The Science of Discworld II

paper-texture and charcoal-swirl oil paintings into the same frame. We think that this frame naturally includes ancient cave-paintings, which we know to be early, so have more claim to being genuinely primitive, if we could only look at them with the eyes and minds of viewers contemporary with the artist. The same problem occurs with Shakespeare,too: we no longer have the ears or minds – the extelligence – of the first Elizabethan age.
    We have to be more than a bit scientific here. We have to consider how we perceive light, sound, touch – what our sense organs tell us. For a start, they don’t, and this is the first lesson. In his book Consciousness Explained , Daniel Dennett is very critical of the Cartesian Theatre 1 picture of consciousness. In this picture, we imagine ourselves sitting in a little theatre in our minds, where our eyes and ears pipe in pictures and sounds from the outside world. In school we all learned that the eye is like a camera, and that a picture of the world is imaged in the plane of the retina, as if that was the difficult bit. No, the difficult bit starts there, with different elements of that picture taking different routes into different parts of the brain.
    When you see a moving red bus, the features ‘moving’, ‘red’ and ‘bus’ are separated fairly early in the brain’s analysis of the scene … and they don’t just get put together again to synthesise your mental picture. Instead, your picture is synthesised from lots of clues, lots of bits, and nearly all of what you ‘see’ as you look around the room is only ‘there’ in your brain. It’s not at all like a TV picture. It is not picked up instantly and updated, but nearly all of that ‘detailed’ surround is invented as a kind of wallpaper around the little bit that has your attention. Most of the details are not present as such in your mind at all, but that’s the illusion that your mind presents to you.
    When we see a painting … except, again, we don’t. There are several ways to convince people that they invent what they ‘see’, that perception is not simply a copy of the eye’s image on the retina. There is, for example, a blind spot on the retina where the optic nerve leaves it. This is big. It’s as big as 150 full moons (that’s not a misprint: a hundred and fifty). Not that the moon is as big, to our eyes, as we usually think – and certainly not as big as Hollywood repeatedly shows it. We ‘see’ the full moon as much bigger than it ‘is’ (sorry, we have to use some trick to separate what’s in your mind from reality out there),especially when it’s near the horizon. The best way to appreciate that is to demonstrate to yourself that the moon’s image is the size of your little fingernail at arm’s length. Hold out your arm, and the tip of your littlest finger more than covers the moon. So the blind spot is smaller than our description may have suggested, but it’s still a big chunk of the retinal image. We don’t notice any hole in the picture we get of the outside world, though, because the brain fills in its best estimate of what’s missing.
    How does the brain know what’s missing from right in front? It doesn’t, and it doesn’t have to: that’s the point. Although ‘fills in’ and ‘missing’ are traditional terms in this area of science, they are, again, misleading. The brain doesn’t notice that anything is missing, so there isn’t a gap to be filled in. The neurons of the visual cortex, the part of the brain that analyses that retinal image into a scene that we can recognise and label, are wired up in elaborate ways, which reinforce certain perceptual prejudices.
    For example, experiments with dyes that respond to the brain’s electrical signals show that the first layer of the visual cortex detects lines – edges, mostly. The neurons are arranged in local patches, ‘hypercolumns’, which are assemblies of cells that respond to edges aligned along about eight different directions. Within a hypercolumn, all connections are inhibitory, meaning that if one neuron thinks it has seen an edge pointing along the direction to which it is sensitive, then it tries to stop the other neurons from registering anything at all. The result is that the direction of the edge is determined by a majority vote. In