So in this lesson I'm going to tell you that's not such a hot idea, that is the idea that this is a side effect of our interest in edge detection, which is certainly a reality of visual processing. And I'm going to show you some more complex examples that simply couldn't be explained in that way. So let's look first of all at our standard instance of equal limited patches, looking lighter or darker as a function of the surround in which they are put. And the point I'm making here is that there is another arrangement of patches and surrounds that creates the opposite effect of the perceptions that you see in response to a dark surround here, and a light surround here. So let’s look at these in some detail. It's called White's illusion, named after the person who first described this about 35 years ago now. And I want you to focus on these patches, there are seven of them here, and they again, are all physically identical, just as these patches are photometrically identical. In fact, they are the same photometric intensity. But what you see here is that these four patches are now surrounded by more light stuff than dark stuff. So there's darkness here, but there's lightness here, darkness here, lightness here. So the greater surround of these patches is lightness, and the greater surround of these three patches, is darkness. That is they're surrounded by dark here, less light here, less light here, more dark here. So then that surround of this one is dark, and that surround of this one is light. But now the ones that have more dark surround are seen as darker than the ones that have more light surround. So that's the opposite of the effect that you saw here. So White's illusion can't be explained just by an edge detecting bias, because this is an opposite effect. It just doesn't make any sense in those terms. So, let's look at another phenomenon. This is, again, a presentation that was devised by psychologist, psychophysicist named Tom Cornsweet. Again, on the order of 30, 35 years ago. And the phenomenon here is that you see, as I see, these two surfaces. Forget the stuff in the background, that's just artistic folderol. But you see these two surfaces as differently light. This one is darker than this one. But this is not, in this case, coming from a surround or an edge. It's coming from these gradients that are referred to as the Cornsweet edge, but it's actually a gradient going from gray to black, and a gradient going from gray to white. And what you see here, and you can see this by just putting your finger interposed, but I'll do it with a mask. What you see here, is that as soon as I take away that double gradient from gray to black and gray to white, this difference in the appearance of the top and bottom panels disappears. So again, let's look at that a couple of times. Darker, lighter, as soon as I take away the edge, that impression, that perception, goes away. You just can't explain that in terms of some simple side effect of how we are interested more in edges and the receptive field properties of neurons. Of course they're involved, but they're not involved in the simple way that one would have liked it to be, so that these things could be explained in a straightforward way. And of course lots and lots of people have tried over decades to explain these things, so far without a consensus explanation. So, let me show you yet another example that can't be explained in any simple fashion. And this is what's referred to in the literature, and what I'm referring to here, as Mach bands. Mach, you probably know, was a 19th century physicist who was a giant in German physics of that era, and is best remembered today by the nomenclature of the speed of sound. He was interested in many aspects of physics, in particular, well among many things, the speed of sound, but he was also interested in vision. So it was possible in those days for the giants in that era to focus on different things, and he was interested in vision, not so much as audition and sound, but he was interested in that. And he described this phenomena that bears his name today as Mach bands. So what is a Mach band? Mach band refers to this area between a dark region and a light region. And the Mach bands are the impression that you'll have in looking at this, and I have and everybody has, that at this point, you'll see a band that's a little bit lighter than this light gray region. And at this point, you'll see a band that's a little bit darker than this region. So these bands, these Mach bands, lighter here, a light band here, and a dark band here, where are they coming from? That's obviously something we perceive. These two graphs just demonstrate the physical reality. So the physical reality is that there is, of course, a gradient here, that's the gradient between points 2 and point 3. But that gradient is uniform, there's no band of greater light intensity here, or of less light intensity here. The Mach bands are perceptual, and they refer to the bump in lightness that you see at the onset of the gradient, and a bump in darkness, the band of darkness, that you see at the offset of the gradient. So again, these are phenomenon that have been studied since the 19th century, and many people have busted their heads trying to explain what could cause these. Again, without a satisfactory explanation, what's clear in all this is that there's no simple way, given the phenomenology of the ways in which we see lightness and darkness, of capturing the range of that phenomenology in any simple way. And that of course raises the basic question of, what's going on? That's really the challenge, and it's the challenge that we are discussing in this course. And the challenge that I’m going to discuss in the next lesson, in the context of lightness and brightness. We'll go on to discuss it in other contexts later.
