Temporary rapidly induced blindness has provided evidence that an older, primitive part of the brain plays a role in processing visual information unconsciously.
This finding by researchers at Houston's Rice University was published in the Proceedings of the National Academy of Sciences online this week (www.pnas.org).
For the study, six volunteers with normal vision underwent more than 600 trials in which they had to look at a target placed at varying locations on a computer screen. For half of the trials, the participants were asked to move their eyes to the location of the target, and their eye movements were measured electronically.
For the other half, the participants were asked to press a button that corresponded with the location of the target on-screen. During the trials, the researchers sometimes tried to distract them with an item shown on the center of the screen. Response time was recorded for each trial.
Prior to the tests, the researchers mapped each participant's visual cortex – the area at the back of the brain that processes what the eye sees – with transcranial magnetic stimulation (TMS), a harmless noninvasive technique using brief magnetic pulses. When applied to the visual cortex, TMS induces temporary, reversible blindness lasting only a fraction of a second.
TMS was administered on 75 percent of the trials at optimal times to produce visual suppression. Participants were asked to report verbally after each trial whether they had perceived the distracting item. On most of the trials when TMS was administered, the participants reported not seeing the distractor when it was presented.
"We found that when we caused the person to go momentarily blind, they were still influenced by the distracting item they reported they had not seen, but only when they made eye movements, " said Tony Ro, associate professor of psychology, who directed the study.
Ro noted that in non-mammals, a portion of the midbrain known as the superior colliculus handles basic sensory information processing, including eye movements related to vision. As humans evolved, the visual cortex at the back of the brain took over the more complex processing of visual information.
The results of the eye-movement portion of Ro's study indicate that part of this older visual system still functions by encoding visual information.
"When we knocked out the conscious part of the brain with TMS, the brain still coded visual information unconsciously for eye movements, but not for more complex tasks, such as the button presses," Ro said.