 Okay, great. You guys are all nice and quiet, so good to start. Okay, so I'm going to be teaching again for a bit, Henry Lester, and I will be trading off as we get more into systems. And we'll start today looking at our first sensory system, which is the visual system. Before we do that, any announcements from TAs? I think the midterm is other TAs here? No. Okay, well, I think the midterm is posted. Is that correct? Okay, so the midterm is up, so take that soon. And I guess tomorrow you have discussion sections where you guys can review and ask your TAs about content that you've covered so far in preparation for taking the midterm. Any questions about course logistics? You guys are all happy with what things are going so far, discussion sections, etc.? Okay, so we're going to go into vision and as we had before, so the PDF of, I think all the three vision lectures, the PDFs are up on the course website, or at least my two are. So we trade off a little bit. Today we basically go over a sort of an overview of the visual system that focuses on just basic principles, the retina, topography, the thalamus, and that's pretty about it. And then on Friday, Henry Lester is going to talk about photo transduction in more detail, how light is converted into electrical potential changes that the brain then uses to construct the representation of the visual world. And then on next Monday, we'll go into more higher level vision and object recognition. And so the points that are important to note, as before, are written down here. So you can take a look at all of these. The readings overlap to some extent, but they're basically these four chapters, 25, 26, 27, 28, in your book. So read those. And here's then an overview of what we're going to try to cover today. So we're going to have an overview, as I mentioned in general of the visual system, and to talk about early, relatively early stages of visual processing, visual perception, what happens in the retina, how central projections from the retina go into the brain, and in particular the first place that they go, the main place and primates that we're going to concern ourselves with, is a portion of the thalamus. Remember all sensory modalities with the exception of smell go through this collection of nuclei in the brain, thalamus. There's a nucleus there, the LGN, which stands for lateral geniculate nucleus of the thalamus, and then only from there does information get relayed on up to visual cortex, which is parenthetical here, because we probably won't get to it, and instead we'll be covering it on Monday. And so Monday, then we go on, so Friday again, Henry Lester will tell you about phototransduction in the retina in much more detail, and then Monday we'll go on and talk about primary visual cortex, higher order visual cortices, and what it is that these regions do, and a lot of that, a lot of our knowledge of there in humans comes from using fMRI, so we'll talk a little bit about high-level vision there. Okay, so the challenge, just broadly, I've seen these before, I think, is, has been, has always been, to explain how stimuli in the world give rise to behavior, and so nervous systems mediate this, and there's a variety of ways in which people have thought about this. Behaviorism treated this as sort of a black box, and you could just come up with a bunch of rules that would describe law-like relationships between stimuli impinging on organism over time, and learning, and so forth, and how that changed behavior. There are a number of reasons that that fell out of fashion in the 1950s, 1960s, 1950s mostly, one being that this doesn't account for how flexible behavior is, and in particular, even if you take the identical stimulus and present it to, you know, a brain, even brains that are very similar to one another, you typically, you often get different kinds of behavior. So it turned out to be very difficult to predict just on the basis of sort of the rigid kinds of rules that people might have put into this black box, and instead people needed to elaborate very rich internal mechanisms inside this black box, i.e. the mind, and cognition, and the whole architectures that cognitive psychology came up with. There's another component of course here that, until also historically, people have left out, but now people are looking at it as a dependent measure that has some value, which is that it seems like in addition to generating the behavior, the brain generates something else, although the only access that you have to that, at least in the case of people other than yourselves, is behavior. But typically from behavior, and in your own case, not on the basis of behavior, you infer that there's some kind of subjective experience that's being generated by this as well. And there's lots of studies nowadays looking at that as a valid source of data. So that's how things look then going forth in cognitive psychology, and of course, well actually mostly really in humans at least, you know, 1990s or so, so fMRI really wasn't invented until the sort of mid 1990s and really didn't take off until the late 1990s. So it's extremely recent. But this is a monkey brain shown down here. But what we want to do now, what we want to do in this course is to unpack this and not just have some box and arrow architecture that the cognitive psychologists would have had that's informed solely by relationships between stimuli and behavior. But in fact, we can look inside and open this up and with electrophysiology and fMRI and other tools that we have available, we can actually start to really figure out how this works mechanistically in the brain. And so all these techniques have made that possible. So to just take a look at this in a little more detail, this is a picture from Earl Miller at MIT who's a researcher who works in monkeys with electrophysiology. And it's oversimplified, but it gives you a little preview of what we're going to be talking about the next three lectures, which is the visual system. So the story that we would like to tell is a monkey, say a trained monkey, the simplest kind of experiment is sitting there, you flash something on a screen and it has to push a button in order to get a reward when it does so. Typical kind of monkey task. So the question is how does the brain link what comes into the eye, the visual stimulus that you as the experiment put on the screen, to the output from the system, which is the monkey pushing the button, right? So, well, we know quite a bit about what goes on in there, at least in the simplest possible case, we could, you know, sort of trace something like a path through the brain that would mediate this. The problem is, of course, that it's not just that path, but from every one of these places, there's, you