I "borrowed" the post below from a friend on another forum. Thought everyone here would appreciate it.
I've just run across an article from January, 2009 issue of Seed magazine regarding a rare sensory phenomenon that I was unaware of.
It seems that man in his 50s suffered two strokes that functionally destroyed the visual cortex of his brain, rendering him completely blind. Well, not completely, as it turns out. Nearly all cases of blindness involve damage of some sort to the eyes or to the optic nerve, or both. In those cases the visual cortex remains functional, but to no avail due to lack of stimulus.
In this case, the man's eyes remain functional but send signals to a dead end, thus what we commonly think of as "seeing"--turning light waves into images in our brains--doesn't happen. What does happen is the surprising bit. The light-gathering and impulse-producing eyes are still feeding parts of the brain other than the visual cortex, and thus the man is still able to "see," but without the actual images that occur normally. From the article (which can be read in full at the link above):
Known as selective bilateral occipital damage, TN’s unusual injury made him the subject of much interest while recovering at a hospital in Geneva. Researchers began examining him and discovered that despite his blindness, he had maintained the ability to detect emotion on a person’s face. He responded appropriately — with emotions such as joy, fear, and anger — to a variety of facial expressions. Observed activity in his amygdala — the part of the brain responsible for processing emotions — confirmed the curious results.
To further test the extent of TN’s abilities, researchers from Tilburg University in the Netherlands devised a simple yet decisive experiment: an obstacle course. They arranged boxes, chairs, and various other objects down a long hallway. The team then asked TN to navigate the course without any sort of assistance. TN was skeptical, as he required the aid of a cane and a guide to get around. But eventually, he decided to participate. Researchers recorded the result in their recent paper: “Astonishingly,” the report reads, “he negotiated [the course] perfectly and never once collided with any obstacle, as witnessed by several colleagues who applauded spontaneously when he completed the course.”
There's a short video on the linked page that shows the man negotiating the hallway obstacle course.