This lecture is on vision. What light is and how the eye tells the brain where light is coming from. Now in humans, vision is an extremely spatial sense. We use it for perceiving everything, from the shapes of objects, to their locations in the environment. And this informs nearly everything we do with vision. It's because vision is a spatial sense that you can read the letters on the screen right now. That requires detecting exactly where the darker regions are and, how they are connected, and how they are spatially arranged. Whenever you recognize someone based on their facial features, you are using the spacial aspects of vision. To detect the eye shape, the expression of the mouth, or the eyebrows, sports activities like catching a ball. Or even simply walking around. In all these circumstances you're using vision space liabilities, to allow you to perform these essential behaviours and forms of perception. So we're going to begin with how this works, and the first issue we need confront, is how does the eye detect where light is coming from? Now this question befuddled scholars for over 2,000 years. In fact, before anyone could really think about we know where light is coming from. The very properties of light itself, the way it travels through the environment, had to get worked out. And we'll retrace the conceptual steps that were taken by scholars dating from ancient Greece on up to the present day. Now the first question that scholars had to confront was, what is the physical substance, or entity, that is involved in vision? Now for background, around 425 BCE, Greek scholars had begun to suggest that everything in the world was composed from four basic elements, Earth, fire, air and water. Democritus and others then argued that all substances were composed of tiny particles, that they called atoms. And these atoms retain the fundamental character of the larger substance, so for example, water, which is slippery would be composed of atoms that retained the property of slipperiness. Air, being transparent, the atoms of air would retain the feature of transparency. So Democritus then extended this concept to vision, and he argued that vision worked by the eye detecting small particles of some kind. Particles that would emanate from objects in the visual scene, and come into the eye. So, seeing a mountain, for example, would mean that the mountain gave off some kind of substance that retained the mountain-like nature of the original object, and entered the eye, where it was somehow sensed. Now, Democritus obviously didn't get the actual nature of light correct, but at least he got the direction in which light travels correct. He argued, in short, that something is coming from the scene and entering the eye. And this theory was called intromission. Now others disagreed. For example, Plato argued that the eyes send something outward to probe the visual scene. Some kind of visual fire that returns to the eye with information about what visual objects are present. So this view, that's something comes out of the eye to have essentially feel the world, is called extramission. And it's a little like modern stories of comic book action heroes, having some kind of ray or beam that comes out of their eye, to probe the world. Now the debate between the extramissionists and the intermissionists raged on for quite a while. Without being resolved, until the works of these early scholars fell into obscurity, as the Greco-Roman empire declined. Now eventually the Islamic Golden Age from about 750 to 1258, common era. During that period of time they were rediscovered and translated into Arabic. And scholars at that time gained access to them and renew the debate. So initially, Plato's extramission theory held sway. Now, at first glance, Plato's theory seems to capture some important aspects of vision. For example, seeing seems like an active process. Something you do to the world. What you see depends on what you're looking. So, for example, I hope that right now you are looking at the screen. But you could simply be listening to the audio, perhaps you're riding the train to work and looking out the window. And so what you're seeing depends on which of these behaviors you're engaged in. It puzzled the early Islamic scholars, as it had the Greeks before them, why you would not always be able to see everything. If intermission were true and particles from all places were constantly entering your eye. So we now know that an essential aspect of vision for humans is our active exploration of the world via eye movements. So eye movements allow us to aim the best part of the eye, the part where we can see with the highest facial resolution. And we can aim our eyes at specific things in our visual scene. Now the central region of the eye is called the fovea and we'll return to eye movements and their importance for vision and spatial sensing in general, later on in the course. Now a second sticking point was Democritus conception of vision is involving a kind of material particle. If such particles were felt inside the eye and conveyed information about such features as the size of the object, wouldn't these particles then scale with the size of the object? How could really large objects, such as mountains, enter small eyes? And how could the same object enter the eyes of many observers at once? So such issues required some kind of explanation. The debate finally began to be resolved a few decades after the turn of the first millennium. So sometime between 1028 and 1038 Common Era, the intermission theory was resurrected and revised by Abu Ali Alhazen ibn Alhazen ibn al-Haytham. Known in the west as Alhazen. Alhazen argued against the Greek version of both extramission and intromission and actually introduced his own theory. So he rejected the arguments of the extramissionists on logical grounds. Noting that Plato and other adherents of extramission had suggested that something must come out of the eye, merge with light, and then return to the eye. And Alhazen argued that since the critical step was the return to the eye, it was not necessary to postulate the initial coming outward from the eye. So that part was not provable. It wasn't required to explain any known property of vision, and should therefore be rejected. But rather than adopting Democritus's original particle-based intermission theory, he shifted the emphasis to focus on light, as the major ingredient in vision. So while Hazen emphasized that light dictates what can be seen, and what can't, and what it feels like. So, he noted for example that staring at the sun causes intense pain, and staring at something bright such as the sun leaves an afterimage when one looks away. The stars can only be seen at night and not during the day, and the only difference between those two times is how much light is present. Similarly, fireflies glow at night, but they are unremarkable insects during the day. And fine details such as small print on the page are more clearly seen when they are strongly lit. And colors of objects can change depending on how much light is available. So in shifting the discussion from some kind of material emanation, some kind of particle, to phenomena that involve light, Alhazen set vision science on the right track.