know, a hundred different arrows going to many places. Nonetheless, the causally most potent path, so to speak, or at least one of them is illustrated here. So there's transduction in the retina. You'll hear more about that on Friday in Henry Lester's lecture. One feature of transduction in the retina is that it is slow, because it involves second messengers. So it's not as fast as, say, audition, since we transduction in audition or since we transduction for some aspects of touch. Take some time for light hitting the retina to be transduced into changes in electrical potential, and then eventually there's action potentials that go from the optic nerve from the eye into the brain. The main place, 90% of these primates go to this place in the brain, part of the thalamus, the lateral geniculate nucleus that we'll take a look at. And then from there they go to the back of the brain in the occipital lobe which you'll remember is concerned with vision. First place they go is primary visual cortex v1. And then from there they go to second-order visual cortex, higher-order visual cortex, and keep going. And stuff happens here. We know a fair amount about what happens. So early on here these neurons in these regions respond not to complex objects out there in the world, but to simple kinds of features that are closer to what's actually just represented in the output from the retina. Edges, changes in contrast, changes in motion, just very simple features. And then from that they construct representations in these higher-order regions to more complex objects. So for instance here in v1 you would find, or lgn, you would find neurons that just respond to edges or dots of light or something like that. Whereas down here you would find neurons that respond to things like faces or some very complex stimuli. So obviously there are faces and things out there in the world that the monkey wants to recognize. It doesn't have that to work with at the level of the retina. It just has a bunch of action potential that don't get described faces. So it needs to somehow infer faces and build and construct a representation of objects. So that's what's done here. And then eventually if it sees that, so you're flashing something on the screen, the task of the monkey is to push a button, it has to link what it sees, the representation of a face or something on the screen, to making a decision to make an action, push the button. So these parts of the brain have to eventually hook up to motor parts of the brain. So they project to the frontal lobe where decisions are made, then here where actions are planned and in front of motor cortex and these go down to the spinal cord. They're motor neurons and they go to the muscles and the thing pushes up the button. So you can see how this roughly works. It's obviously much more complicated but we will try to give you a flavor for how it works by going through these systems. The visual system here, later on in the class, we'll talk about the motor system and you'll see how this this part here with the spinal cord and the muscles also works. Any questions about the broad scheme here over simplified as it is? But it makes sense to you, right? So that's basically what we want to do. We want to have a diagram that explains this and understand the computations that are being carried out in each of these regions. So one you know one thing that you might say is well all the information really that you have to work with is there at the level of the retina. So why not just hook up the retina to the muscle and you're done. Why bother having a big brain there, right? So presumably there's a need for this and you can't just do that or if you tried doing that it would be much less flexible and indeed if you did that you hooked up the retina to the muscles directly. You would have something that's like a reflex and you could have some patterns but they would be extremely inflexible. It would be very difficult to learn and you wouldn't have anything like the flexibility that you have with this architecture in here. So the question is exactly what do all these extra processing steps in the big brain buy you? What exactly are they doing computationally? So Charles Sherrington who won the Nobel Prize back in the 1930s wrote this book here The Integrative Action of the Nervous System and classified different sensory modalities in terms of sort of their relationship between the body of the animal and stuff out there in the world. So there are teloreceptive ones so there is like remote sensing. So you're sensing something that's far out there that is not at the body surface or internal to you and so vision and hearing are sensory modalities concerned with that. They're objects that are distant to you. You don't have sort of you know direct modes of access to them instead you have to use spectral reflectance of light rays that bounce off of those objects that come into the eye and from that you construct you know the apple of the blackboard or the clock on the wall or whatever is out there. Same with audition there's something way out there and it emits the sound and it's pretty far away and so the computational challenge here for the brain is how to construct a representation of objects that are out there given that at the level of the sensory interface with the world the retina or the cochlea in the ear for hearing you only have certain kinds of information to work with from which you have to make often ill-posed inferences about what kind of distal stimulus would have caused that that stimulus that kind of pattern of light on your retina for instance. Proprioceptive is different so that's different information that you have to work with is not stimuli out there in the world but this is concerned with your sense of limb position in the body so there's lots of sensors in there that we'll talk if you'll hear about when lectures on the motor system. There's extraceptive so this is to some extent also concerned with objects out there in the world but they're not far from your body but they're touching your body so this is the sense of touch so that you know the whole body surface basically and you can explore objects with that and of course with many objects you explore them with multiple sensory modalities and interesting questions would be which sensory modalities give you more reliable evidence than others so you might think for instance that if you hear some kind of sound what do you do well you try and look there to see what's making the sound because vision usually gives you more reliable evidence than does hearing and you know if you can't see it very well you might walk up to it you might want to touch it and so all these different behaviors of yours would be an active exploration of the world