UCSD Researcher ‘Fills In’ Blind Spots : Science: The brain makes educated guesses to compensate for natural gaps in our vision, professor reports.
Few of us realize it, V.S. Ramachandran says, but the fact remains: there are gaping holes in the way human beings view the world.
At the outer perimeter of our field of vision, the UC San Diego psychology professor explains, there are natural blind spots caused by the way the optic nerve attaches to the eyeball. For years, scientists have puzzled over why we don’t notice these “holes in the world.”
In today’s edition of the British science journal Nature, Ramachandran and a colleague offer the first research designed to explore one common theory. The reason we don’t know what we’re missing, the article concludes, is that the human brain “fills in” the gaps, making educated guesses to complete the visual picture.
“The brain actually takes the stuff around (the blind spot) and makes a statistical estimate,” Ramachandran said in an interview Wednesday. “You make a guess, and that’s what you see.”
Don’t believe it? On a piece of paper, draw a black spot about half a centimeter in diameter (about the size of a pencil eraser). To the right about 10 centimeters (about half the pencil’s length), draw a tiny dot. Cover your right eye and stare at the tiny dot as you move the paper in front of you, Ramachandran says, and, at a certain distance, the larger spot will disappear--hidden in the blind spot of your left eye.
For years, some scientists have speculated that this phenomenon occurs because the brain simply “ignores” the information that it lacks. But in Thursday’s Nature article, Ramachandran and University of Bristol professor Richard Gregory make a case for the competing theory: that neurological sleight of hand fills in the blanks.
With the help of computer-generated video patterns, Ramachandran, Gregory and a UCSD undergraduate, Will Aiken, designed experiments that simulated blind spots. When volunteers were asked to stare at the artificial blind spots--a small gray square on a screen filled with “snowy” interference, for example--they reported that the spots disappeared within seconds, leaving the perception of a completely snow-filled screen.
The reason, the researchers believe, is that the brain cells that recognize the gray square’s existence get tired after a few seconds of staring. The brain then draws upon the surrounding visual information to guess at what should fill that space.
In another experiment, volunteers were shown a snowy television screen with a red background and a small gray square. After staring at a tiny dark spot on the screen, viewers said the square appeared to turn red and then became filled with snow--suggesting, the researchers concluded, that there are two section of the brain involved in the process: one for color and form, the other for motion.
The research, however, found limits in the brain’s ability to interpolate. Although excellent at filling in simple types of patterns, the brain apparently flunks when it comes to other types of omissions. When a blind spot obscures the corner of a square, for example, the brain is unable to imagine the missing corner and complete the shape.
“It’s not super-clever,” said Ramachandran, who speculates that the brain’s “fill-in” mechanism evolved not just to remedy blind spots but also to save neuroelectrical energy.
He said that, to save on “computing” time, the human brain initially determines the boundaries of objects, then swiftly sketches in the rest.
“If you’re looking at a chair or table, the brain is interested mainly in the outlines--like a cartoonist,” he said, noting that there have been attempts to mimic such programming in robots. “Surface interpolation . . . tells you where it is, what its boundaries are. Then you use a quick sleight of hand to fill in the rest.”
That same mechanism, he believes, enables people with significant visual impairments--such as glaucoma or damage to the visual cortex in the brain--to remain oblivious to their diminished vision.
“If you take patients with glaucoma, they have holes in their world--you can diagnose glaucoma by finding those holes--but the patient doesn’t notice,” Ramachandran said. “It’s always been a puzzle. We’re saying they fill it in.”
