Good day. So this lecture is on perception, and I'll be talking about a series of things. The first here are some examples to begin with.
Perceptions are relative, and hopefully you can see the pictures on the right. We've got a square with green and white and pink squares, and the x is actually the pink x that crisscrosses the square. is literally the same pink color. It's the same color waves. So the perception of color, objectively speaking, in terms of the wavelengths and the activation, is identical.
And yet, if you're like most people, you see the upper part of the X in that square as brighter, but in the lower part of the X, the same pink is perceived as basically a different color. darker, almost more red, at least to me in terms of my perception. And that's just a visual illustration of the importance of context in perception.
And so it's not just the objective stimuli that encounters our sensory systems, like the wavelengths or the intensity of the touch. It's probably more functional and valuable. from an evolutionary perspective, from a functional perspective, that we know whether colors differ in a given context and how they differ or how size or intensity or speed differs in the context in which you're seeing it rather than just having a like an absolute level of color waves or meters per second or how much something weighs objectively. It's the relative nature of perception, I think, has evolved over thousands of years and speaks to functional implications more so than it does to, you know, accurate, detailed, objective perceptions.
And we can look at size as well. So that was color and size is very similar. Adjacent to the phrase, the circles in the center are exactly the same size. believe it or not.
In the first configuration of circles you have a circle in the center and then smaller surrounding dots or circles. And then the set of circles to the right have you have a center circle and larger surrounding circles. And I don't know if you're if you're like most people but if you look at the left set of circles the center circle looks very large and certainly looks larger than the center circle in the right set of circles.
So that, again, is just a simple example of the contextual nature of perceptions. So we're probably more interested in knowing if the objects, within a set of objects, if we're looking at something at animals, say. If you're looking at a set of animals, it might be important to know which animals are the large ones and which animals are the small ones, rather than knowing exactly. how large or how small the animal is. This would be certainly true for predators.
I don't know if you guys have watched wildlife shows, but it's typical for a predator to look at the prey and they often, you know, attack the smaller creatures, the slower ones, the smaller ones, the weaker ones, because they're easier to kill and eat. So there's sort of functional reasons why our perceptions and interpretation and... and what we focus on is relative to the surrounding objects or context in the environment. I don't know if you remember, hopefully, we just watched the sensation lectures on color perception and we talked about cones in terms of being sensitive to different wavelengths and we also talked about ganglion cells activating sometimes to low firing rates and sometimes to high firing rates.
And so the opponent process theory also applies to the color black and white. And what we have here on this right set of squares, we have squares and then we have lines dividing those squares. But if you're like most people, and certainly myself, is when you look at the corners where four squares meet on a white surface, what I'm seeing, and I think most people, is gray spots. And this is just another example of... the opponent process theory of color perceptional in this case gray perception or the contrast process And this is just another example of how our perceptions aren't exactly objective, they're more relative.
So we need to know about change, the detection of change, and rapid changes are important. I'll encourage you to watch a video that further emphasizes the importance of context-dependent perception. It's a really cool video.
It's like one minute long. I love showing it in class. typically, but what I've done is include a link to the same video.
But what you're going to see is a woman take a square, a little object, a tile object that can move, and she puts it in light, and then she puts it under shadow. And you'll be surprised, I think, at how dramatically your perception of how dark and how light that square is, just depending on whether it's in the shadow or whether it's in the light. And this is just another example or evidence that we tend to see the world in a context-dependent, relative way. So we want to interpret how we see in the light and the sounds and the tastes and the pressure that's encountering us, that's impacting us, is interpreted in a contextual or relative way, rather than in an absolute sort of metric where we're measuring, you know.
objective levels of stimuli way okay this is about uh um another example of how we see things from a contextual perspective so we have a sort of a monster at the front or whatever a creature at the front and a creature at the back and just sort of i would say which creature seems scarier to you the front guy or the back guy so why If you're like me, the back guy looks a lot bigger than the front guy. So maybe I would want to tangle with the little creature rather than the big creature. In fact, the two creatures are exactly the same size.
What's happening here is that there's this thing called size constancy. So that when we look at the surrounding context and... objects, we assume that they're maintaining their natural size. So to be more specific, we see what we see is a hallway made of bricks and floor tiles. And in the normal world, like if we were standing in a hallway made of bricks and floor tiles, the size of the bricks and the size of the floor tiles would not change.