that would bring you information from different sensory modalities as you try to obtain information about objects in the world and then of course you know we don't do this so much but if you watch the babies what they will do is if they see or hear something well they crawl up to it they touch it and then they put it in their mouth right so they go right sort of to the closest form of interaction with the object it's not always a good idea but at least then you know right away if it's good to eat or not so chemoreceptive senses smell and taste well we'll talk about as well and then finally one that's off that has been neglected historically but it's probably very important is all the all the information that your brain gets from all your internal organs so your brain gets information from your lungs from your heart from your blood vessels from you know from your bladder lots of information much of most of that is not consciously registered by you so in general you wouldn't unless you're having a heart attack or something you wouldn't know you know that wouldn't really know how fast your heart is beating you wouldn't be very aware of it nonetheless your brain needs all this information for homeostatic regulation of these organs all the time so there is your brain is always plugged into your body even if you're in the sensory deprivation tank or something you're getting lots of interceptive information all the time mostly below the level of conscious awareness so here's the question then for vision and we will have analogous ones for other sensory systems so for those of you that have the PDF or have looked at the PDF of the lecture what's the answer to this question what is seeing so if you had to describe you know to an alien that did not have vision as a sensory modality what vision is what would you say you can't just say well you know it's like stuff that you see you know because they wouldn't have that what you know what could you what definition could you give anybody want to say what it is have you looked at the PDF do you have the PDF or independently of the PDF do you want to take a stab at the question yes olivia good okay so indeed that is the answer on the PDF to know what is what is where by looking close close to that this is the answer that a famous vision scientist david marr in his book vision wrote back in the 1980s and it's a good answer because it illustrates some of the computational problems that vision is trying to solve and indeed gives us insight into what different parts of the brain are doing when you see something so this part here to know is obviously a little problematic so typically you would infer this from the behavior of an animal but then this part here is informative so you have to identify what something is and there is a sort of a pathway in the brain that will be taking a look at that is concerned with object recognition so it figures out what things are in the world so if you look around you right now you can recognize what objects are you know tables faces clocks etc so that's one kind of information and it's processed mostly in a stream that goes ventrally from visual cortex into the temporal lobe concerned with object recognition to figure out what objects are in addition to knowing what something is you don't just see faces and chairs and clocks but you also know where they are located in space and relative to one another so spatial location where things are is obviously pretty important too because it allows you to act in the world and that turns out to be processed by a different visual stream that goes dorsally from visual cortex into the parietal lobe so your brain needs to figure out a lot of things but to first order these two are the broadest processing streams object recognition concerned with figuring out what things are and then some stream concerned with figuring out where things are in space to guide action towards them where things are and you do this not by taking a picture like a camera would but by actively exploring the visual world with your eyes your eyes are always moving around and so you do this by looking so these things emphasize that vision consists of separate separable problems you have to do figure out what something is where something is and you do it actively by looking okay so this just says what I said um David Maher in this book um thought that one good way of trying to gain insight into how the brain does this was to decompose the problem of vision into these three different so called levels of analysis so at the very top is a sort of computational theory what's the broad goal so this would be a goal that animals even with eyes and nervous systems very different from yours as long as they have a similar kind of environment would all need to solve so you know flies have compound eyes that look completely different for me their nervous systems are totally different but many of the same computational problems would be similar because they should they share a common world with the same laws of physics the algorithm that nervous systems implement this gets more detailed and this again there's a lot of similarity across different species and nervous systems but there are also a lot of idiosyncrasies so some animals have solved these computational problems with algorithms that are different from what other animals have used and then the actual hardware implementation how you actually do this in eyes and brains differs a lot across different animals so eyes evolved independently many many times and many different kinds of eyes and the many different nervous systems but many many animals can use light to you know find out something about the world so that that's one way of thinking about it what are all the cues available so there are many and we're not going to really spend any time in detail going through all of these but it's it's worth just listing them and you know having a list for for how this might work so where something is projected onto the retina because of the optics of the lens gives you information about where it is located in the visuals visual field out there how bright it is it's like related to the luminance the ratios of wavelengths in your retina that are activated by it gives you information about the color and so forth so there's a particular cues that the visual system can then use of make use of in order to infer these properties here on visual stimulus and the other thing to point out as you might imagine is that if you look at the retina in general there will be a lot of correlation between receptors that are spatially adjacent so as we'll see and the visual system will work away through this in just a second there's a lot of topography but neurons that are similar to one another if you