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“It is much more important to know what sort of a patient has a disease than what sort of a disease a patient has.” - Sir William Osler

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    Friday, November 09, 2007
    Blind Spot

    No, this isn't a metaphor for anything. It's a literal blind spot, and we all have one in each eye. I tracked down this website, which nicely demonstrates the phenomenon, for a fellow blogger who's been having some eye problems recently. The learning point here is that if you develop a partial visual deficit, in time your brain will learn to ignore it and "fill in" the missing piece in your field of vision. If you go to the website and perform the simple experiment there, you will see what I mean.

    The retina in the back of your eye is a sheet of nerve cells that pick up energy from light coming in through the front of the eye; the individual cells join together to form the optic nerve, which leads to the brain. The optic nerve is in fact the only part of the brain that can be visualized without surgery; when your doctor looks in your eye, a big part of what he or she is looking for is the optic nerve, which can tell us if the brain is under pressure (think brain tumor). The blind spot is the point in the retina where the optic nerve leaves the eye to travel to the brain, and it's "blind" because there are no nerve cells there - the gap where the nerve leaves the eye socket leaves a deficit. So we should notice two gaps or deficits in our visual field, one to the left and right of center, but we don't. Binocular vision solves most of this problem, but even using one eye you won't notice it unless you have something very specific to use as a landmark. That's why the experiment linked above works.

    I find vision incredibly interesting - for instance, why do we see in black and white at night and in color only when there is more light? The retina contains two kinds of cells, rods and cones. Rods are more sensitive to light than cones are, but cannot sense color. In limited light conditions, therefore, the cones will not fire but the rods will. We can thus see, but only in black and white.




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