Before I start the post, a little something for your viewing pleasure:
Why am I showing you a video of a man walking around obstacles? If you read the title of the video, you already know (darn Blogger and YouTube giving away the punchline!). But I'll tell you anyway.
This man is blind.
That's right, blind. As in, unable to see. But look at him avoid the obstacles! If he's blind, how's he doing it?
This blind man is able to avoid the obstacles thanks to a phenomenon known as blindsight. Blindsight exists because not all blind people are actually blind. Some of them can see. They just don't know they can see.
To understand blindsight, you need to know how visual information is processed in the brain. Here's a picture of the brain's visual pathways:
|This picture shows how visual information moves from the eyes into the brain.|
Visual information leaves the eye via the optic nerve. The optic nerve splits, and sends information to the lateral geniculate nucleus (LGN) and superior colliculus (SC). Both areas get the same information from the eyes. In other words, both areas "see" the same thing.
The pathway from the LGN to the visual cortex is the conscious pathway. When you see things, and know that you see them, that's visual information moving through the LGN to the visual cortex.
When visual information moves through the SC however, you aren't conscious of it. It doesn't go to the visual cortex, and you don't know that you see it. But it's still very important. Most animals have an SC. Not all animals have a visual cortex. Amphibians have an SC. They don't have a visual cortex. Humans have both.
Imagine you're a frog. A black dot buzzes across your vision. Do you sit there and ponder it? What is the meaning of the black dot? Is it food? If it's food, is it tasty food? Are you hungry enough to eat the black dot raw? Or do you feel more like a dragonfly stir-fry instead? Or are you hallucinating, and the black dot is only a figment of the imagination. And if the black dot is only a figment of your imagination, what does that mean about you? What's the meaning of life, anyway?
Or, instead of pondering for an age and a half, do you just eat the black dot?
You, Mr. or Miss. Frog, will stick out your tongue and snatch that black dot, because it's a fly and flies fly and you want to eat it before it flies away.
But you don't think about it.
My point, with the whole black dot story, is to give you an example of a time when an animal needs to react to visual information very quickly, without time for thought. Eating something speedy, for instance. For humans, who don't often catch tasty flies on their tongues, avoiding danger is a time when we would want to react fast without thought. Or, walking around an obstacle that might make us trip and slam painfully to the ground. If you add thought into the mix, your reactions will slow down. Humans can think fast, but not that fast, especially when a baseball is heading for our foreheads at 100 mph.
It's important to have an area of the brain that can respond to danger with lightning speed, faster even than thought. That's the SC. When you dive out of the way of a speeding baseball, it's the SC. If you've ever thought, "Wow! I can't believe I jumped out of the way so fast!", you can thank your SC. It doesn't need you to think and tell it what to do. It sees danger, and it acts.
Back to blindsight...Imagine a scenario where an injury or a stroke wipes out your visual cortex, which is part of the LGN pathway. What does this mean?
Well, your eyes are still intact. You can still see. But your conscious visual pathway is broken. You don't know that you can see.
But - and here's the key - your SC still works. Which means that unconscious danger-avoidance pathway still works. So if a doctor starts to lead you down a obstacle-packed hall, just like in the video at the top of the post, you'll be okay. Your SC will keep you from running into things. If someone then asks why you made that sudden right turn, your answer would be something along the lines of, "I don't know. It just felt right." Because you don't know. You're acting without conscious thought. It's only later that you realized you moved.
It doesn't work for all blind people, only those with damage to the conscious visual pathways in the brain. You need working eyes to have blindsight.
If there's a take-home message to this post, it's this: Not all blind people are completely blind. And blindness doesn't always happen in the eyes. The eyes can work perfectly, and the damage is in the brain.
Another take-home message would be: The brain is awesome. But you already knew that.