don't do anything else will be very highly correlated because objects tend to not be too discontinuous in the world and so because of the way the of statistical regularities in the world there's an apple or something out there then all the neurons that are kind of close to that and represent the apple will all be you know representing red round whatever the properties are of that apple and in the retina and in other sensory systems one thing that sensory systems do is try actively to decorrelate spatially adjacent neurons in order to maximize the information that they can transmit so let's just go through the different stages of processing so transduction happens in the retina and photoreceptors and that's just converting light that's coming in into changes in electrical potential from there on that is everything that your brain has to work with so it just has to work with whatever electrical potentials are generated in the retina we'll take a little more look at that today and then Henry Lester will talk to you more about it on Friday then there is sort of early perception where so these are computational processes typically early in time and early in the visual process in the streams of visual processing neuro anatomically that are concerned with representing what's out there but not yet linking it to any anything that's particularly synthetic or constructive so just seeing you could think of it as you know what a baby if its eyes were like your eyes would be able to see or what other animals you see a frog or something could see and if you showed it a face so presumably you know a cat or a frog an animal with a reasonable visual system if you showed it a face of obama say would have some representation in terms of the lines and you know where there are changes etc it wouldn't necessarily recognize this as a face and it certainly wouldn't link this to knowledge that this is the president of the US so it would have early it would have just visual representation that's not yet linked to any other knowledge of the world which is something that your brain would do so this is early on and then what would happen is that later on your brain would link this with other information that it has stored and he would recognize the face as the face of a familiar person president obama or your friend or whatever it is so but there's stages that you have to go through this doesn't happen in one step and these are dissociable we know that these are to some extent not to first order discrete stages of processing because you can have damage to the brain in certain patients that can dissociate these so you can have damage to higher order parts of the brain where people fail to be able to recognize faces they can't recognize that this is Barack Obama in extreme cases they can't even recognize it's their own face in a mirror but they can see the face just fine it's not that they don't see the face you just don't know what it means and they can't link it to its meaning they can distinguish it if you show two faces side by side they can perfectly well tell them apart but they can't recognize who it is so recognizing who it is isn't it's not sufficient to just represent the information that's there in perception you would need to link that to other kinds of information in particular information that's stored broadly speaking in memory that that you've learned you've learned that this face happens to be the face of the president of the US and his name happens to be Barack Obama and all the other knowledge that you have stored but that's not in the in the early perception so your brain would just have a visual representation of the face that that that then needs to be linked to lots of other information so that you recognize it as the face of Barack Obama etc and of course this you know this happens very automatically and effortlessly in most cases but it nonetheless depends on discrete processing stages we know that they're discrete to some extent because we can dissociate them and then after that there's many other things that come into play so you can make judgments on the basis of what you recognize so you might think well I see a face I recognize it as Barack Obama I remember that that's the president of the US and I think he's not a very good president or whatever judgments you have and then you might plan an action on that and eventually link this to behavior okay so basic very broadly speaking this would the stages these stages of processing would map onto something like sensation knowledge about the world forming a belief and then acting and making a decision on the basis of that belief so there are various tasks there are also words for tasks that map onto these discrete stages of processing and then as I mentioned there are disorders that you have in patients who've had a stroke or something to the brain that allow you to some extent to dissociate those processing stages so the very simplest task would be detection so in this task I would simply ask you was there something there or not so I show you take a face on a screen and all you have to say was there something there or not there's no recognition there's no memory needed it's very simple discrimination is a little more complicated and you can do this across time or you can do it contemporaneously but this would be is this face the same as this other face so I'll show you two faces you have to do a bit more you can't just say there was something there you have to represent the details of the face in order to distinguish it from other often very similar looking faces but you don't need to recognize any of them yet categorization you still have to do more you have to you know say these are female faces these are male faces these are happy faces these are angry faces and then recognition would mean that you could link the face say to a name to all the conceptual knowledge you have that of that person and then of course you know that the name is the final one and so these can be to some extent dissociated and you don't need disorders to dissociate them it happens to me and it probably happens to you all the time that you see a person you recognize them and you know who they are but you can't come up with the name you have you know the name but you can't link your recognition of them to the name you just say that's you know and you serve on the tip of your tongue but you can't come up with the name if someone tells you that that's comes up with a name then you can recognize it as the correct name so it's not that you didn't know it but you just couldn't link these and there's some disorders that have that in the extreme case where people can tell you everything about a person except their name there are people who have amnesia so they would have