How does information get from the eyes to the brain?
A couple of times a week as I’m sitting at my desk I’ll glance out of the window and see a being that my partner and I call Shed Cat. Shed Cat got their name because they are a cat that likes to sit on the neighbour’s brown wooden shed. Shed Cat is (a) cat-shaped, (b) black and white, and (c) usually climbs up onto the shed to survey their kingdom and wash their bum.
Ta da, Shed Cat! Sorry that this image looks like iteration 5 of a DeepDream process; the weirdness seems to be an artefact of using the zoom function on my phone’s camera. Funnily enough, this is kind of appropriate - as you’re about to learn, human vision is also extremely weird.
A Shed Cat sighting is a pretty mundane part of my life, but every time it happens something amazing and mysterious is also occurring: as I look at Shed Cat upon their shed throne, my brain is able to tell what is shed and what is cat. Now, you’re probably thinking, “Clare, that’s not particularly amazing or mysterious. Most of us can tell a shed apart from a cat because, e.g., sheds do not have bums and therefore cannot wash them.”
Very true. I’ll do you one better. Any time any of us sees anything, something amazing and mysterious is happening. So today, I’m going to tell you a bit about why vision is absolutely incredible. And I do mean “a bit” – this is an introduction, not a masterclass.
A light at the start of the tunnel
Everything I see begins with light coming into my eyes through the lenses and corneas and hitting the retina, the wall of cells at the back of the eye. The light hitting the retina is upside down and back to front compared to what’s actually out there in the world, because, well, physics.
Things being upside down and back to front on the retina isn’t super important – as we’ll see, the journey from eye to sight has much more complex problems for your brain to solve – but it is a good reminder that the physical facts of the world and your perception of the world generally have a cordial but pretty distant relationship.
Perception is a more-or-less useful hallucination that allows you to interact with the world. It is not how the world is.
If you’re now experiencing an existential crisis, welcome to the world of perception research. Every psychology department I ever worked in was absolutely stuffed with researchers having quiet meltdowns about how much our brains lie to us.
Anyway! At my retina, the light is picked up by light-detection cells called photoreceptors. Most of our photoreceptors are of two types, rods and cones. Broadly speaking, rods pick up information about movement, distance, and brightness, while cones pick up colour and fine details of shape.
This means that last time I saw Shed Cat, my rods picked up that they were about 25 metres away, bright with dark patches, and moving. My cones picked up that Shed Cat was not just bright-and-dark but specifically black-and-white, and (once they’d filtered into conscious awareness) the fine details of shape that meant I could recognise Shed Cat as Shed Cat and not, say, a geographically adventurous black-and-white ruffed lemur.
Not a cat.
Relay leg 1: To the ganglion cells!
“Ganglion” is a fun word, isn’t it?
Cones react only to light of certain colours and rods react to all light, but that reaction is exactly the same for each of them: a chemical cascade that creates a relay of nerve impulses.
Listen in on the rods and cones in my retina and you’ll hear many of them (chemically) shouting, “HEY THERE’S SOME LIGHT ON ME” or “HEY IT’S DARK OVER HERE” or “HEY I FOUND A RED THING”. It’s the ganglion cells’ unenviable job to gather together this chaotic information from little groups of rods and cones and do some maths to figure out whether to pass the message on.
Imagine one ganglion hears all the rods that report to it shouting “HEY IT’S DARK” consistently. Probably not important because it means nothing’s changing. No need for the message to go any further.
Now imagine a different ganglion cell hears several of its rods, all in a row, shouting “HEY IT’S LIGHT WAIT NOW IT’S DARK” one after the other. That means something’s bright and moving in a certain direction. Possibly important. Could be an enemy. Could be cake. Pass the message on to the next leg.
Relay leg 2: To the optic chiasm!
It’s pronounced with a hard ch, like chaos.
Like many humans, I have two eyes at the front of my head. This means that most of what I can see is visible to both eyes at once.
Don't believe me? If you also have two eyes and/or can find someone nearby who does, here’s a simple way to test whether I’m telling the truth. Look around and find an object that's ahead of you and on your left. Now close your left eye. You should still be able to see the object with your right eye, unless you’ve picked something really far to your left.
Seeing most things in front of us with both eyes is really useful for depth perception, but there are downsides.
First, the range of what we can see (our visual field) is quite restricted - we see what’s ahead, but not much above, below or behind without moving our heads. Things would be very different if we were configured like horses, who generally have one eye on either side of their heads, giving them massive visual fields.
Brief pause here for a shout out to the Legend of Zelda team - I’ve been playing Tears of the Kingdom for the last few days and have only now realised that they’ve coded visual fields in, and not just in the horses. Bokoblins have eyes at the front and are relatively easy to sneak up on, lizalfos have eyes at the side and are much harder to sneak up on.
Second, my brain now has to sort out what's to my left and what's to my right, because I can see things on my left side with my right eye and vice versa. This has to be sorted out if I want to use what I see to interact with the world, so our second handover point on the relay is a left-right sorting house for nerve impulses called the optic chiasm. Here, information from the rods and cones is sorted into “things on the left side of me" and “things on the right side of me”, regardless of which eye it originally came through.
Basically, any information that’s come from photoreceptors closer to your nose does the optic chiasm switcheroo, and any information that’s come from photoreceptors closer to your ears stays where it started. In the end, information about what I can see on my left ends up in the right side of my brain and information about what I can see on my right ends up on the left side of the brain. This sounds like a weird swap, but it aligns vision and action. When I reach out to something with my right hand, it’s (mostly) areas on the left side of my brain that are responsible for that action, and vice versa for my left hand and the right side of my brain.
Relay leg 3: To the lateral geniculate nucleus!
Fun fact: geniculate means “shaped like a bent knee”
We’ve sorted out left and right, and from now on those things will remain sorted as we move on to more fine-grained organisation. Just bear in mind that most times I mention a part of the brain, there are actually two of that part – one on the left, one on the right. Psychologists and neuroscientists tend to talk about most brain parts in the singular since they usually have exactly the same function regardless of which side of the body they’re on – just like your nostrils, big toes, gonads, or any other body part you have two of.
The next leg of the relay brings us to the lateral geniculate nucleus, or LGN, which is an information sorting house. We can make some rough groupings of what kind of information a nerve impulse contains depending on whether it originally came from a rod or a cone.
Rod impulses contain things like movement, distance, and brightness, while cone impulses contain information about colour and fine details of shape. Each of these aspects of vision is packed up and sent off to a part of the visual cortex, a large area of brain at the back of your head. In general, packages with different types of information go to different areas of the visual cortex.
At this point in the journey from eye to seeing, Shed Cat has been deconstructed into their constituent perceptual parts and is now in multiple areas of my brain. So has their shed throne. So has literally everything else I can see out of the window, from the neighbour’s hedges to my bike under its silver rain cover.
Now, how the hell did my brain manage to take all of those separate packages and reassemble them correctly into one (1) moderately distant, black-and-white, moving cat without accidentally creating one (1) moderately distant, silver, stationary shed or one (1) nearby, black-and-white, moving hedge or any other incorrect combination?
That’s the binding problem! Or at least, a binding problem. How do we solve it? That’s a story for another day.