Your Eye, Your Brain and How We Hallucinate

Would you like to have a hallucination? It’s really very easy, we can all do it together right now.

Step 1: close one eye.

There you have it! You are now hallucinating. Wasn’t that great? It’s awesome, I know. What? You didn’t notice any hallucination? Okay, allow me to explain. The human eye is a sack of crap, it’s really badly designed, probably because it wasn’t designed at all but evolved from more primitive structures over millions of years.

The light that enters your eye first passes through your cornea, then your lens and then through all the jelly like substance that fills the bulk of your eyeball, the vitreous humour. At the back of your eye are the rod cells and the cone cells, the cells that detect the light and turn it into an electrical signal. To get that signal to your visual cortex at the very back of your brain obviously requires it to travel along a nerve, collectively called the retina.


Here’s where we encounter a major problem: all the nerves for those individual cells lay on the inside layer of the eye over the top of the receptor cells. All the light that they detect has had to pass through that tangle of nerves first. To get those nerve cells out they all group together at the back of the eye to form the optic nerve, but where the optic nerve leaves the eye there is no space for any receptor cells; this is our blindspot.

I imagine most of you have heard of the blindspot, but what has this got to do with hallucinating? Well, one of the major functions of your brain is to give you the illusion of an unbroken stream of reality and consciousness. It filters out the vast majority of the sensory input it receives, else we would be overwhelmed, but it also fills in gaps where it thinks important data are missing.

When both eyes are open your visual fields overlap and there is no problem with blindspots. When you close one eye, though, you should be left with a spot of nothingness just off centre because you literally cannot detect light there. This isn’t the case, though, because your brain hallucinates its best guess to fill in the gap. If you don’t believe me I can prove it to you.

Close your left eye. Hold up your right arm straight in front of you with your index finger raised. Start to move your finger to the right, your eye will naturally want to follow the tip of your finger but don’t let it, stare straight into the background. After about 6 inches you should notice that a portion of your finger disappears into your blindspot. With practice you’ll be able to lose most of your finger.

It isn’t the fact that we lose our finger in the blindspot that’s impressive, that makes sense. What blows the mind a little is that all the background in that area is a complete work of fiction that your brain is generating in real time. It’s a guess, normally a good one, but nonetheless it is not real, it is a hallucination.

This isn’t just a fun parlour trick to try out the next time you’re at a party, there is an important lesson to be learnt here. What you perceive as reality is a construct. You do not see everything your eyes see, hear everything your ears hear nor feel everything your skin feels. You also feel, hear and see plenty that your body has not detected at all. This is just one example of literally dozens of ways that your brain feeds you the information it thinks is necessary to stop you being eaten by lions. It is tuned to over simplify and is prone to false positives. The next time you catch yourself saying, “I know what I saw,” think again. You don’t know; you just don’t.


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