memory problems they can recognize and perceive objects in the world but they can't remember anything and then various forms of so-called agnosia we'll take a little more look at these on Monday where you dissociate perception from recognition so people could discriminate faces but they wouldn't be able to recognize them for instance okay so let's go through what the the visual some basic properties of the visual system probably the most important one is topography so topography remember is a mapping in the brain to the world and in the case of the visual system the topography is retinotopic in some other sensory systems the topography is something else but so it's retinotopic so there is a map of the visual world on the retina it's just inverted and that's just due to the optics of the eye so it's because of a lens here that you know it's just like shown here you have an inverted image of this cup that is there on your retina that topography on the retina in turn is preserved at higher stages of processing so you also have topography in the lateral geniculate nucleus and you also have topography in v1 and indeed you have some topography it starts breaking down more as you go to higher visual regions they're maps of the visual world in your brain as a consequence of the optics of the eye basically any questions about that that's clear to people okay and so the eye looks like this you have a cornea you have a lens in our eye and this has a refractive index that's greater than water and so it bends light just like a glass lens wood in your camera only much less so and projects an image then an inverted image of the world onto the retina and so then what the retina needs to do is to transduce this into a pattern of electrical impulses that your brain has to work with there are many different eyes that evolved some pictures of them and as I mentioned these animals often have similar computational problems to solve in some cases the algorithms are even similar but the hardware hardware can be very different so for instance the cephalopod the nautilus here doesn't even have a lens it just has a pinhole eye very very simple insects like flies or dragonfly I guess this is have compound eyes you know other animals have weirdly shaped pupils I guess a goat or something up here octopus and for instance here some animals have very very specialized eyes spiders are good for this so if you look at spiders at that hunt well all spiders hunt but like wolf spiders or jumping spiders are very good they have different eyes that do different things and one interesting thing so for instance with respect to spiders their eyes are fixed in their exoskeleton and so they can't move their eyes unlike you can move your eyes to explore the world so how do they explore the world well they can move around their whole body around but another way that they can do that is to move the retina rather than moving the eye and actually this is what you see here so in these these spider eyes here this vertical slot this vertical line here is the retina and if you watch these you can actually see this and jumping spiders if you have like a binocular microscope it doesn't have to be too high powered you can move your finger around and you can see in the little spider that this slit will move from left to right and scan like a flatbed scanner it'll sort of scan scan the world so they have a a vertical retina here that can that can scan rather than moving the pupils here's one of the earliest drawings in this case of a bird retina by Romoni Kahal that makes the point that the retina has a very crystalline very specific organization it's maybe not quite as beautiful as the cerebellum but nonetheless it has enticed many people to work on it because of how how nice it looks under a microscope Marcus Meister I hear as was a professor in biology for instance works on the retina and tries to figure out you know given this nice architecture how this looks if we stick electrodes in here what can we find out about the computations that these differently shaped neurons are doing and contributing to vision all right so here's some factors about the retina the one main one immediately to note or two main ones one is that the cells that transduce light into electrical potential the photoreceptors that you'll hear much more about on Friday come in two big flavors cones which can distinguish very fine things that are concerned with color vision and rods that are much coarser but much more sensitive and to give you your night vision there are many many more rods 100 million or so then there are cones 5 million or so there are only about a million axons that leave the retina and go into the brain so right away you can see that there has to be a massive convergence in particular of the rods onto the output of the retina so each axon each optic nerve fiber that goes into the brain must get input from many many photoreceptors as a consequence of that convergence these can be quite sensitive so all you need is you know a few rods to detect like a single photon of light and that might be sufficient to give an action potential going into the brain so there's a lot of convergence and there are many more rods which are concerned with the most sensitive form of vision like night vision then there are cones kind of what you would expect so if we magnify a little bit of the retina so here's our eye here's the lens up here and here's the retina down here it would look like this and there are a couple of weird features about it one main one is that light comes in and has to go through all the processing layers until it gets to the photoreceptors so the photoreceptors are down here they're down towards the inside of your retina close to something called the pigment epithelium that's concerned with recycling these photoreceptors and nourishing them because they have a very high metabolic rate and then on top of that are all these other parts of the retina and up here there would be retinal ganglion cells and axons that then would go run on the inside of the retina and then exit here where the optic nerve is this is where the blind where your blind spot would be so as a consequence of that the light has to go through all of these different layers until it gets to the photoreceptors your retina is inside out compared to how you might think it would be best to engineer it if you wanted to have the best access of light to the photoreceptors and so it's illustrated again here this is now flipped with respect to what we just saw so light is coming in here photoreceptors are up here and then there's these layers of processing down here these are retinal ganglion cells either the cells that send that make action potentials and send them out through axons that