Mystery of "Blindsight" Lets Some Blind People "See," Study Shows

John Roach
for National Geographic News
November 1, 2005
An innovative research technique is providing insight into why some blind people are able to sense and describe objects they cannot see.

The phenomenon of "blindsight" occurs in some people who suffer injuries to the primary visual cortex, the region of the brain considered essential for sight.

Blindsight allows people to use visual information they get through their eyes even though they have no consciousness of the visual experience, said Christopher Mole, a postdoctoral fellow in philosophy at Washington University in St. Louis, Missouri.

"But that of course is quite hard to show in the lab," he said.

A team of psychologists at Rice University in Houston, Texas, may have found a way to directly study blindsight in the lab.

They are using electromagnetic stimulation on the brains of people who can see to render them partially and temporarily blind.

"The way it works is an electric current inducts into the brain via a magnetic pulse, and that causes a disruption of underlying neurons in the brain," said Tony Ro, a member of the Rice team.

"What this technique allows us to do essentially is in a safe and noninvasive way shut down a portion of the brain temporarily," he added.

Ro and colleagues report their technique and findings in the current issue of the journal Proceedings of the National Academy of Sciences.

Mole said the Rice team reports "compelling proof" for blindsight.

Unconscious Pathway

Blindsight is most prevalent among people who suffer damage to the primary visual cortex, such as in some stroke victims, Mole explained.

However this is never "clean" or specific damage—other parts of the brain are also impaired. Studies with these patients are therefore difficult, he said.

To study blindsight directly, researchers often purposefully and permanently disrupt the primary visual cortex in monkeys and other mammals, a method that would be unethical to use on humans, Mole said.

"What [Ro's team] has done is cleverly manage to interfere with the brain in a totally temporary way … It doesn't have any long-term lasting effects at all," he said.

The technique devised by the Rice researchers induced blindness for a fraction of a second in people who ordinarily have good vision.

During the state of temporary blindness, an object was flashed on a screen in front of the test subjects' eyes.

In one experiment the object was either a vertical or horizontal bar, and the subjects were asked to guess the bar's orientation. In the second experiment the researchers flashed a colored disc, and subjects were asked to guess the color.

In both experiments the blinded volunteers correctly guessed the characteristics of the objects at much higher levels than chance alone.

This fits the definition of blindsight and raises the question of how it is possible.

"What we believe is happening is people are able to discriminate orientation and color—as our experiments showed—by processing routes into the brain that aren't consciously accessible," Ro said.

"We believe there are pathways that go from the eyes into the brain that bypass the normal routes tied to conscious processing of information."

Ro added that the study supports the theory that these pathways go to a visual center in the brain that is more sophisticated than the visual centers common to all mammals. This suggests the pathways may be unique to higher-order species.

The test results also show that volunteers were more accurate when they were more confident in their guesses.

"It's unclear what that reflects, but what we think it reflects is that this unconscious processing system can contribute to feelings of certainty," Ro said.

In follow-up experiments the team will test why people feel varying levels of confidence in their guesses. Perhaps the unconscious processing routes are stronger in some people than others, Ro said.

Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).


© 1996-2008 National Geographic Society. All rights reserved.