Blindsight is a medical phenomenon in which patients who are clinically blind actually show response to and predict aspects of visual stimuli at rates well above chance. To a blindsight patient's conscious perception, he cannot see. However, if forced to guess the characteristics of a visual stimulus, or to, say, navigate an obstacle course, the patient does so with a high degree of accuracy, even without any conscious knowledge that there are objects in the way! If you don't believe it, watch this video, annotated by Scientific American, of the famous patient TN before reading on: http://bcove.me/eq5aqeoh.
Blindsight isn't just some esoteric anomaly - delving into its neurological basis reveals several profound facts about the nature of human experience. As we will see in more detail, our "awareness" is not the all-encompassing, front row seat to reality that we often assume it to be, and "vision" is really a reconstructed experience synthesized by our brain.
The first step in unearthing the mystery of blindsight is to understand that the visual system consists of two separate components: the optics (all of the physical machinery in the eye designed to capture and absorb light) and the brain (all of the neural machinery designed to process, organize and bring to conscious awareness an image of the world). Once light passes through the machinery of the eye and makes its way to the retina to be absorbed and translated into neural language, it is sent along the optic nerve up into the brain. From the optic nerve, the information about the light can go to several different places. Most of the information eventually makes it to V1 visual cortex, a region located at the back of the brain, where it receives further processing and, most importantly, somehow finds its way into our conscious awareness (neuroscientists do not know how this is accomplished, but do know that V1 is the neural correlate of visual awareness). In most blindsight patients, it is this V1 area that has suffered isolated damage. However, with an optic system still intact and with other brain regions that process visual information still functional, a blindsight patient is still able to capture light, absorb it into his neural system and process it to a limited degree. The key is that, while still able to do this information processing, with the loss of V1 the patient has lost the ability to be consciously aware that his brain is actually still "seeing". Thus, patient TN's brain still receives visual information about the obstacles in front of him and is able to respond to them even though TN "himself" is not consciously aware of the obstacles.
Blindsight shows in dramatic fashion that there is a dissociation between conscious and subconscious awareness. It is strange to think that, at the same time that your brain is aware of something, "You" can be left out of the loop! In another experiment with a blindsight patient, a woman was shown two side-by-side pictures of an identical house, except that one was drawn to be on fire. When asked which picture she preferred, at first the woman insisted that she had no way to tell. But when forced to "guess", she consistently chose the picture of the house that was not on fire, and when asked to explain her choice, she had no idea why she had made the choice!
Secondly, blindsight shows that our conscious experience of vision does not come from our eyes, but from a lump of neurons situated all the way at the back of our skull! Of course, what we "see" is ultimately rooted in what our eyes capture from the physical world, but it is the brain that conjures up the experience of, for instance, what "green" looks like. And really, dreams show us that we don't need our eyes to "see" at all! The brain can just play a movie that is screened into our conscious perception, and in a sense this is what it does all of the time, adding subjective coatings to our visual experience like "greenness".
Modern advances in neuroscience continue to show us that the human experience is a lot more than "meets the eye".
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