Blindness can be caused by damage to the brain, even when the eyes function perfectly. Stroke victims and military combat veterans sometimes lose their vision because of damage to the visual cortex. But many of these patients retain some visual processing ability, even though they are not consciously aware of seeing anything. This phenomenon was first reported in veterans of World War I, and is called blindsight. It has many applications for research on artificial vision, rehabilitation for cortical injuries, and psychotherapy.
The May 2010 issue of Scientific American describes a patient known as TN who successfully navigated a hallway full of obstacles, without the ability to see them consciously. A film is available online (see References). His remarkable performance indicates that the human brain is much more adaptable in its visual processing skills than we have previously imagined.
Brain plasticity offers hope for treating a wide variety of disorders. A recent book by Dr. Norman Doidge explains how current research is changing our perception of how much the brain can recover from damage.
The blindsight experiment I found most interesting was performed on patient GY, who had cortical blindness in just one eye. Researchers presented him with images of people demonstrating various emotional states. They measured his emotional response by electromyography and monitoring of pupil dilation. When fearful images were presented only to the “blind” eye, his physiological response was stronger and faster than when the images were presented only to the eye with normal sight. This experimental result contradicts the theory of cognitive behavioral therapy, which claims that our emotional reactions to our environment depend primarily on our interpretation of sensory data. Patients with blindsight have no possibility of interpretation, because they have no conscious awareness of what they are “seeing”.
For artificial vision, the ultimate goal is to feed data from a camera to the brain and produce visual sensations comparable to what healthy eyes provide.