Idiot Brain by Dean Burnett Page A

object in bright light will look, on the retina, very different from a red object in dark light, but the secondary visual cortex can seemingly take the amount of light into account, and work out what color the object is “meant” to be. This is great, but it’s not 100 percent reliable. If you’ve ever argued with someone over what color something is (such as whether a car is dark blue or black) you’ve experienced first hand what happens when the secondary visual cortex gets confused.
    It goes on like this, the visual-processing areas spreading out further into the brain, and the further they spread from the primary visual cortex the more specific they get regarding what it is they process. It even crosses over into other lobes, such as the parietal lobe containing areas that process spatial awareness, to the inferior temporal lobe processing recognition of specific objects and (going back to the start) faces. We have parts of the brain that are dedicated to recognizingfaces, so we see them everywhere. Even if they’re not there, because it’s just a piece of toast.
    These are just some of the impressive facets of the visual system. But perhaps the one that is most fundamental is the fact that we can see in three dimensions, or “3D” as the kids are calling it. It’s a big ask, because the brain has to create a rich 3D impression of the environment from a patchy 2D image. The retina itself is technically a “flat” surface, so it can’t support 3D images any more than a blackboard can. Luckily, the brain has a few tricks to get around this.
    Firstly, having two eyes helps. They may be close together on the face, but they’re far enough apart to supply subtly different images to the brain, and the brain uses this difference to work out depth and distance in the final image we end up perceiving.
    It doesn’t just rely on the parallax resulting from ocular disparity (that’s the technical way of saying what I just said) though, as this requires two eyes to be working in unison, but when you close or cover one eye, the world doesn’t instantly convert to a flat image. This is because the brain can also use aspects of the image delivered by the retina to work out depth and distance. Things like occlusion (objects covering other objects), texture (fine details in a surface if it’s close but not if it’s far away) and convergence (things up close tend to be much further apart than things in the distance; imagine a long road receding to a single point) and more. While having two eyes is the most beneficial and effective way to work out depth, the brain can get by fine with just one, and can even keep performing tasks that involve fine manipulation. I once knew a successful dentist who could see out of only one eye; if you can’t manage depth perception, you don’t last long in that job.
    These visual-system methods of recognizing depth are exploited by 3D films. When you look at a movie screen, you can see the necessary depth because all the required cues discussed above are there. But to a certain extent you are still aware that you’re looking at images on a flat screen, because that is the case. But 3D films are essentially two slightly different streams of images on top of each other. Wearing 3D glasses filters out these images, but one lens filters out a specific image and the other filters out the other. As a result, each eye receives a subtly different image. The brain recognizes this as depth, and suddenly images on the screen leap out at us and we have to pay double the price for a ticket.
    Such is the complexity and density of the visual-system processing that there are many ways it can be fooled. The Jesus-in-a-piece-of-toast phenomenon occurs because there is a temporal-cortex region of the visual system responsible for recognizing and processing faces, so anything that looks a bit like a face will be perceived as a face. The memory