Visual Perception

I'd like now to distinguish two words, which sometimes you'll probably hear me use interchangeably, but actually have a subtle difference: sensation and perception. In one sense, there's the things that we really hear, and shall we say, the immediate senses. This is what in psychology is called sensation.

So with your eye, the photons actually come into the back of your eye, hitting your retina and the nerve cells, noticing those and creating a signal. When you hear, little hairs in your ear are getting wiggled by the sound and then passing that into your brain. So that is the raw sensation. That's the absolute raw sense of what's there.

However, our brains do lots of work with that. They process what are those raw sensations in order to give that sense of that feeling of actually what's there. There's a couple of examples of this you can think of. So, for instance, if you... Looking at your room, just glance at the room you're in, you'll have a sense that you can see it all, but actually your eyes dance around and builds it up slowly.

So that raw sensation at any one moment is probably that everything in the periphery is fuzzy. But you don't see that. What you believe you see is something that's sharp everywhere. Because if you ever need to know about a bit of the room your eye will glance to it and it will become sharp at that moment. So your perception of the room and your visual perception is that it's large, it's everywhere, and you can see it all. Your actual sensation is much, much tighter than you can see clearly.

Because these differ and because they are about the meaning of the world... We are *meaning-giving creatures*; we're trying to find meaning and sense in this world. You can sometimes use this creatively to create effects, but you could also sometimes get, shall we say mistaken effects because of this. And optical illusions use this very much in order to fool your eyes because your eyes are trying to create sense of the world and you can sometimes fool them.

So here's two optical illusions, both of which are based on linear perspective. There's the Ponzo one. So if you look at that picture, it's a ladder with two blocks, and it will look as if the block at the top is bigger than the block at the bottom. The reason for that is your meaning bit of your brain is saying, "It's doing, the ladder's doing this," so it's probably sloping away from you. If it's sloping away from you, then something that takes the same amount of visual space

further away must be bigger. On the right is the Müller-Lyer illusion. So the Müller-Lyer illusion is two lat[eral] lines and then two arrowheads and one has got arrowheads going in and one's got arrowheads going out. And if you look at it, you probably will see the one with the arrowheads going out as being a longer line than the one at the bottom. In fact, they're absolutely the same size and the same length.

However, your eye sees the one at the top longer because it looks a bit like the back edge of a box – so, again, in terms of perspective, if the line's going... It's like looking at the back of the box whereas the ones at the front is more like if you're seeing the front end of the box. And of course, again, your eye is saying that probably means the one on top is further away. Although it takes the same amount of distance on the back of your eye, that probably means it's actually larger. Now all that's happened, when I say "probably", you're not reasoning this out.

This is all happening tacitly, unconsciously. You've got no idea it's going on. And it's very hard, even though you know those two are the same length to actually be able to see that. Now, it was thought for a long time that a lot of us... and some of these illusions really are very basic; they're built into our – shall we say – our most primitive base being, the things that we've had for, you know, 10-20 thousand years,

probably longer as part of the way we see the world. And it was thought in particular the Müller-Lyer illusion was one of these. Something that's very, very basic. I know I found problems with this for many years because linear perspective, it's actually hard to think, shall we say, what in the wild corresponds to linear perspective. And sure enough actually, what's been realized relatively recently is that most subjects of psychological experiments are what's been called WEIRD.

By "WEIRD", it's White, Educated, Industrialized, Rich and Democratic. So basically, think of US psychology students as subjects of most psychology experiments – or British ones for that matter. So you actually end up with a very select set of people

for which we believe everything is true of. And of course, actually that turns out not to be true. Crucially, an awful lot of things which appear to be very fundamental psychological effects turn out to be *cultural*. And crucially, this thing about the Müller-Lyer illusion is one of the ones that is not the same. So if you have a... there is a relatively small number of experiments where this is the case, but there are some experiments that have been done with people who have been brought up in

the middle of forest jungle environments, but without built straight lines, concrete and brick structures. And when that's the case, the Müller-Lyer illusion doesn't work. They see the two lines as the same length. This is a learned effect that's happened from childhood, where because you live in a built environment, you end up with particular illusions. So these are partly about sensory perception, but perception is based partly on

– shall we say – built-in parts of your brain. But also the culture in which you live changes the relationship between raw sensation and perception. So, you know, optical illusions can fool you. However, actually, if you think about it... So there's the kind of optical illusion like Müller-Lyer, but also you've probably seen those trick photographs where you think something's bigger than it is

or you see something and it says, what's this? And you've got no idea. And then there's some small tweak and it's zoomed out and "aha, of course". Now the interesting things about those is partly it's a perception thing that your eye can get fooled. But they're often deliberately chosen at a very unusual eye position. So you may take a photograph of something so that certain things line up, that then make it confusingly look like something else. And you only have to perhaps move your head a little bit.

You know, if it was a real thing that you were looking at, you would move it, you would move your head. And suddenly it would all make sense. Our perception systems are designed for the real world. They're not designed for static images on screens. We've learned to deal with them – I said culture comes in here. So our brain is trying to make sense of this all the time. And that can sometimes mean you can get these optical illusions that can be confusing. But also that can work to our advantage because we're trying to make sense.

So this is something you can think about in design, both things that can go wrong, but also things that can go right.

