Idiot Brain by Dean Burnett

don’t really notice, like the gap between the frames of an animation. (The saccade is one of the quickest movements the human body can make, along with blinking and closing a laptop as your mother walks into your bedroom unexpectedly.)
    We experience the jerky saccades whenever we move our eyes from one object to another, but if we’re visually following something in motion our eye movement is as smooth as a waxed bowling ball. This makes evolutionary sense; if you’re tracking a moving object in nature it’s usually prey or a threat, so you’d need to keep focused on it constantly. But we can do it only when there’s something moving that we can track. Once this object leaves our field of vision, our eyes jerk right back to where they were via saccades, a process termed the Optokinetic reflex. Overall, it means the brain can move our eyes smoothly, it just often doesn’t.
    But why when we move our eyes do we not perceive the world around us as moving? After all, it all looks the same as far as images on the retina are concerned. Luckily, the brain has a quite ingenious system for dealing with this issue. The eye muscles receive regular inputs from the balance and motion systems in our ears, and use these to differentiate between eye motion and motion in or of the world aroundus. It means we can also maintain focus on an object when we’re in motion. It’s a system that can be confused though, as the motion-detection systems can sometimes end up sending signals to the eyes when we’re not moving, resulting in involuntary eye movements called nystagmus. Health professionals look out for these when assessing the health of the visual system, because when your eyes are twitching for no reason, that’s not great. It’s suggestive of something gone awry in the fundamental systems that control your eyes. Nystagmus is to doctors and optometrists what a rattling in the engine is to a mechanic; might be something fairly harmless, or it might not, but either way it’s not meant to be happening.
    This is what your brain does just working out where to point the eyes. We haven’t even started on how the visual information is processed.
    Visual information is mostly relayed to the visual cortex in the occipital lobe, at the back of the brain. Have you ever experienced the phenomenon of hitting your head and “seeing stars”? One explanation for this is that impact causes your brain to rattle around in your skull like a hideous bluebottle trapped in an egg cup, so the back of your brain bounces off your skull. This causes pressure and trauma to the visual processing areas, briefly scrambling them, and as a result we see sudden weird colors and images resembling stars, for want of a better description.
    The visual cortex itself is divided into several different layers, which are themselves often subdivided into further layers.
    The primary visual cortex, the first place the information from the eyes arrives in, is arranged in neat “columns,” like sliced bread. These columns are very sensitive to orientation, meaning they respond only to the sight of lines of acertain direction. In practical terms, this means we recognize edges. The importance of this can’t be overstressed: edges mean boundaries, which means we can recognize individual objects and focus on them, rather than on the uniform surface that makes up much of their form. And it means we can track their movements as different columns fire in response to changes. We can recognize individual objects and their movement, and dodge an oncoming soccer ball, rather than just wonder why the white blob is getting bigger. The discovery of this orientation sensitivity is so integral that when David Hubel and Torsten Wiesel discovered it in 1981, they ended up with a Nobel Prize. 9
    The secondary visual cortex is responsible for recognizing colors, and is extra impressive because it can work out color constancy. A red