Question: how come drawn illusions can trick your eyes? :)

Ciarán O'Brien answered on 17 Nov 2014:

It’s a combination of our eyes being rubbish at evolving, and our brains being too eager to fill in the blanks!

We see things because light bounces off objects and enters our eyes, and gets focused by the pupil to land on the retinas at the back. The retina is a carpet of nerves that are sensitive to different colours of light, and when light of the right colour hits the right nerve, it sends a signal to the brain. There are millions of these nerves sending signals all the time, and your brain sorts them out to create a picture of what you see in your mind.

There are problems though. Our eyes didn’t evolve very efficiently for a start. The light sensitive nerves in our retina lead back to the brain through tiny nerve fibres, and in humans the retina is kind of back to front, with the nerve fibres facing outwards! All the fibres meet at one point where they lead back into the rest of your body as the optic nerve, and in that area there’s no room for light sensitive nerves. This is called your scotoma, or blind spot.

In octopuses, the retinas evolved the other way round, and they don’t have a blind spot because the nerve fibres are on the right side and don’t have to go through the nerve endings to reach the rest of their bodies.

This diagram from Wikipedia shows the differences between vertebrate eyes (like humans) and the eyes in an octopus:
http://en.wikipedia.org/wiki/Blind_spot_(vision)#mediaviewer/File:Evolution_eye.svg

So there’s a fairly large portion of our eye which doesn’t see anything at all. But you don’t notice any gaps in your vision unless you try a blind spot test like this:
http://visionaryeyecare.wordpress.com/2008/08/04/eye-test-find-your-blind-spot-in-each-eye/

And the reason you don’t notice any gap is your brain. It is ABSOLUTELY TERRIBLE for using any old nonsense to fill in the blanks. Your brain looks at the whole picture your eyes send back, blind spots and all, and then fills in the blind spot with what it thinks is there, based on what’s around it.

Your brain lies to you all the time like this. The dodgy construction of your eyes and your brain’s tendency to bluff its way through by straight up making stuff up to fill in the blanks is the reason people see all sorts of things that aren’t there from UFOs to angels. Don’t trust everything your brain tells you. It’s good enough to get you through the day without getting killed, but if you trust it you’ll end up swearing blind that leprechauns are real to the psychiatrist listening carfully and taking notes 🙂