Free Idiot Brain by Dean Burnett Page B

attached to it, and the brain recognizes this.
    In the periphery of the retina, the receptive fields can be quite big, like a golf umbrella canvas around the central shaft. But this means precision suffers—it’s difficult to work out where a raindrop is falling on a golf umbrella; you just know it’s there. Luckily, towards the center of the retina, the receptive fields are small and dense enough to provide sharpand precise images, enough for us to be able to see very fine details like small print.
    Bizarrely, only one part of the retina is able to recognize this fine detail. It is named the fovea, in the dead center of the retina, and it makes up less than 1 percent of the total retina. If the retina were a widescreen TV, the fovea would be a thumbprint in the middle. The rest of the eye gives us more blurry outlines, vague shapes and colors.
    You may think this makes no sense, because surely people see the world crisp and clear, give or take the odd cataract? This described arrangement would be more like looking through the wrong end of a telescope made of Vaseline. But, worryingly, that is what we “see,” in the purest sense. It’s just that the brain does a sterling job of cleaning this image up before we consciously perceive it. The most convincing Photoshopped image is little more than a crude sketch in yellow crayon compared to the polishing the brain does with our visual information. But how does it do this?
    The eyes move around a lot, and much of this is due to the fovea being pointed at various things in our environment that we need to look at. In the old days, experiments tracking eyeball movements used specialized metal contact lenses. Just let that sink in, and appreciate how committed some people are to science. §
    Essentially, whatever we’re looking at, the fovea scans as much of it as possible, as quickly as possible. Think of a spotlight aimed at a football field operated by someone in themiddle of a near-lethal caffeine overdose, and you’re sort of there. The visual information obtained via this process, coupled with the less-detailed but still-usable image of the rest of the retina, is enough for the brain to do some serious polishing and make a few “educated guesses” about what things look like, and we see what we see.
    This seems a very inefficient system, relying on such a small area of retina to do so much. But considering how much of the brain is required to process this much visual information, even doubling the size of the fovea so it’s more than 1 percent of the retina would require an increase in brain matter for visual processing to the point where our brains could end up the size of basketballs.
    But what of this processing? How does the brain render such detailed perception from such crude information? Well, photoreceptors convert light information to neuronal signals which are sent to the brain along the optic nerves (one from each eye). ¶ The optic nerve relays visual information to several parts of the brain. Initially, the visual information is sent to the thalamus, the old central station of the brain, and from there it’s spread far and wide. Some of it ends up in the brainstem, either in a spot called the pretectum, which dilates or contracts pupils in response to light intensity, or in the superior colliculus, which controls movement of the eyes in short jumps called saccades.
    If you concentrate on how your eyes move when you look from right to left or vice versa, you will notice that they don’t move in one smooth sweep but a series of short jerks (do it slowly to appreciate this properly). These movements are saccades, and they allow the brain to perceive a continuous image by piecing together a rapid series of “still” images, which is what appears on the retina between each jerk. Technically, we don’t actually “see” much of what’s happening between each jerk, but it’s so quick we