Tuesday, 25 February 2014

How vision really works

You probably never really thought about this before, and neither had I really until I read about it about a month ago. But it turns out that your eyes are pretty much the least important player in vision. You might be pretty sceptical at this point, so I want you to close your eyes and imagine a boat. Done it? Okay, good, you just saw something without using your eyes!

At this point you're probably wondering where I'm going with this. ell I'm about to show you how much work has to be put in to be able to see something properly and hopefully you'll realise just how impressive the machinery  in-between your ears really is.

First, let's start with how we can go from an array of photoreceptors in the retina to a complex image of the world around us in 3D. First you have the two types of photoreceptors in the primary visual system; rod cells can detect low level light but they do not give very good acuity and they do not give colour vision, cone cells can detect colours and have very good acuity but they need high levels of light intensity. When these photoreceptors detect light, they send a signal to an area called the primary visual cortex (PVC) at the back of your brain, which you can see below.

Visual cortex Picture Slideshow
The primary visual cortex (PVC) sits just at the back of the brain, with much of the occipital lobe devoted to visual processing in some way.

The PVC deciphers the signals that came from your retinas, works out what's in your field of vision and then sends the signals to numerous other places within your brain focused on memory, decision making and anything else that could create your "conscious awareness of the world". This all occurs within around 200 milliseconds, pretty impressive right?

To create a visual representation of our world so quickly, our brains use a number of assumptions about how the world works so that it can take shortcuts. For example, because we want to be good at spotting other humans and also predators our brains are primed to spot these faces everywhere. Because of this face bias, we are prone to seeing faces where the do not really exist (Jesus on your toast, anyone?). If you want to learn more about these assumptions, look up Gestalt psychology. 

The assumptions our brains make can sometimes be wrong, I'll put down a few examples below, but we've all got our own favourite visual illusions so please post a link to your favourite in the comments if you like. 

This is no photo shop manipulation, but a real room with such a crazy layout that when girls of the same height stand in different corners they appear to be completely different heights. Hint: the bigger girl is higher up and closer to the observer than the smaller girl.

If you look at this mask you'll quickly see that when you look at the back of the mask, it suddenly appears to be pointing out towards you and spinning in the opposite direction.

So now you can see that while it's easy to pick up information, deciphering it requires much more work and we have very sophisticated systems to allow us to create a very vivid and detailed picture of our lives. You really see just how impressive this is when you step away from 2D still images and move on to 3D moving images. Take this clip below, see if you can keep track of the apple in this video.

Now remember how I said that the information that your PVC deciphers enters your consciousness? well that is a two way street, and your consciousness can greatly influence what you see. If you've seen this kind of thing before- you'll be thinking of gorillas in our midst, which you can see below.

Now this isn't just a clever lab trick, this happens all the time in our daily lives (obviously we don't notice), as you can see from this final clip.

What is out there to be seen and what we actually see are completely different- and they are massively influenced by our consciousness.

If you want to learn a bit more about the neuroscience behind how vision works, this is a good starting point

I hope that I have illustrated the real processing power of the human brain and how easy it can be to understand a little bit more about what is going on behind the scenes in our brains.

Please feel free to comment, share or follow this blog and I'm always open to topic suggestions.

Also check out this website for a really great tool that allows you to easily see the power of the visual system. 


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