The Atheist's Guide to Reality_ Enjoying Life Without Illusions - Alex Rosenberg [70]
Fortunately, when it comes to the persistent questions about our minds, we are not going to have to wait the 200 years or so required to formulate this theory. To rule out introspection’s answers, all we need is what cognitive neuroscience can already tell us about neural circuits in the brain, along with the startling results of some pretty interesting experiments. Once we clear the deck of illusions, we can read the right answers to our most urgent questions off from physics and from the neurobiology it ordains.
BLINDSIGHT—THE TRICK
WE CAN ALL PULL OFF
Here is the sort of absolutely obvious thing consciousness tells us has to be true: To tell what color a thing is, you need to look at it, you need to be at least momentarily conscious of its color. If it’s yellow, you have to have to the sensation of yellowness, right? Wrong. And if consciousness can be wrong about that, it can be wrong about anything.
The most startling evidence of how unreliable consciousness is comes from the phenomenon of “blindsight,” seeing things when you don’t have a conscious visual experience of them. This is so weird it’s worth repeating: you can see stuff without consciously experiencing it, without having the subjective, private, in-your-mind sensation of its shape, color, position, size, texture, and so on. But if you can see stuff without having these experiences, then an idea that has always struck us as dead certain must be completely wrong. This is the idea that to see things you have be conscious of them, you have to have a visual experience of them.
How do we know that there is any such thing as blindsight? Signals from the eye hit the brain at the primary visual cortex. A great deal is known about how this part of the brain processes signals from the retina. It has been clinically established that damage to it causes blindness. But more than a half century ago, experiments on monkeys whose primary visual cortex had been destroyed (probably no longer a permissible experiment) revealed that the monkeys could nevertheless detect shapes and movement. When presented with visual stimuli they should not have been able to “see,” they behaved in ways that showed they could respond to the stimuli as if they had seen them.
Though mysterious, no one thought that these experiments had much relevance for human vision. Then a patient (named DB in the subsequent reports) had his right hemisphere’s primary visual cortex removed at a London hospital to relieve symptoms caused by a brain tumor. After surgery, DB was subjected to the same experiments the monkeys had been subjected to, with the same startling results. Because DB could talk as well as act, he was able to confirm that, being blind, he had no visual experience of colors. But when asked to reach for something yellow among a set of colored objects, he succeeded in doing so. When asked whether the yellow object was on the left or right, he pointed in the correct direction, though he reported that he could not see anything yellow anywhere in front of him. When asked to point to a square object and ignore a round one, he was able to do that, too, though he had no conscious experience of anything round or square in his visual field. When asked whether a grillwork grating in front of him had horizontal or vertical lines, he gave the right answer, though again, he had no visual experience of (he couldn’t “see”) the grating itself. When told of the experimental results, DB didn’t believe them. He insisted he had just been guessing—that’s what the introspective feeling told him he had been doing. DB was the first