Hello and in this lecture we will focus on the visual system. Some of you may have taken some intro to sci classes, so for some of you this context may already be something that you're familiar with. But the reason for discussing the visual system and in the next lecture perception, it is because Perception represents the way we see, feel, hear the outside world.
And so from a cognitive ergonomics perspective, in order to be able to use systems to interact with systems, with machines, in all sorts of contexts, all sorts of applications, it is important to understand how the visual and perceptual system work. To start this lecture it is important to make a distinction between three different concepts sensation perception and attention Sensation is the process of detecting and encoding stimulus energy in the world. This could be in the form of vision, so visual information that is present around us or auditory information in the case of hearing. So that represents sensation.
So detecting this information that is in the outside, that comes from the outside world. It's just receiving that information and not organizing that information. The next step is perception.
So whereas in sensation we're just detecting that there is visual information, auditory information around us, in perception we're organizing that information and interpreting the sensory information to give meaning to it. So for example, If you see an object in the real world, like a circular object, so a circle, that has two smaller circles at the top. and a line toward the bottom, that configuration together may look like a face.
And so in that case, although that object may not be a human face, but we're organizing those... shapes, the circles and a curved line, we immediately organize them as being part of one bigger object which we perceive as a face. So whereas in the sensation component we just detect objects in the outside world, in the perception we try to give these objects meaning.
Because we're used to see faces and we're accustomed to seeing human faces, whenever we see a configuration that share characteristics with human faces we tend to perceive that configuration as a human face itself even though it may not be a human face. So in the perception stage we assign meaning to the sensory information. The third process is called attention. It is the process of selecting objects or events in the perceptual world for conscious analysis. There are a lot of different phenomena, there is a lot of different information in the outside world.
Visual information, auditory information and all type of other information out there that we perceive through our senses. Even if you're in a quiet room, you may be, you're watching this video, but there's probably cars driving by outside your house or outside the room that you're in. Also the laptop that you're using, there's a fan in it that may make some noise.
If you're sharing your house, your apartment with other roommates, you may hear roommates in the other room or your family in the other room. Maybe you have a phone there and you're receiving a text or a notification. So even though it may seem like a quiet room, so not a lot going on at the moment, but if you think about it, there actually is a lot of information pleading for your attention.
but if you're trying to pay attention to this video even though there's a lot of information out there you're still capable of paying attention to this video so you're trying to focus your attention on one source of information this video and you're um sort of drawing out the noise and the visual information from all the other sources. That's what attention does. There's a lot of information, a lot of perceived information out there.
the noise from the other room, the noise from the street, other visual information, other objects that you can see visually in the room. But attention, it represents a filter that you put on all this information that helps you just select what you're interested in. So the three processes, sensation is the process of detecting and encoding stimulus, perception is the process of making sense of these stimulus, and attention is the process of filtering. whatever information we're interested in.
So you can see these three processes in a hierarchical fashion. Sensation at the bottom and then perception and attention at the top. While sensation and perception may not require attentional processes or conscious processing, attention requires active cognitive engagement, because you're deciding to pay attention to something and ignore something else.
So the visual system, it's a complex system that we're not going to be investigating and looking at in great details here. We're just going to look at it from a very high overview. The physical stimulus entered the visual system through the eye. The eye has a combination of different lenses, of different components that adjust the light coming into the eye.
On the back side of the eye there is the retina. The retina is a surface, again, that is located in the back of the eye. and is where there is initial processing of the visual information entering the eye.
The retina then processes some of that information somewhat and relay that information back toward the... rest of the visual system through the optic nerve which is located at the bottom of the eye. The optic nerve then leads the information through a series of processing stages and eventually through to the primary visual cortex.
So the human color spectrum is limited. Here in this picture for example you see that what's visible to the human eye is between roughly 400 and 700 nanometers. So what's below this threshold is called the ultraviolet range and the human eye is not, you know, we cannot see, you know, that range and above it's the infrared spectrum so we cannot see that either. So the human, this spectrum is very different depending on species. So there are certain species, a certain animals that are have a broader range of visible you know like visible spectrum and other species they have like you know like just different spectrum For example, here you have the bees compared to humans.
Because of the differences in sensitivity to colors, humans can see colors like orange and red that bees cannot see. But the same token, you know, bees are more sensitive to higher frequencies in violet and ultraviolet. So the retina represents one of the first stages of processing for visual information. The retina as I said is located in the back of the eye so the light enters the eye and there is a first series of lenses that adjust the how you know the delights that enter you know the eye and then the light goes back and hits this layer of photoreceptors called the retina The retina has two main areas, a central area which is called the fovea and the remainder of the retina which is called the periphery.
