shape based simply on the timing of visual changes.
They used a computer image, made up of patches, that resembled a chaotic marching band whose members move about the field in a random way. In a rectangular region at the center of the field, however, all of the patches repeatedly changed their direction of movement simultaneously, at irregular intervals. (For a demonstration, see the researchers' Web page.) People viewing the test, the researchers found, could discern the rectangle well enough to say whether it was oriented vertically or horizontally.
It is reasonable that timing would be a binding cue, says New York University neuroscientist Anthony Movshon, because when a real object moves, all its parts generally begin to move at the same time. What's more, in some natural situations, timing alone, rather than a shared direction of motion, might be the main cue. "Imagine a disturbance in a forest, created by a predator moving around in a tree," Movshon says. A synchronous change in the movement of leaves on that branch may be the only clue to the predator's presence--making the ability to spot it vital.