constitute the optic nerve that goes into the brain that the layers of all of those axons that keep getting bigger and bigger as they get closer to the optic nerve are closest to where the light comes in and then there's the retinal ganglion cells then there's other cells and from farthest away from where the light comes in and the inside of the retina are the photoreceptors so it's inside out there what this shows if you know a little bit more this is a bit jumping ahead you can you would be able to tell me which roughly which parts of the which part of the retina this is an illustration of so it's not the fovea which is where you have the highest spatial resolution because at the fovea you have only cones so the fovea has no rods and just has cones you have very good color vision there you have very good spatial resolution the rods increase in number as you go more peripherally this is why if you want to see something that's very faint like a very faint star at night you can't look at it directly it'll go away because if you foveate it your fovea only has cones and those are not good for night vision you have to look off to the side a little bit and then you can see the star because of the rods that it is projecting onto so these are the photoreceptors they transduce light they have graded action potentials and there's then a lot of different processing layers that eventually give rise to action potentially with the brain and all of these have different names that are illustrated here one question we don't have much time to devote on this although Henry will mention a bit more on Friday is why you see in color so you have three different kinds of cones that have different photo pigments and the relative combinations of those is what allows you to see in color most mammals can't see in color they don't have trichromatic vision as you do most mammals do not and so a dog doesn't a dog would see only this year and so if you showed in particular most animals are red green color blind and so if you showed a picture like this to a dog it would be you know would look more like this and so one question it seemed it's very sort of prominent in our conscious experience of the world to see lots of colors and one question that's not the answer to which isn't well known it isn't known is why we see in color in particular red green color like this one answer maybe like what's illustrated in this picture here I don't actually know what these are maybe cranberries or something currents very currents one answer maybe that it helps us to break camouflage in order to identify something that was very important in the diet of our ancestors which is ripe fruit so when fruit get ripe they turn from green to orange or red and it would help to be able to see that and so that in the kind of trichromatic color vision that we have that allows us to distinguish between red and green allows us to do that of course some people in particular in males because the genes for the color pigments the absence that code for these are on the X chromosome some people are color blind so red green color blindness is affects about one percent of the population so if an ophthalmologist ophthalmologist when you go to the eye doctor would look inside your eye to check that everything's fine and they see all the blood vessels in there and we can superimpose onto this where the phobia and other parts would be so the blood vessels all come out where the optic nerve exits the retina so that's called the optic disc so there are all the blood vessels come out and then they go on the inside of the retina to nourish the retina and this would also be where your blind spot is because this is also where all the retinal ganglion cell axons exit as the optic nerve and they have to do that by going through the retina since they're on the light side of the retina on the side of the retina where the light hits down here some distance away from that you have a region it's a little darker it's called the macula this is a problem in older people where you have macular degeneration if this degenerates it's very bad because it happens to contain the region of the retina that has the highest density of photoreceptors in particular it has all cones that are all squished together very tightly and so you have really good spatial resolution because the spatial packing of the cones is so tight and because there is topography because of the way that the lens projects the visual world onto the retina so what you try to do when you read for instance is you move your eye so that the text that you want to identify falls on this part of the retina the phobia if you don't do that it's going to look blurry and so the spatial resolution is something like this if you fix it if you look at the center here then the idea is maybe it doesn't work for everybody here that you should be able to read all of these letters equally well so this exercise shows you that right around your retina you have much higher spatial resolution because you can read these tiny letters whereas if you're looking here at the center then you need really big letters out in the periphery in order to be able to distinguish them equally well and that's because the cone packing is so tight in the middle but falls off as you go out so this baby monkey here just illustrates the point that you explore the world with your eyes so this monkey is like many animals that would be very interested in other animals the monkey is very interested in the human face just like we are very salient stimulus and so it's exploring the visual world with its eye and head movements and trying to fixate the face in order to process the information so what your brain or what your retina has is a picture something like this that you're making eye movements rapid eye movements, saccades all the time your resolution is greatest in the center and blurry as you go out and so this is what it would kind of look like if you made a movie of how things look at the level of the retina now it doesn't look like that when you look around the world so presumably your brain has lots of mechanisms to compensate for these rapid eye movements and make sense of the world in a much more stable way but so this is one the basic aspect then of the active part of vision that we had in that little answer to what is seen you might detect something out there in the periphery you have great sensitivity there because you have lots and lots of rods they're good for very faint things they're also good for detecting motion so if there's something faint there's something moving off to the side you're very good at detecting that there's something there but you can't recognize it because you're unable to get sufficient resolution to construct a good image of what