How does the human visual processing system works? Now, I want to walk you through a model of what happens when the light enters our eyes and how it is processed by the eye and the brain. In order to do that, we have to start from the eye itself. I'm going to describe only the very essential parts of the eye. Of course the eye is much more complex than what I'm going to describe. So, light is coming from the world through either sources of light or surfaces that are reflecting light coming from other sources. So, at any given time what we see in front of us, is the effect of light coming through our eyes. Now, the eye as a number of structures. First of all, there is the cornea, which is basically a lens through which the light comes inside the eye. We also have a pupil, that is basically like a camera that can open or shrink and according to how much light is coming in. So gets larger and larger when the light is dimmer and dimmer and it gets smaller and smaller when there is more light. Now, when the light is reflected through the cornea, it's actually reflected to the back part of the eye to a region that is called retina. The retina is the place that is of the eye that is in contact with the optic nerve. The retina again which is the back of the eye is full of photoreceptors. What is a photoreceptor? Well, it's kind of cell that transforms signals coming from the eye to signals that go to the brain. Now, let's look more closely at the content of the retina. The retina is packed with these photoreceptors which you can think of like light sensors. There are two main types of sensors, there are rods and cones. Rods and cones are complimentary. Rods are necessary and they are active in low light conditions whereas cones are active and necessary in normal light conditions. Now, an interesting aspect of the retina is that the resolution that humans have in the surface of the retina changes according to the location. More precisely, there is a small region of the retina that is called the fovea. This is a region of the retina, that has a much much higher resolution than the rest. How large is the fovea? Well, one way to figure out to get a sense of how large the size of the fovea is, is to basically look at the size of your thumbnail in front of you at arm's length and when you focus your attention on it, the size of your thumbnail is roughly the size of the fovea in your eye. Okay. You can try. The resolution is enormous in this very small region. Just to give you an example, in a normal monitor, there are lots of pixels and around 40 pixel per centimeter and the kind of resolution that humans can have is about one tenth of one pixel so, it's incredibly high in these specific region. Of course, is not the same in the rest of the eye. So, this is a diagram that shows you how the resolution changes in different regions of the of the retina. As you can see, resolution is incredibly high in the area around the fovea but it decreases very very sharply as you move out of this central region of the eye. So now, light comes in, it's initially reflected by the lens that we have in our eyes, goes to the retina, the retina as this non-uniform distribution of cons and rods and of resolution. Now, these information is captured by the sensors and transmitted to the brain through the optic nerve. What happens next? So, here I'm going to very briefly talk about a model of processing of visual information, taken from Colin Ware's book. The same book that I mentioned a moment ago. So, this is an idealized version of what happens when light comes in. So, light comes through the eye. First of all what happens, is that there is what is called Feature Perception. This is happening at the first stage of processing. There are very important properties at this stage. The first one is that, the detection of these low-level features is incredibly fast and it's parallel. What do I mean by parallel? It means that it doesn't really depend on where these features are, they are perceived all at once. They are also stored in a sort of buffer that is called Iconic Memory, where information is very briefly stored and it's very transitory. It means that it stays there for a very brief moment. Whatever is captured in the Iconic Memory, is then transferred in a second stage through another processing step. In this processing steps, the low-level features are aggregated together to create patterns. So, at this stage, this stage is characterized by a number of factors. First of all, perception of these patterns is sequential is no longer parallel and because of that, is much much slower. Second, the perception of these patterns is highly influenced by attention. Whereas in stage one, whether something is perceived or not is much less influenced by whether something is attending to something or not. In the third and final stage, the patterns that have been detected through the processes stage one, are transformed into objects that are retained in the working memory. This is a very small number of objects, the estimate is that we can't really store more than six or seven objects or more precisely what is called chunks of information, at any given time. This information as I said, is stored in what is called the Working Memory. The memory that we need in order to reason about certain thoughts. I want to conclude by highlighting a very important aspect of this whole process. The important aspect is the fact that, at any given time our eyes receive an incredibly large amount of information. But, this information is processed through many stages and reduced enormously and simplified enormously. So, even if we perceive, even if we have a lot of in incoming information from our eyes and our sensors at any given time, the visual processing system does its best to reduce this disinformation as much as possible and retain only what is important for the given task. In the next video, we're going to discuss this aspect in much more detail.
