RENEE MONTAGNE, host:
What it means to see is a scientific and philosophical question raised by the following story. It involves a man who is totally blind, and yet he can use his eyes to navigate the world. NPR's Joe Palca has this report on the mysteries of blind sight.
JOE PALCA: The story revolves around a physician in his fifties, known by the initials TN, to protect his privacy. A few years ago TN had two strokes, one on each side of his brain. Both strokes were in the part of the brain called the occipital cortex. That's the primary place the brain processes vision.
TN's eyes work just fine, but without the occipital cortex, he had no conscious ability to see. Beatrice de Gelder heard about TN and decided to explore the nature of his blindness. De Gelder is a Dutch neuroscientist. She says there's no question TN considers himself blind.
Dr. BEATRICE DE GELDER (Neuroscientist, The Netherlands): If you would ask him: Am I holding something in front of your eyes, yes or no? He will say I don't see it, I cannot tell whether it's yes or no.
De Gelder believed that TN might still have some visual function, because even though his brain's primary visual processing area was gone, he still had other visual centers intact. But no matter what tests they tried, or what questions they asked, they couldn't find any evidence that TN could see anything. So, they tried something different.
Dr. DE GELDER: We said, rather than asking questions about what he sees, or does not see. Let's take the big risk of building a little maze in the corridor here and see whether he can navigate that.
PALCA: So, they made a kind of obstacle course and then told TN to walk down the corridor.
Dr. DE GELDER: We didn't give any information about obstacles or anything. So, he was not aware that there were obstacles.
PALCA: The obstacles were things like a wastebasket and a stack of papers. There was also a tripod.
Dr. DE GELDER: They all had different shapes and sizes.
PALCA: So, if he had walked straight ahead, he would have bumped into them.
Dr. DE GELDER: Oh, yes, absolutely.
PALCA: But he didn't walk straight?
Dr. DE GELDER: He never touched any. We were like, totally amazed.
PALCA: Because this was after days of testing, that confirmed that he had no sight at all. TN couldn't figure out why the researchers were amazed by what he did.
Dr. DE GELDER: Since he hadn't seen any obstacle, he was not aware of having achieved anything sensational.
PALCA: Did he give any explanation for why he didn't walk in a straight line?
Dr. DE GELDER: No. He was not aware of not walking in a straight line.
(Soundbite of laughter)
PALCA: Wow. That's amazing. You can see a video of this at npr.org. De Gelder thinks what is happening is the signals from the eyes are being processed by more primitive centers in the brain. After all, having a cortex to process vision is rather a new invention in evolutionary terms. Many animals do just fine hunting prey and avoiding predators, with little or no cortex to rely on. De Gelder says many of these old centers still exist in the human brain.
Dr. DE GELDER: They are still present there, and they can be called upon to be used.
PALCA: It's just they don't provide a conscious experience of seeing things. It may seem odd that our brains could be doing something we're not aware of, but Charles Gross says it really isn't. Gross is a neuroscientist at Princeton University. He says our brains do a variety of things, without our conscious control, things like regulating blood pressure, or adjusting hormone levels. Gross says something similar is going on with this blind sight.
Dr. CHARLES GROSS (Professor of Neuroscience, Princeton University): There is visual information, it comes in, it's processed, it's used to control behavior, but you have no conscious access to it.
PALCA: It's hard for people with normal sight to really understand how this unconscious seeing would work, but it does provide a whole new shade of meaning to the phrase "seeing is believing." Joe Palca. NPR News, Washington.
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