Text and video credit to NPR; if you have the time and interest, it's better to read and view this story at the primary link, where the text is more detailed and the video has voiceover (the embedded YouTube version is mute).
A few years ago, TN had two strokes, one on either side of his brain. The strokes severely damaged the part of the brain primarily responsible for vision, known as the occipital cortex.This is not a case of psychologic blindness where someone believes they are blind; it is true cortical blindness. It's startling to realize that there are parts of the brain capable of processing visual information that are not in the occipital cortex. This doesn't help people with retinal blindness, but it does have implications for those like TN who are cortically blind. The question is whether his cerebral cortex (or wherever his "consciousness" is processed) can be made aware that his brainstem (or wherever) is receiving retinal impulses.
Extensive testing of TN confirmed that even though his eyes were just fine, he was completely blind. He couldn't see objects held in front of him and used a cane to get around. Ask him if he could see, and TN would reply, "No, I'm blind."
But neuroscientist Beatrice de Gelder wanted to study TN further… they laid out an obstacle course in a hallway. The obstacles consisted of everyday objects… he was not aware that there were obstacles."
So TN walked down the hall, but instead of walking straight ahead, he carefully stepped around each of the obstacles...
TN has what is known as blind sight, according to de Gelder. Even though the primary part of his brain that processes visual information is destroyed, he still has more primitive parts of his brain intact, and these are capable of doing some visual processing. After all, one of the most basic functions of the visual system is to help an animal avoid obstacles or predators. TN still has some visual abilities — he's just not aware he has them…
It also raises questions about what other capabilities the brain has, such as those displayed by autistic savants (if you did not see the video in my post on Daniel Tammett last September, it is definitely worth a view).