How much brainpower do our senses take up?

James - Thanks David. Just what proportion of our brain is dedicated to our mind's eye? And how does this stack up to the rest of our senses? Is it even possible to disentangle our vision from our sense of smell or touch when we look at brain scans? Well, I asked Holly Bridge, Professor of Neuroscience at the University of Oxford, for her help with this one.

Holly - It's very easy to divide the brain up into different regions that respond to each sense. There's no question about that. I'm a vision scientist, so I know that the back of everyone's brain is dealing with visual information. And that will be different depending on whether I present you, say, pictures of faces or a picture of a waterfall flowing over a cliff. That will excite different bits of the visual system, but it will all be at the back of the brain. Similarly, if I present you with music, I can pinpoint where that's going to be, which is going to be very distinct from those visual areas.

However, in order for us to function as humans, generally, our senses are going to work together. So if we need to reach for an object, we need to know what that object feels like, we need to be able to see it, and we need to be able to feel where our arm is. So we need to be able to integrate that information.

James - Nobody said brain science was easy, I suppose. Nevertheless, it sounds like we're going to be able to give you a fairly straightforward answer, David. Focusing on sight then, as you picked out, what numbers have you got for us, Holly?

Holly - Estimates range for sight as being between about a third and a half of the brain will give you some sort of activity to something that's presented visually. But if you think about when things are presented visually, what that might do is it might also evoke memories of when you've previously seen something like that. So if you look at the areas that are activated to a particular visual presentation, the definition of what is visual is a little bit loose. But nonetheless, I would say that at least a third of the brain is processing visual information. If you consider what the visual system is doing, it is quite phenomenal. The input to our brain is essentially spots of light. Those spots of light, which give us a sort of representation of where things are, need to be somehow transformed into this amazing perception of the world that has colour and it has shape and it has objects and it has faces. How that happens, we think there's probably about 50 different visual areas. So if I present you pictures of faces, I know that we will see sort of three different areas come up in your brain because you're responding to faces.

James - Quite the justification, I'd say, for why sight takes up so much brain capacity. So how much room is there left for the other senses?

Holly - We'll probably consider that audition is the next most important to us because it can alert us to danger. So if we are not paying attention to something, we're trying to cross a road and suddenly a car honks at us, that's really important for alerting us to what's going on. We can get a very fast reaction. The touch system is obviously most useful for things that are very close to us. Our touch system works very closely, of course, with our motor output. From that point of view, it's also very important.

James - In terms of brain capacity, taste and smell take up less in us humans than our other senses, but that's not to say they aren't vital for some people's quality of life, as many of us will no doubt remember from a run-in with COVID. But David, your intuition was correct. Sight does indeed occupy the largest part of our brains with estimates suggesting around 50 individual sections totting up to between a third and a half of the real estate up there.

Thanks for the question and to Holly Bridge, Professor of Neuroscience at the University of Oxford, for her help with the answer. Next time we'll be answering this one from Donald. He asks, ‘how big is a photon?’