So central area fovea and a surrounding area which is called the periphery. On the retina we can see a two type of photoreceptors, cones and rods. These are two types of photoreceptors.
Not only they have a different physiology, but because of their different physiology, they have different characteristics in the way they process the information. So the cones are more prevalent in the center of the retina, in the fovea. They're sensitive to light, colors and movements.
So you may notice for example that We have higher sensitivity to colors in the center of our visual field than compared to the periphery of our visual field. And that is because the cones which are sensitive to colors, they are located in the center of the retina. Because the visual acuity, so our sensitivity to colors, to light, to movements is higher in the center of the visual field, because again the cones are more prevalent in the phobia, usually the information that falls on the cones, so in the center of the visual field, is what we pay attention to. That's why if you look straight...
That's the area of our visual field where our attention is allocated. On the other hand, rods are the photoreceptors that are more common in the periphery of our visual field. Rods are predominantly used for night vision and they're less sensitive to color changes. Information processed by rods is typically not attended to. For instance, if we see something moving, we can perceive it.
If there is something moving, try to put a finger in the periphery of your visual field and try moving your finger closer. Even if you move your finger closer, it's more difficult to see changes in movements when the finger is in the periphery of a visual field. On the other hand, when it hits the center of your visual field, even if the speed at which you're moving your finger is lower, but that movement is better defined that if the same movement at the same speed was captured in the periphery. And this is because the fovea is where cones are presented are more prevalent and they are more sensitive to movement.
On the other hand, rods are less sensitive to movements, to colors, and they are more prevalent in the periphery. Another demonstration to understand You know, these photoreceptors and their different characteristics is whenever you move, you walk from, you know, a room to a dark room, let's say a movie theater, you may see that at the beginning you're more sensitive to, even if it's dark, you're better able of seeing shapes in the periphery. than in the fovea.
There is a period, an adjustment period, that it takes for you to develop more sensitivity to objects in the dark, in the center of your brain. field. So if you walk into a dark room you start seeing shape sooner and quicker when they're in the periphery and that is because again the rods are more sensitive to night vision and it takes longer for the cones to be adjusted to the dark.
As I said, cones are more prevalent in the center of the retina, so in the fovea, than in the periphery. In the periphery we have more rods. So once the information is captured and processed initially in the retina, that information is then sent upstream to a neural population.
Imagine that a group of Photoreceptors send information to a group of neurons upstream in our central system. Cones have low convergence. What does that mean?
It means that one population, one group of cones, they send information to one group of neurons. So the ratio is 1 to 1. On the other hand, rod have a higher convergence. meaning that many groups of rods connect to only one group of neurons. These different convergence between cones and rods also play a role in the way, as I said at the beginning, we are more sensitive to colors.
movements in the center of the visual field. So cones are more sensitive to that, but also because they have a one-to-one convergence, so one group of cones send information to one group of neurons, in that case the information relayed by... From the cones to the visual system to the neurons upstream There is more power, say, it's better defined because of this one-to-one ration.
On the other hand, because the ration is many rods groups to one neuron group, in that case, some of the information processed by rods may get lost. when it's processed upstream. Okay? So, this is one of the reasons why we have better visual acuity, so better sensitivity to details, for cones that we have for rods.
So just to recap. cone vision, cone based system, higher acuity and color vision in bright conditions there is a low receptor neuron convergence so one or a few photoreceptors synapse or connects to one neuron group on the other hand, rods is more sensitivity in the dark and there is higher convergence The trichromatic theory says that unlike rods, cones are also more sensitive to colors. So we can distinguish between three different types of cones. Short cones, medium cones and long cones.
Short cones are more sensitive to the color blue, medium cones are more sensitive to the color green, and red cones are more sensitive to the color red. So whenever you have somebody who is color blind or has color deficiency, it's because they have one of these three cones populations that are either damaged or they're not functioning properly. So for instance in the condition known as monochromacy there's a total absence of either two or three of the pigmentous retinal cones.
So those three cone populations, the red, the green and the blue, either all of three, so none of the three are working properly or only two are, sorry, or only one is. In the condition called dichromacy, only two of the three groups of cones function properly. So here, for instance, you see an example of what the same picture would look like in the condition of monochromacy, so there is no colors whatsoever, or in the condition of dichromacy, where you see that the leaves that should look green instead look very kind of yellow, pale yellow.
