Our eyeballs are inside-out!
A neuroscientist at #scicommcamp told us a really interesting fact about the human eye. It's apparently designed in a completely illogical way (at least, it seems so at first glance)! The photoreceptors are in the of the eyeball, with a whole network of nerves on top of them, blocking light from getting to the receptors. So our eyes aren't as sensitive as they could be.
Here is a picture of the way light travels through the eye. Light enters through the iris, is focused in by the lens, and the focused light eventually hits the retina at the back of the eye. But when you zoom in on the retina, you start to see the problem.
When light hits the retina, it must travel through a web of nerve cells before reaching the photoreceptors. The human eye has two different types of photoreceptors: rods detect light, but are color blind. Cones are not as good at capturing light, but can detect colors. Once the light is detected by the rods and cones, it is then passed forward to the nerve cells, eventually making its way to the optic nerve, which leads to the brain where the visual data is processed.
While this might seem like our eyeball is designed inside out, a recent study showed that there's a reason our eye is designed this way. It turns out that some of those cells that stand in front of the retina are actually useful in directing light. Muller cells are more densely concentrated than any of the other cells in front of the retina. They are transparent, and very good at focusing red and green light. But they scatter blue light out to the side. Since the cones are mostly concentrated in the very back of the eye, and the rods are distributed at the edges, that means that more blue light ends up going to the color-blind rods, and more green and red light is sent to the color-sensitive cones.
Why is that particular distribution of light good for us? Since the eye usually receives more blue light than red and green, we have fewer blue photoreceptors than red and green photoreceptors. So scattering blue light off to the sides doesn't really affect the amount of light (or the color of light) that we're seeing. But focusing the red and green light to the color-sensitive cones helps improve our daytime color vision!