it is so the first thing that you do is you move your eyes or your head or both so that you fixate this so this something out there in the periphery will grab visual attention that directs a mechanism that moves your eyes sometimes your head so that you foveated so whatever was moving there is now no longer in the periphery but you're looking straight at it and then you can process it with your cones and identify it and get maximum information of course if you're too far away you would move closer and so forth and do whatever it takes to get the information that you need to be able to identify what it is so it's illustrated here with this plot in terms of eccentricity on the x-axis so the fovea here is at zero and as you get more to the side either to the side of your head temporarily or towards your nose nasally that's plotted on the x-axis and on the y-axis is the density of different photoreceptors so remember the rods sensitive for night vision they're very dense in the periphery but they plummet in fact they're none at the fovea the cones by contrast are essentially absent in the periphery and they really shoot up and get really high in the fovea and how this might look if you looked at the retina in a schematic way with the rods being red and the cones being these little greenish yellow ugly things is illustrated on the top so in addition to just these ratios of rods to cones changing what changes is the size of the cones the cones are pretty big and sparse in the periphery and they get really really dense and smaller so that you can pack them very tightly together at the fovea let me skip this one okay so this so as before there's a couple of boxes with text here in the PDF of the lecture that summarize the main points that you need to know so this is this little answer that we had here visions to know what's where by looking different species might implement this in different ways have any different eyes but they might all have to solve same computational problem the retina is very complicated already there's lots of processing that we haven't really talked about and then there's lots of different kinds of computations that happen in the retina one main one is that there's huge divergence and especially in the case of rods there are many many rods a hundred million there are only one million axons exiting the eyes so there's at least a hundred to one compression ratio there's some divergence as well so at the fovea you can have cones diverging to some extent and there's more complicated processes like decorrelation in the retina as well okay what happens next so from the retina we go into the brain the retina projects to several places in the brain you need to know just the names of these very vaguely not much detail about all of them but except for the thalamus so most of the input most of the output from the retina most of the retinal ganglion cells in your retina and in monkey retina about 90 percent project to a nucleus in the thalamus which is in the middle of the brain called the lateral geniculate nucleus some of them also project to other areas so they project to something called the optic tectum which you may remember from the development lecture this is the one that the the counterpart to visual cortex that animals like goldfish or frogs have this is the part of the brain that Roger Sperry studied in his chemo affinity theory so the superior colliculus is the mammalian homologue of that so fish and amphibians it's called the optic tectum in us it's called the superior colliculus but that's the homologue that we have so there are also projections from the retina in your eye to the superior colliculus in your brain it's pretty small what does this do it's not concerned with object recognition so this doesn't help you to recognize objects it's probably it probably contributes nothing to your conscious visual experience either but it is concerned with making rapid eye movements and head movements so it has something to do with very rapid reflexive kinds of movements in order to detect stimuli which is something that you might imagine you would want to do in a very quick and perhaps separate way independently of conscious vision and object recognition so that's what that part does your retina also projects and again a small percent only to nuclei in the brain that are concerned with regulating how big your pupils are so if the light is very bright your pupils will constrict if it's very dark they will open up as you would expect and there are mechanisms in the brain there's a circuit that regulates that that involves other kinds of nuclei there are parts of your brain that those of you that have experienced jet lag know about that you know we have circadian rhythms and so the light dark cycle that you experience and trains lots of bodily rhythms and there's a part a nucleus in the hypothalamus the suprachiasmatic nucleus scn that gets input from the retina that has to do with circadian rhythms one feature that's important and different from many other sensory systems is that there is no feedback at all from the brain to the retina so the retina projects purely feed forward into the brain there's no feedback and that's completely different from every visual processing stage subsequently so at the thalamus at the level of the thalamus at the level of visual cortex there is a huge amount of feedback more feedback than feed forward but not at the level of the retina retinal ganglion cells only project to the thalamus they get no feedback from the thalamus so the visual system you remember from the first discussion section where we looked at a human brain we were able to identify a few things the brains weren't in as good a shape as this brain but you remember we saw the olfactory nerves cranial nerve 1 that were stuck on the front those are those things up here and we were able to identify as here the stumps of the optic nerve so those are those white things we're looking at the bottom of a brain here there's the front the top there's the back the bottom so we've turned the brain upside down the spinal cord would be coming out at you if it were attached to this brain and these would carry the million axons from each eye so the eyes would be somewhere up here and then the optic nerves would be running in so the stumps of the optic nerves are here parts of them cross at this point called the optic chiasm such that the optic tracts which are now the continuation of the optic nerve after some of the fibers have crossed are arranged such that the left optic tract has all the information from the right half of the visual world and the right optic tract has all the information from the left half of the visual world that's not the case at the optic nerve the left optic nerve has