Watch your finger… disappear!
Hey you! Yes, the one reading this blog! I’ve got a magic trick for you…I’ll make one of your fingers disappear, quickly and painlessly!
Now before you frantically look around for a knife-wielding finger collector, don’t worry – it’s perfectly safe! I’ll even reattach your finger for free! All you have to do is the following…
- Close your left eye.
- Fully stretch out your left arm in front of you, palm facing away.
- Make a partial fist with your left hand, with your index finger pointing upwards and your thumb pointing to the right (making an ‘L’).
- Fixate your vision on the top of your left index finger.
- Stretch out your right arm and mirror your left hand. Let the tips of your thumbs touch.
- Without moving your gaze from the top of your left index finger, observe your right index finger.
Ta da! With any luck, your right index finger should have disappeared without a trace! Ignore the weird stares from the people around you and give yourself a pat on the back.
What on earth just happened to my finger?
Nothing, really! Everything that just happened was in fact a result of your visual system and something it contains called a blind spot. It’s normal!
Blind spot? Uh… do I need to do a head-check or something?
The blind spot I’m referring to is actually the result of the structure of the retina, tissue found at the back of your eye. The retina converts the light information entering from the front of your eye into information your brain can process. It’s made up of multiple layers. Simplified, the information given by light travels through 3 layers of cells in this order:
- Photoreceptors – These cells are the ones that react in the presence of light, starting the whole process!
- Bipolar cells – Think of these as intermediate cells, just passing along the information
- Ganglion cells – These are the cells linking the eye to the brain! Most of the length of these cells, called axons, form the optic nerve leading out of the eye connecting to the brain.
Knowing that, it’s natural to assume the arrangement of the eye to look like this:
Well surprisingly, human retinas don’t look like that! Our photoreceptors are closer to the back of the eye, and our ganglion cells are closer to the front of the eye. Why? We have no idea – it’s a weird trait we’ve evolved to have! And yes, this means light needs to travel through the rest of the retina to reach the photoreceptors!
Spot the blind spot!
A consequence of this structure is that the axons of the ganglion cells now need to travel through the retina to reach the back of the eye, and eventually, the brain. Looking at the diagram above, you can see that this leaves a large ‘gap’ between the cells making up each layer. This region is filled by the optic nerve head.
So what does that mean? It means that any light landing in this region never encounters a photoreceptor – there’s no room for one! This leads to a hole in your vision called… the blind spot!
Both of your eyes have a blind spot each, but you don’t see two empty spots in your vision. This is because the visual fields of each of your eyes overlap over these regions, accounting for the missing visual information. It can only be ‘detected’ when you close one eye.
But even then, you don’t see a blank region in your visual field. Think back to your disappearing finger – the area where your finger should be isn’t actually empty! That’s because your brain fills in this space using the information of the surrounding area. Amazing!
To find out more information, check out these links below!
Finger trick: http://www.michaelbach.de/ot/cog-blindSpot/index.html
Alternative blind spot test: https://visionaryeyecare.wordpress.com/2008/08/04/eye-test-find-your-blind-spot-in-each-eye/
Great video with a more in-depth explanation, plus more: https://www.youtube.com/watch?v=L_W-IXqoxHA