The next time you miss the forest for the trees, blame your jittery eyeballs. The small, involuntary movements our eyes make when they focus help the brain discern the finer details of an image, according to new research. Although the findings leave several questions unanswered, they mark an important step toward settling a 50-year-old controversy.
Most animals with sharp central vision, such as humans, monkeys, and cats, make microscopic eye adjustments when they fix their gaze. These jitters wiggle the image on the retina, and scientists know surprisingly little about why this happens. In the 1950s, vision researchers used cumbersome techniques involving rotating mirrors to negate the jitter when volunteers stared at an image. The volunteers began to see a featureless gray field rather than the image at hand, so scientists concluded that jittering kept the image from fading. But it wasn't clear how, or if, the jitters served other functions.
Boston University neuroscientist Michele Rucci and colleagues revisited these questions using a less awkward approach: They used a computer to track the eye's movements. The researchers showed six subjects one of two images on a monitor: a gray background with either thick or thin slanted lines at its center. For each trial, the computer either held the image steady or moved it in tandem with the subject's miniature jitters. The subject then had to tell the researchers which way the lines on the image slanted.
Rucci found that resolution made all the difference. When viewing the thick lines, the subjects could identify which way the lines slanted regardless of whether the image moved or not. But the subjects' ability to determine the lines' orientation dropped 16% when they viewed fine-lined patterns that moved in tandem with their eyes. The result shows the eye's jitters help the brain pick out fine details, the kind involved in locating a single tree in a forest or a berry in a bush, says Rucci. "Vision isn't like a camera, where you take a picture and the brain processes it," he explains. "The actual process of looking ... affects what you see."
The research, published tomorrow in Nature, "makes a very compelling case" for the role of eye movements in discriminating details, says neuroscientist Richard Krauzlis of the Salk Institute for Biological Studies in San Diego, California. Vision scientists are still better at measuring and cataloging eye movements than explaining what they do, he says, so Rucci's study offers a welcome new piece to the puzzle.