Now in this second lesson I want to come back and emphasize the discrepancies between luminance and lightness. That is the discrepancies that are universal in vision between measurements that you make with a photometer or some other instrument, a photometer in the case of luminance and lightness. The lightness that we see, or darkness that we see and report of a surface, or the brightness that we see and report in response to looking at a light source like a lightbulb. So let me begin by showing you again this very simple example, call it a standard example because, it's used so commonly in courses on vision. But, not in exactly the way that I'm using it here today to explain how we get around the inverse problem, but these two patches are physically identical. And when you see them as they're presented here on the same background, they look the same. But I already showed you that when you put them in different backgrounds, they look different, and I point out again that this is an instantaneous effect. It's not that it takes some time to see this difference in lightness. You instantly see as soon as I put on or take off the surround, you instantly see that now they appear different. Even though photometrically nothing has changed and the little circles in terms of their luminance. What's changed is just the context in which they're presented. A dark surround in this case, a lighter surround in this case, and now you see that this circle looks lighter than this circular patch even though again they're absolutely identical. So when you look at a demonstration like this, it's fine and convincing enough when you see it on your computer screen or your cell phone screen, but there's nothing like seeing it for yourself in physical reality. So what I've got here are, well these happen to be pieces of cardboard, but you can use any pieces of paper that you take out of a magazine or any surfaces that are on the one hand lighter, and on the other hand, darker. And you can take two patches that you cut out of another piece of paper that are exactly the same stuff, in this case, a gray, and you can put these patches that are identical. I mean you know they're identical because you cut them out of a piece of paper and you can put them on these other pieces of paper and see that again you see this effect. So this is not, again, I want to convince you that this is not something that's cooked up by psychologists, fooling you with illusions. No, no this is something you see in reality, and you see this every time you are making a judgement, consciously or unconsciously, of lightness or darkness. So you can take these little patches, again, and put them on the same surround as I've done here. Put them on the dark surround. The look the same, put them on the light surround. Again they'll look the same, but seeing them on the light surround, you can convince yourself very quickly and seeing them on the dark surround. In each case gives you a different impression of the identical grays that you're seeing from these physically identical patches. So I encourage you to use a little demonstration like that for yourself to make sure that you understand that these are not in any way phony. These are not illusions, this is just the way we see lightness and darkness all the time. And why that's happening is obviously important to explain. What's going on here? What's the possible explanation of these things? And a bunch of them have been presented over the years, but one kind of, or one direction of explanation would be, well, these are not very big effects. Maybe they just indicate the kind of imperfection of our visual system, and it's really nothing to worry about. We understand that biology is imperfect and we're just going to accept that this is a, shall we say, a trivial glitch in the mechanism of vision. But it's obvious and I've shown you this before as well but now we're going to use it for a more pointed purpose. It's obvious that that's not the case, that you can see the same identical patch in terms of measured luminance. As something that approaches either black or white which is in no means a trivial distinction. I mean black and white, these are the opposite poles of our sense of lightness and darkness. And I want to show you this again by concentrating on the apparent lightness of these patches. And I think all of us see it as this patch being very much lighter than this patch. This patch being very much lighter than this patch. And this is some thing here that approaches a very light gray, something approaching white-ish. And this is quite a dark gray, something approaching blackish. Well it won't surprise you, I think, to see that when I take the context away, when I take out the information in the scene that these patches are in fact physically identical. Each one of these areas is photometrically the same, and again this is an instantaneous effect. As soon as I take off or put on the mask, that takes away or adds back the information in the scene, you see this change, and the change is dramatic. So you can't get away with an explanation, or no vision scientist can get away with an explanation says well, this is really not too important, it's obviously important, it's changing our perception in a very dramatic fashion. So there are other ways of presenting this kind of explanation that I've just told you. In simplest terms, could be the visual system isn't perfect. One way of thinking about this that has been in the literature for decades is that this effect depends on our interest in edges. And that when you see a stimulus, let's go back and look at this one. When you see a stimulus like this, the edge here and the edge here is obviously different. This edge is gray to black. This edge is gray to some lighter color of gray. So one way of thinking about this is in terms of the receptive field properties of neurons that we talk a little bit about earlier. Let me come back and remind you that the receptive field of a neuron when you record for an individual neuron is the area of space that it responds to, the area in visual space that it responds to. And the properties that it has, does it respond to, lightness darkness, does it respond to color, does it respond to orientation? You can test the whole variety properties of people have. But with respect to this, here is a boundary between a relatively dark and a relatively light region. And what's been found universally in receptive fields in the simplest imput layers of V one, the primary visual cortex that we talked about, is that the receptive fields are organized in a center surround fashion. When a center that the cell fires more robustly, when illumination is directed at the center of the field, indicated by these little plus signs. And less robustly that it's inhibited that the activity of the cell is inhibited by light directed at the surround. So this center surround organization has been a characteristic that's known, really, since the 1950s, or before in different animals, but known in mammals since the 1950s. And you can see how this organization, receptive fields, might provide an explanation with this kind of effect. Again in sort of the domain of things aren't working properly or quite properly. This is a sacrifice that the emphasis on detecting edges by virtue of this organization of receptive fields, that a side effect of that is the difference that we see. So, again, look at this boundary. And these are receptive fields of a single cell indicated in different positions in respect to this boundary. So here it's completely the cell's responsiveness, the cell's receptive field is now completely in the dark area. And now it's a little bit in the dark area, the centers of the dark area, but there's a little bit of the surround that's in the light area and so on down through this array of diagratic representations, the receptive fields. And you can see that at each one of these positions, A, B, C, D here that are represented in this graph in terms of the response rate, how excited or not is the cell. That the cell's excitation is going to vary as a function of its position across this light-dark boundary. It fires a little bit less at B for reasons that should be obvious. The inhibitory surround is a little bit more in the light area than in the dark area. That's going to change the firing rate of the cell, the baseline firing rate of the cell, and it's going to be less. Here at C, halfway in, halfway out, comes back to normal, by normal I just mean the baseline firing rate. And here at D, where the inhibition is largely out but the center is completely out, there's going to be an increase in firing rate. And then when it's completely out in the light area, a decrease back to something that approaches baseline. So what is all this about? Well, this is a way of saying that, look, because cells' receptive fields of neurons in the primary visual cortex are organized in this particular way that one's known for a long time. That you're going to have a difference in a firing rate of the cell as a function of its position across a light dark boundary. And we're interested in light dark boundaries because they indicate contrast boundaries, the edges of objects that we're particularly interested in. Seeing the world, it's well known that we are particularly interested in boundaries. I showed you that before in talking about the way in which the eye moves by eye movements all the time. And you will remember that the eye moves to focus on contrast boundaries, not on areas. Remember we looked at a face, not on areas like the cheeks that don't have much contrast, they focus on where's there a lot of contrast like the eyes, for example. So this would be an explanation of this phenomenon in terms of a side effect of our interest in edges. But that's not really a viable explanation, at least nowadays. As I said, it was around for a long time in the literature, still in the literature, but I think people don't regard this as sufficient explanation. And if you look at other examples, they simply undermined the simplicity of what we would like to imagine might just be a side effect of our obvious interest in looking at edges more than looking at boundaries that don't have contrast.