I'd like now to distinguish two words, which sometimes you'll probably hear me use interchangeably, but actually have a subtle difference: sensation and perception. In one sense, there's the things that we really hear, and shall we say, the immediate senses. This is what in psychology is called sensation.

So with your eye, the photons actually come into the back of your eye, hitting your retina and the nerve cells, noticing those and creating a signal. When you hear, little hairs in your ear are getting wiggled by the sound and then passing that into your brain. So that is the raw sensation. That's the absolute raw sense of what's there.

However, our brains do lots of work with that. They process what are those raw sensations in order to give that sense of that feeling of actually what's there. There's a couple of examples of this you can think of. So, for instance, if you... Looking at your room, just glance at the room you're in, you'll have a sense that you can see it all, but actually your eyes dance around and builds it up slowly.

So that raw sensation at any one moment is probably that everything in the periphery is fuzzy. But you don't see that. What you believe you see is something that's sharp everywhere. Because if you ever need to know about a bit of the room your eye will glance to it and it will become sharp at that moment. So your perception of the room and your visual perception is that it's large, it's everywhere, and you can see it all. Your actual sensation is much, much tighter than you can see clearly.

Because these differ and because they are about the meaning of the world... We are *meaning-giving creatures*; we're trying to find meaning and sense in this world. You can sometimes use this creatively to create effects, but you could also sometimes get, shall we say mistaken effects because of this. And optical illusions use this very much in order to fool your eyes because your eyes are trying to create sense of the world and you can sometimes fool them.

So here's two optical illusions, both of which are based on linear perspective. There's the Ponzo one. So if you look at that picture, it's a ladder with two blocks, and it will look as if the block at the top is bigger than the block at the bottom. The reason for that is your meaning bit of your brain is saying, "It's doing, the ladder's doing this," so it's probably sloping away from you. If it's sloping away from you, then something that takes the same amount of visual space

further away must be bigger. On the right is the Müller-Lyer illusion. So the Müller-Lyer illusion is two lat[eral] lines and then two arrowheads and one has got arrowheads going in and one's got arrowheads going out. And if you look at it, you probably will see the one with the arrowheads going out as being a longer line than the one at the bottom. In fact, they're absolutely the same size and the same length.

However, your eye sees the one at the top longer because it looks a bit like the back edge of a box – so, again, in terms of perspective, if the line's going... It's like looking at the back of the box whereas the ones at the front is more like if you're seeing the front end of the box. And of course, again, your eye is saying that probably means the one on top is further away. Although it takes the same amount of distance on the back of your eye, that probably means it's actually larger. Now all that's happened, when I say "probably", you're not reasoning this out.

This is all happening tacitly, unconsciously. You've got no idea it's going on. And it's very hard, even though you know those two are the same length to actually be able to see that. Now, it was thought for a long time that a lot of us... and some of these illusions really are very basic; they're built into our – shall we say – our most primitive base being, the things that we've had for, you know, 10-20 thousand years,

probably longer as part of the way we see the world. And it was thought in particular the Müller-Lyer illusion was one of these. Something that's very, very basic. I know I found problems with this for many years because linear perspective, it's actually hard to think, shall we say, what in the wild corresponds to linear perspective. And sure enough actually, what's been realized relatively recently is that most subjects of psychological experiments are what's been called WEIRD.

By "WEIRD", it's White, Educated, Industrialized, Rich and Democratic. So basically, think of US psychology students as subjects of most psychology experiments – or British ones for that matter. So you actually end up with a very select set of people

for which we believe everything is true of. And of course, actually that turns out not to be true. Crucially, an awful lot of things which appear to be very fundamental psychological effects turn out to be *cultural*. And crucially, this thing about the Müller-Lyer illusion is one of the ones that is not the same. So if you have a... there is a relatively small number of experiments where this is the case, but there are some experiments that have been done with people who have been brought up in

the middle of forest jungle environments, but without built straight lines, concrete and brick structures. And when that's the case, the Müller-Lyer illusion doesn't work. They see the two lines as the same length. This is a learned effect that's happened from childhood, where because you live in a built environment, you end up with particular illusions. So these are partly about sensory perception, but perception is based partly on

– shall we say – built-in parts of your brain. But also the culture in which you live changes the relationship between raw sensation and perception. So, you know, optical illusions can fool you. However, actually, if you think about it... So there's the kind of optical illusion like Müller-Lyer, but also you've probably seen those trick photographs where you think something's bigger than it is

or you see something and it says, what's this? And you've got no idea. And then there's some small tweak and it's zoomed out and "aha, of course". Now the interesting things about those is partly it's a perception thing that your eye can get fooled. But they're often deliberately chosen at a very unusual eye position. So you may take a photograph of something so that certain things line up, that then make it confusingly look like something else. And you only have to perhaps move your head a little bit.

You know, if it was a real thing that you were looking at, you would move it, you would move your head. And suddenly it would all make sense. Our perception systems are designed for the real world. They're not designed for static images on screens. We've learned to deal with them – I said culture comes in here. So our brain is trying to make sense of this all the time. And that can sometimes mean you can get these optical illusions that can be confusing. But also that can work to our advantage because we're trying to make sense.

So this is something you can think about in design, both things that can go wrong, but also things that can go right.

If we want to design effectively for humans   working with computers, clearly  we need to understand humans.