Stroke, trauma or surgery can cause so-called cortical blindness by knocking out a portion of the visual cortex, the part of the brain that receives input from the eyes. Blindsight is thought to occur because the brain is capable of rerouting the visual information that the eyes are still receiving. Examples of the effect include being able to guess the color, motion and even emotional expression of visual stimuli placed in the blind spot at better than chance rates. Some groups have tried to identify improvements in blindsight after repeated stimulations, but such studies have generally involved introducing stimuli near the edges of the blind spot, which leaves open the possibility that subjects could have skewed the data by shifting their gaze.
Hoping to eliminate this source of uncertainty, vision researcher Arash Sahraie of the University of Aberdeen in Scotland and his colleagues asked 12 cortically blind people to practice their blindsight for stimuli smack in the middle of their blind spots. Daily for three months, participants using home computers repeatedly guessed which of two stimuli was presented to them first--either a flickering circular field of black and white lines, designed to maximally stimulate their cortices, or a gray dot. After the at-home training, the participants improved in a laboratory test of the same ability compared with tests given before the training, according a report appearing online September 25 in Proceedings of the National Academy of Sciences USA. On average trainees added 5 to 10 percent to their fraction of correct guesses and reported being consciously aware of the correct answer about twice as often. When tested for blindsight in another part of their blind spot, away from the center, participants reported no change in awareness and only a modest improvement in their ability to distinguish stimuli. "These are very encouraging results," says Sahraie. A clinical trial of the technique will be needed to determine if the technique improves the blind's quality of life, he points out.
According to neuropsychologist James Danckert of the University of Waterloo in Ontario, who was not involved in the study, "this represents a remarkable demonstration of plasticity in the adult brain. The changes occur a long time after neural damage was sustained." To get the biggest improvement, though, trainees might have to work on their whole blind spots, he notes.