In the case that only two of the three cons population functioning, so in this case of dichromacy, we can have different conditions. What you see up here is a picture of a function. photos under normal condition. There is also this condition called pronotopia, a condition where only the long cones are missing. where the medium cones are missing and trichinopoeia where the short cones are missing.
So in this case the short are for instance the blue cones that are more sensitive to blue and so whenever in this this third condition that the short cones there are more more sensitive to blue are missing, you see that again this very little like blue, you know, in this picture. And here's another example of how the same picture of these hats, colored hats, would look based on those conditions. So as I said, the retina is the first layer of photoreceptors where the information the visual information entering the eye is processed so it's the first station of processing information then is sent upstream to different neural population bipolar cells ganglion cells and These different neural populations have different processing and process the visual information further. There are some, for instance, some of these populations are more sensitive to certain movements.
So if my movement is, let's say, an object moved horizontally, some of these populations are more sensitive to horizontal movements. On the other hand, some of these populations are more sensitive to certain color contrast. So what you see here...
is like a sectional view of the brain and you'll see you know the eyes here there's the retina you know what you see in blue and red here and the same blue and red for the red right eye so as we said earlier the information enters the eye from this case from up here and then gets processed into the retina and then upstream there are those population of say ganglia cell or bipolar cells that further process the information So one point here in the center it's what is called like optic chiasm and what it does is relays the information to some information is related to the same part of the brain for instance for the information that enters the left eye and for the on the temporal retina so that information is in the blue is processed on the same side of the brain that is uh you know that that of the same that the eye that enters it on the other hand The information is processed by the left eye, but instead of entering the temporal retina, it enters the nasal retina. That information upstream is sent to the right half of the brain and vice versa for the right eye. So this processing is necessary for an example like binocular vision and eye and coordination. So there's some adjustments then from the information that enters the...
i am get processing to the retina then that information undergoes more and more processing and then it will get split either it goes to the right side of the brain or the left side of the brain depending on uh you know the area of the retina that it enters the eye and then upstream it will eventually be processed into the primary visual cortex. So in the primary visual cortex we can identify two paths of processing. They're called the where and the when paths. So for instance here in the primary visual cortex The ventral or WATH path is useful to process visual information to detect color, texture, shape, and size.
On the other hand, there is another path or stream, the where stream, and it's more useful for spatial processing. So processing of features like location, movements, spatial transformation, spatial relations. So you may wonder, How do we know of the existence of two different paths? Well, one of the reasons, one of the sources, one of the ways that we know that information is processed to two different paths is because of case studies and neuropsychological studies. In some of these studies, some of these patients may have sections of the wear path damaged.
So remember the wear path is useful to process features like spatial characteristics of an object, movements, distance. So whenever sections of this path get damaged, we see conditions like dyslexia arising. So whenever it is required to focus on certain letters, so think of letters as targets. So whenever you have a text in front of you, we follow the reading from left to right and top to bottom. So in that case, this is kind of flow to the way we're reading information.
And patients with these conditions, they have difficulty following that information, like spatial information, so left to right and top to bottom. Those conditions, they are so because they are associated with a damaged wear path, so this inability to detect movements and complete visual search, for instance, that follow a certain route. On the other hand, there are conditions associated with when the Watt path gets damaged. To remember that the Watt path is useful for detecting features like shape, color, and patients that have conditions wherein the Watt path is damaged, they're unable to identify objects. The wear path, so the path associated with movement detection, is still intact.
So, there are patients who are still able to, you know, if you throw them a ball, they're still able to catch it because they still have the ability to detect movements and act on it by, you know, catching a ball. You tell them to identify a ball, you know, if you put a football in front of them and you ask them what it is, they may not be able to identify objects. They're able to see movements because the wear path is intact.
but they're not able to identify or to detect to identify what object it is because the system that is necessary to see shapes colors is damaged So this is the last slide of the deck for today and next time we'll look at perception. So today we looked at sort of an overview of the visual system. The important sections of this light deck are the retina, so pay close attention to the different photoreceptors in the retina, the cones and the rods, to the different type of processing that they do on visual information. Also pay close attention to the last section of the slide deck where we talked about the where and the what path. and what kind of information they process, what kind of information they're sensitive to, and how damages the two paths result in specific conditions.
So as I said, next lecture we'll talk about perception and we'll look at how this information that is being acquired is actually being organized by our brain.