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…

  1. Close your left eye.
  2. Fully stretch out your left arm in front of you, palm facing away.
  3. Make a partial fist with your left hand, with your index finger pointing upwards and your thumb pointing to the right (making an ‘L’).
  4. Fixate your vision on the top of your left index finger.
  5. Stretch out your right arm and mirror your left hand. Let the tips of your thumbs touch.
  6. Without moving your gaze from the top of your left index finger, observe your right index finger.
With your arms fully stretched forward, here’s what your hands should look like. The ‘X’ represents where you should be looking. What’s happened to your right index? Source: Author’s own.

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:

  1. Photoreceptors – These cells are the ones that react in the presence of light, starting the whole process!
  2. Bipolar cells – Think of these as intermediate cells, just passing along the information
  3. 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.
A cross-section of the eye facing right. The retina can be found at the back of the eye. Source: National Eye Institute via Flickr.

Knowing that, it’s natural to assume the arrangement of the eye to look like this:

Author’s own

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!

Author’s own

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.

In a front-on view of the back of the eye, the optic nerve head looks like a white disk. Source: Richard Masoner via Wikimedia Commons.

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


16 Responses to “Watch your finger… disappear!”

  1. Jonathan Lay says:

    Thanks a lot Owen!

  2. Jonathan Lay says:

    Thanks John, hope you didn’t make yourself look too weird in front of your lab haha

  3. Jonathan Lay says:

    Thanks for the kind feedback Lixing!

  4. Owen Stanley says:

    Interesting blog post Jonathan, the writing is very easy to read and engaging.

  5. John Nguyen says:

    What a great post Jonathan, I really enjoyed that activity you described. A great practical and fun way to explain this phenomenon.
    Cheers!

  6. Jonathan Lay says:

    Thanks Marco, the things our brains can do also never ceases to amaze me – and we still don’t know so much about how it works!

  7. Marco M. says:

    The fact that our brains somehow manage to fill in the gaps left by this blind spot using visual information around it blows my mind. Nice post!

  8. Jonathan Lay says:

    Thanks Nicole, I’m happy to hear it was an entertaining and easy read!

  9. Lixing XU says:

    Your post is so engaging and very easy to read! Well done!!

  10. Nicole Nguyen says:

    This is definitely one of the funnest blog posts on Scientific Scribbles I have come across! Even though I knew what outcome your little magic trick had, it didn’t stop me from trying it out! The best thing was that you explained it so clearly and gave a visual as well, making it so easy to follow along. Great work!

  11. Jonathan Lay says:

    Thanks Megan! Maybe I should focus on magic tricks for all future posts!

  12. Jonathan Lay says:

    Thanks Isabelle! Glad to hear the diagrams made sense 🙂

  13. Jonathan Lay says:

    Haha thanks for taking the time to read it Andrew!

  14. megany says:

    This was a very engaging post! I liked how you included the “magic trick” at the beginning, it was very captivating; interesting read!

  15. Isabelle Foo says:

    Give me my finger back, Jono!! Other than that, this was a good read! Your writing style is very captivating and the diagrams makes your science a lot easier to understand (:

  16. Andrew Butler says:

    Hello Jonathan,

    So i did your little trick and my right fingers not coming back!?!?!?! What have i gotten myself into!?
    Kidding of course.

    On a serious note , your post has a very engaging opening. I couldn’t help but not finish reading it after i lost my finger !
    The information is very well presented, the dot points made it easy to follow and the images were nicely chosen too.

    Overall it is a really nice post .
    Thank you for taking the time to write it 🙂

    Kind regards,

    A