The research, published the week of Oct. 31 in the Proceedings of the National Academy of Sciences' (PNAS) online Early Edition, suggests the brain has more than one pathway along which visual information can be sent.
For the study, the researchers induced temporary, reversible blindness lasting only a fraction of a second in nine volunteers with normal vision. Transcranial magnetic stimulation (TMS), a harmless noninvasive technique using brief magnetic pulses, was applied to the volunteers' visual cortex -- the area at the back of the brain that processes what the eye sees - to interrupt the normal visual pathway. The volunteers looked at a computer screen, and during their momentary blindness, either a horizontal or a vertical line or a red or a green dot flashed on the screen.
Researchers then asked the study participants whether they had seen a horizontal or a vertical line; because their primary visual pathway had been shut down, the participants reported that they saw nothing. However, when forced to guess which line had appeared on their computer screen, the participants gave the correct answer 75 percent of the time. When the participants had to guess whether a red or a green dot had flashed on the screen, they gave the correct answer with 81 percent accuracy.
"This high degree of accuracy for both the directional orientation and color tasks was significantly above chance," said Tony Ro, associate professor of psychology and principal investigator for the study. "Even though the human primary visual cortex activity was temporarily shut down, it's clear that detailed visual information was still being processed unconsciously."