Are your eyes playing tricks on you?

Are your eyes playing tricks on you?

Try these experiments to find your blind spot and understand the way your eyes work.

We recently reported on the discovery of a giant eyeball on a beach in Florida and thought it would be fun to have a look at our own eyes.  In particular, we will look at our blind spot (scotoma in scientific language) and try some experiments that prove that our eyesight may not be as perfect as we think.

What is a blind spot?

A blind spot is a gap in your vision.  Normally we can see quite a lot of things at the same time, even if some of them are a bit blurry.  When you are focussing on one thing in particular, you will still see other things "out of the corner of your eye."

That's how you know to duck if a ball comes out of nowhere towards your head.  Your eye and brain spot the movement at the edge of your vision and you duck by instinct (hopefully).

So let's try an experiment...

 

Blindspot

 

In this experiment we will make one of these letters disappear.  You can look at the A and B above on the screen, or get a blank piece of paper and write the two letters about 15 cm apart.

Now, close your left eye and with your right eye look at the letter A. Start with your head about 50 cm (20 inches) away from the image.  Making sure you are really focussed on the letter A, move your head forward very slowly.  As you do this, the letter B will suddenly disappear from the corner of your eye.

Blind spot experiment number 2

 

linebs

 

This time close or cover your right eye and look at the red dot with your left eye. Like before, from a starting distance of about 50 cm, gradually move closer to the image. At a certain distance it will seem like there is just one blue line instead of two.

Your brain is filling in the gap all by itself and you are seeing something that isn't actually there!

There are some other experiments to try on Professor Eric Chudler's Neuroscience for Kids website.  Have a look for yourself.

What's happening?

So why are our eyes playing tricks with us? First of all, it's important to understand how our eyes work.  Have a look at the BBC video below:

So you see, when the light enters your eye it hits the back of your eyeball where nerves pick up the signal and send it to your brain.

What about the blindspot?

Eyeball

Looking at the diagram above you can see that the optic nerve connects to the back of your eyeball.  Where it first attaches itself there is a section of your eye that cannot detect light.  This is the blind spot (circled in red).

Normally we don't notice it, but when we only use one of our eyes (like in the experiments above) then it becomes much more obvious.

It's hard to imagine that we might miss something that's right in front of our eyes, but we can!  All the more reason to really pay attention when you are walking near or crossing a busy road!