Believing is Not Seeing: Visual Science and Reality part 1

I’ve had to break this post down into two posts as it got a bit long.

So before I start on this bit I want everyone reading to take part in a very small experiment:

Put a hand over your left eye so you can’t see anything with it. Now lean back from the computer screen and stare at the cross on the left in the image below.

As you stare at the cross, move closer to the image (you can get pretty close).

When you get around 10 cm from the screen you should notice that the spot on the right suddenly disappears. When you move a little bit closer the spot reappears. Strange huh?

If you succeeded then you have just found your blind spot (no not the one on the car. This is your own personal blind spot). If you didn’t then try again a bit slower. If you still didn’t succeed then keep on going and I’ll explain what was supposed to happen.

It will be worth looking at how we see and how that’s transferred to the brain as well as how many different ways we see the world and how many cells are involved before I start. If you haven’t already.

So the retina is the bit in the eye that see’s the world. Likewise it is filled with hundreds of thousands of little neurons (nerve cells) that are essentially seeing little bits of the world. Like pixels on a computer screen. All of these neurons together will make up everything we see. But they need to get to the brain first. So to get to the brain they come together and form the ‘optic nerve’ which carries information from the retina to the brain.

You can see where the optic nerve starts in this picture of a mouse retina. I circled it in red and put in a small arrow to point it out:

Just to remind everyone, the green dots are the cells that see light in the retina.

That small black dot is the start of the optic nerve, called the optic disk. It is also a visual dead zone. Because it’s a big bulky structure it can’t absorb light and therefore can’t see anything. That is your blind spot, and when you did the test above and you stopped seeing the black dot, that’s because it was facing the optic nerve.

You had to cover one eye because the optic nerve is in a different place for each eye, so there are no blind spots when both eyes are open (kind of).

OK so one of the first things we can take from this is: ‘Wow I never realised I was blind in one spot.’

It’s a pretty weird realisation when you have had total faith in your sight all your life. But that’s not really very important. The important thing is this:

Cover one eye again and look around the room. Can you see the blind spot? The answer is probably not. There is no gaping hole in our sight that we can see. So why does it work just for the little picture above? Well it doesn’t.

You always have a blind spot, but instead of it appearing like a hole, your brain fills it in with what it thinks makes the most sense. If you do the test again you’ll notice that it’s not a static or black hole that appears, but one that matches the background of this webpage. Your brain has taken the apparent background and filled the blind spot with this so that it looks like there is something there.

Let's pretend it's not there...

This is really important for sight. Because a hell of a lot of what you see, you aren’t actually seeing. It’s the brain filling in gaps.

That’s what these posts are about, the brain filling in gaps, and how we only see a fraction of what we think we see.

The next post is almost finished already so should be up some time tomorrow. It’s about movement, colour, and how we can only see a tiny amount of what we think we can see.

This will hopefully help me to explain optical illusions straight after!

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  1. Believing is Not Seeing: Visual Science and Reality part 2 | Science Defined

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