WASHINGTON – Even when the brain can't consciously tell what the eyes are seeing, it may still be able to sense what's there. The finding suggests that the brain may have "blindsight" — an alternate way of processing visual information.
That might help scientists better understand the nature of consciousness and suggest ways to restore some types of vision loss, according to Tony Ro, a psychology professor at Rice University in Houston.
Not all vision researchers accept the idea of blindsight, however, and other studies into the concept are under way.
Ro and his research team studied what could be sensed by volunteers who were temporarily blind. Their findings are reported Monday in the online edition of Proceedings of the National Academy of Sciences.
In the study, temporary, reversible blindness was induced in volunteers by using magnetic pulses that affected the visual cortex, the area in the back of the brain that processes what the eyes are seeing.
A computer screen was in front of the volunteers. In one test, during their momentary blindness the screen flashed with either a vertical or horizontal line. In a second test a red or green ball was shown on the screen.
When the volunteers were asked what they had seen during the temporary blindness, they said they saw nothing, the researchers reported.
But, the researchers said, when the patients were told to guess which way the line was oriented, they were right 75 percent of the time. And they got the color of the ball right 81 percent of the time. Random guessing would be expected to result in a 50 percent correct rate.
Some of the participants said they were guessing randomly and were surprised with their high success rates, the researchers said. Others reported they had a "feeling" about what had been there.
Asked to rate the confidence of their guesses, the higher confidence ratings tended to correspond with more accurate guesses.
"These findings demonstrate that while certain brain areas are necessary for awareness, there is extensive processing of information that takes place unconsciously," Ro said via e-mail.
He said the results, "suggesting the existence of alternate visual processing routes that function unconsciously, may provide some hope for people with damage to the primary visual cortex."
Dr. Edmond Fitzgibbon of the National Eye Institute said the report adds weight to the idea of a second pathway for experiencing sight, but he added that the idea remains controversial and many researchers don't accept it.
The new study is "intriguing," Fitzgibbon said. But he added that there can be perception that people aren't aware of, such as subliminal suggestions briefly flashed during a film.
"If you're not aware of it, does that really mean you didn't see it?" he asked.