Mental Blur in the Blink of an Eye

When your eyes dart from the keyboard to the computer screen, then out the window, your brain must recompose the picture for each shift. In two papers in today's issue of Nature, separate groups report that in the fraction of a second before the eyes flick, your brain distorts the positions of objects. A more complete understanding of this distortion could someday help engineers design instrument panels for cars and airplanes that are easier on the eye.

Vision researchers have long known that during the tenth of a second before an eye movement, the brain's perception of the position of a single point of light starts to shift in the direction the eyes are about to move. John Ross of the University of Western Australia in Nedlands and David Burr of the Università di Roma in Italy put eight different people in front of a computer screen surrounded by white cardboard to block out external stimuli. Each subject stared at a dot on the screen and was told to shift focus to another dot that appeared 20 degrees to the right. The researchers found that if they briefly flashed a green bar between the two dots during the tenth of a second before the eyes moved, the subject would perceive the flashed bar to the right of its actual location. The closer they put the flash to the new point of focus, the more distorted the perceived location. "There's a lot of evidence now that very little is kept accurately" during the transition, Burr says.

The other research group, led by John Schlag of the University of California, Los Angeles, also saw a shift and a compression of the visual field, but with a slight difference: In Schlag's experiments, objects appeared to move only in the direction of the glance, whereas Burr saw shifting in both directions, but always toward the target. Schlag suggests that a difference in the experimental setup--his subjects saw their stimuli against a completely dark background--could explain the variation in results. He thinks the lighted background in Burr's experiment could give the subject more frames of reference, which in turn changes the way the brain responds to the shift in focus.

Christopher Bockisch, a neurologist at the Smith-Kettlewell Eye Institute in San Francisco, says these experiments are a significant advance in understanding how the eye works, and they may suggest better designs for airplane consoles and other environments where people must handle many kinds of visual input simultaneously. He suggests, for instance, that there may be some minimum brightness for a landing light outside a dark cockpit window in order for a pilot to see it in its accurate position.