How can you experience this yourself?
You will need:
- A piece of paper
- A pen/pencil
How to do it:
- On your piece of paper, draw a cross on the left and a dot on the right, approximately 10cm apart.
- Cover your left eye with one hand and hold the paper at an arm’s length with the other.
- Focus on the cross with your right eye (but notice that you can see the dot too) and slowly bring the paper towards you.
- Stop when the dot disappears – this is your blind spot!
- You can take this further by drawing a line through both the dot and cross:
- Try the same steps again, but notice what happens to the line when the dot disappears.
You can try this with other patterns running through the dot as well. How complex can you make it?
What should happen?
The dot should disappear when you bring the paper a certain distance from your face – the distance will vary based on how far apart the dot and cross are. If you draw a line through the dot and cross, when you find your blind spot, the dot should disappear, but the line usually looks like it keeps going.
How does this relate to pain?
Our brains do a lot of processing before we become aware of the world around us. To cope with the amount of information that is constantly being received, the brain uses ‘cheat codes’ to process it quickly. This includes taking shortcuts, filling things in, cutting things out, making best guesses, and making assumptions. In this demonstration, the brain fills in your blindspot using the information that it has – what you can see around the blindspot. It makes the best guess that it can using the information available. When the dot is in the blind spot, the brain fills in the space with what looks like blank paper, because that’s what is around it. When there is a line going through both dot and cross, the brain usually assumes that this line continues through your blind spot – so that’s what we see.
The brain uses these cheat codes all the time, so the idea that it fills things in for you does not only apply to what you can see – it also applies to other senses. The brain makes the best guess that it can based on the information that it has, and sometimes it gets things wrong. In persistent pain, the brain concludes that the body is in potential danger, even when it is not, and alerts you to this with pain. Knowing this can help us to make more informed decisions about what is best for us. You can read more about this here.
How does this demonstration work?
In your eye, there is a small area that connects your eye to your brain. This area cannot detect any light – it is your ‘blind spot’. Your brain uses the light from around it to work out what you should be able to see and fills it in, so you generally don’t notice. When there is something small and dissimilar to the area around it, the brain tries to fill in the gap, but gets it wrong, so the dot disappears.
Note: you can reverse the left/right instructions and swap the dot and cross around to do this with your left eye.
More information
You can see things because the cells at the back of your eye detect light and send information via your ‘optic nerve’ to your brain. Your blind spot exists because the optic nerve does not have any cells that detect light, leaving a small circle missing from your vision. When you see through both eyes, the blind spot in each eye is filled in using information from the other eye, so you don’t notice it. When you look at something through only one eye, the brain fills in the blind spot using the information from around it, making a guess so that you usually still don’t notice. However, in this demonstration, when the light reflecting off the dot hits your blind spot, the brain fills your vision in based on the information around it – the blank page (or the line) – meaning that what you see doesn’t quite match what is really there.