Vision system puts eyesight in blind spots.

Vision system puts eyesight in blind spots

The brain abhors a vision vaccum. Consider a phenomenon long noted by scientists: Each eye possesses a blind spot in the retina corresponding to the head of the optic nerve, yet no black spots mar our view of the world.

The way in which the brain compensates for these natural holes in our field of vision remains largely unexplored. But a new study suggests the brain's visual system may create a physiological representation of visual information surrounding blind spots -- one that automatically paints a coherent scene by filling in the optical void.

A theory that the brain simply ignores visual blind spots -- and operates as if they do not exist -- cannot explain the responses of healthy volunteers to artificially induced blind spots, assert the psychologists who conducted the study. Vilayanur S. Ramachandran of the University of California, San Diego, and Richard L. Gregory of the University of Bristol in England describe their investigation in the April 25 NATURE.

The pair designed a series of experiments employing computer-generated designs with delected sections serviving as artificial blind spots. The tests indicate that visual "filling in" of a missing patch occurs separately for color and texture: that after it disappears, the blind spot continues to influence motion perception unconsciously; and that patterns beyond the immediate boundary of blind spots participate in visual makeovers.

In one test, the researchers directed four volunteers to watch a computer screen with a background of twinkling dots in different shades of gray. Each viewer focused just to the right or left of a small, blank gray square near a corner of the screen. On 10 consecutive trials, all reported that within an average of 5 seconds the square vanished and the twinkling dots took its place.

In another experiment, the computer displayed a pink background with twinkling black dots, and a small gray square with black dots moving horizontally across it as if on a conveyor belt. When the same volunteers focused just to the right or left of the square, they reported this gray patch also faded in a matter of seconds, but in two stages. First, the gray region in the square vanished and the surrounding pink seeped in; then the moving dots disappeared, and a few seconds later the twinkling dots took their place. Thus, separate "fill mechanisms" for color and texture may exist, perhaps in different parts of the visual cortex, the researchers propose.

Brain cells that extract information on the boarders of the square appear critical to the fading and filling-in process, they add. In some of the prior trials, once filling in occurred the gray square with black dots was wiped off the screen and replaced by a smaller replica within its former, now invisible borders. Participants saw the new square pop into view, fade and fill in within a few seconds.

However, information from a pavedover blind spot still exerts an effect on perception, the scientists note. Using the same computer-generated arrangement, they waited until volunteers reported that the gray square had faded completely, then switched off the square and replaced it with an identical square shifted slightly to the left or right. Participants reported that the second square seemed to move or arrive at its location, rather than just appearing out of nowhere. The first square apparently stimulated an unconscious illusion of motion, Ramachandran and Gregory maintain.

Blind spots can reach beyond their borders for visual filler, the researchers report. Another computer pattern showed a circular gray disk straddling the vertical border between a pink and a yellow region. Volunteers looked just to the right or left of the disk. When a thick red ring surrounded the disk, the color red gradually filled in the entire blind spot; when concentric thin red rings surrounded the thick red ring, the bling spot appeared filled with red rings.

Neural pathways feeding into and out of the visual cortex may create the representation of visual information that fills in the blind spot, the scientists suggest.