What would happen is... we would know that as they were getting further and further away from us, they would look smaller and smaller. But because of that, our brain would automatically recognize, OK, the bricks along the walls are exactly the same size, whether they're up close or whether they're far away.
And similarly, the floor tiles, they're the same size. It's not like the bricks are smaller and smaller in this picture and the floor tiles are getting smaller and smaller. No, the bricks and the tiles stay the same size.
size constancy. So in the front guy, he looks really tiny compared to the bricks and the floor tiles. In contrast, the guy at the back looks very large, relatively large compared to the floor tiles and the surrounding bricks.
So he must be substantially larger. When in an absolute kind of sense, when we actually take the two pictures and put them side by side, he's exactly the same sort of visual impact on our retina and the same physical size. So it's our brain that's automatically taking into account that hallway bricks and floor tiles don't shrink as they go away.
So therefore, if this guy is big relative to the surrounding context, it might be a very big guy or creature. So that's just a visual illustration and little discussion. the concept of size constancy and how our brains are looking in the world, evaluating things not just based on pure sensory input, but taking into account world knowledge and patterns and our understanding of the world and imposing that to some degree on our sensations and giving us different types of perceptions.
Okay, just another example. It's a little bit of a a picture or illusion. So once again we could talk about size constancy.
So this front guy he's standing sort of we can see a series of poles or pillars that are holding up this building and we have like the pillars you can see are gradually getting smaller as it looks like as you're looking farther away into the picture, right? And so our brain says, well, the building itself is not shrinking. It's not like as it goes farther away in the picture, the size of the arches are getting shorter and shorter and the size of the pillars are getting shorter and shorter and everything's just shrinking.
No, we simply think that we know that buildings are constant in their size and that's what our brain is assuming. So when we look at the guy in the back, we just say, well, he looks smaller. He's taking up less space on our retina because he's farther away. And we know he's farther away because in part because of size constancy that parts of the building that that are up close are larger and parts of the building that seem to be farther away are smaller. And therefore, the guy would also be smaller.
But in fact, if you look. if we took the picture of the guy and brought him forward, much, much tinier if we put them side by side in exactly the same context. So the actual size of the two people is dramatically different.
But because we have learned over time in evolution to adjust for surrounding contextual input and the context, we can understand that the human, the back human is different. probably a normal-sized human and is looking, is interpreted and perceived as relative to the surrounding context. So that's another example of the importance of context in our perceptions and how we sort of automatically take into account this different types of size and background information.
This is about another perception example. I don't know if you remember from the monocular depth cues, but we talked about parallel lines and in terms of depth perception. But here's the simple question for you. Like look at these two pictures. Are these two buildings at the same angle?
I don't know. To me, they look like they're at a different angle. In particular, I think that the one on the right is sort of at a sharper angle.
sort of tilted more to the right than the one on the left. But I also know that I went to the internet, found a picture of the Leaning Tower of Pisa, copied it, pasted it into this PowerPoint slide. Then I copied it again, Ctrl-C, Ctrl-V, which is paste, pasted it and slid it beside itself. So I in fact know that this is exactly the same image that I sort of copied from the internet. exactly the same image side by side and yet based on my perception I think they don't look quite the same I think one looks like the angles look different so that's sort of crazy I know in fact that it's exactly the same image it's a direct car and yet it doesn't look that way to me strange right So here's the deal, and we sort of talked about this before, this convergence or linear perspective or converging lines.
Parallel lines, we have learned in the world growing up that if two things are parallel, as they go away from you, as they recede of depth, we see them as converging. However, if we see parallel objects that seem to have a natural sort of three-dimensional depth, But as they move away, if they don't appear to converge, then our brain makes an automatic adjustment. It says, well, they're not converging as they're supposed to, so they must be diverging. So when we go back to this previous image, at the sensation level, the angles of these two images are identical.
But our brain is imposing, changing how we see and interpret the image, saying, well... they're supposed to be kind of converging as they move away and they're not so therefore they must be diverging so it's actually our brain and our expectations and our understanding of the world is making us see these lines as sort of these the same image with the same building looks like they're at different angles but that's for depth that's all about depth perception these i also copied this one image from the internet of this girl red dressed girl i guess and I just copied and pasted same process slid it beside each other and in this case they look the same there's no no distortion here because both images there's not an inherent sense of depth. in these two pictures if you have any questions please let me know uh post them on the general discussion board then everybody can see them and i can answer and everybody can hear my answer anyway i'll talk to you again and maybe see you during one of the q a sessions thanks