information from the left eye that has both left and right visual fields represented and the right optic nerve has information only from the right eye that has both right and left visual fields represented so they cross in such a way at the optic chiasm that rather than corresponding to eye the optic nerves now map to visual field left and right visual field and they go into the thalamus here and all these other places that I mentioned that you don't need to know about in detail from the thalamus there are then projections which they've dissected part of the sprain for you to be able to see to occipital cortex back here primary visual cortex these projections one thing to appreciate is that these projections here from the thalamus back to v1 are much bigger there are many many more axons here then came in from the eyes in the first place and that's typical so you have lots and lots of connections and lots of processing in the brain that looks much denser than the information you get from the world because in a sense your brain is sort of creating information by the computations that it does if you flatten the monkey brain and sort of map out the different parts of visual of the visual system that's shown here in this famous picture from a guy called David Van Essen who was once here at Caltech teaching this course actually so here would have the retina they would go in through the optic nerves they would mix in a certain way they go to the thalamus lateral geniculate nucleus of the thalamus primary visual cortex and then there would be many other visual cortices with different parts of visual cortex that are all concerned with processing different aspects of a visual stimulus some are more concerned with processing color some are more concerned with processing motion and so forth so let's take a quick look at where these are in the brain again here are your four lobes that we've seen before cerebellum and the brain stem and you remember if we take a look at the medial part of a hemisphere here so this is the right cerebral hemisphere we're looking at its medial aspect here's the frontal lobe here's the parietal lobe like here's the occipital lobe and in this on the upper and lower banks of this sulcus here which is called the calcurin sulcus lies primary visual cortex which is also known as broadman's area 17 so these things you need to know you need to know the name of this the calcurin sulcus you need to know that this corresponds to primary visual cortex and that corresponds to a cytoarchitectonic region called broadman's area 17 so this is the first cortical area that gets visual input and it has a map of the visual world and then next to that you have higher order visual regions v2, v3, v4 and so forth so one theme you remember is that neocortex is organized into maps these are topographic vetonotopic in our case and that's the higher order cortices are adjacent to the primary ones we just saw that here okay I remember that in the case of vision as I've just told you all information has to go through the thalamus before it gets to cortex and that's the case for all sensory modalities except for smell the thalamus looks like this if we have the brain here here's the corpus callosum here's the thalamus it's a collection of a whole bunch of nuclei schematized down here and in your brains it looks sort of like these nuclei here the ones that you need to know about is the lateral geniculate nucleus that's concerned with vision the medial geniculate nucleus we'll cover in a later lecture that's concerned with hearing with audition and then these ones here are concerned with touch vpm and vpl the ventral posterior medial and ventral posterior lateral nuclei so a specific nuclei of the thalamus that relay information from sensory modalities to the primary sensory cortices visual cortex auditory cortex some kind of sensory cortex the lgn lateral geniculate nucleus looks like this there are layers to it six layers in total it's topographic retinotopic so if you marched your electrode like i'm doing with this pointer across this the tissue of the lateral geniculate nucleus you would find neurons that respond to stimuli located in particular parts of visual space there's a map of the visual world and then in addition this structure begins to create other maps so it has it maps for instance input from which eye it's coming contralateral eye up here ipsilateral contralateral ipsilateral and so forth and it starts to map input from different retinal ganglion cells down here ones that are concerned with more fast kinds of processing ones up here that are concerned more with color and detail and so it begins it begins in the retina and it's more clear in the lateral geniculate nucleus and we will see it on monday in in the visual cortices that there are different processing streams and as i mentioned two really broad ones are concerned with our answers to that initial question so there are different parts of this lateral geniculate nucleus and different processing streams from the projections of this region into cortex that have to do with figuring out what an object is object identification and where it is located in space for instance so different functions okay to just finish off you should just look at these pictures in the pdf and also in your book it's fairly straightforward so i'm not going to there's no point going in detail here but just look at the pictures you know this will map out for you in detail where if you have a stimulus in the world that stimulus falls on the retina and how that then projects to the lgn and higher up the most informative picture sorry it has little animations in there the most informative picture is is this one here somewhat overwhelming but you can work your way through this so if you show someone an image like this if you're staring at the center here then all these differently colored quadrants that are up or down or left or right or central or peripheral coded by color will fall onto different parts of the retina so if you're staring at this in the middle the yellow and the green parts will fall near your fovea so that's right in here yellow and green and then the red and the blue will fall more outside the fovea and they will fall in different parts of the retina because of the optics of the eye and the retinotopic mapping we have and then the retinal ganglion cells from their project to different parts of the thalamus and different parts of visual cortex so work your way through this and look at the book so that you know how topography in the brain is organized perhaps just to go back to this one picture one to first order a lot of things are flipped so the image on your retina is flipped already to begin with but this is also the case in visual cortex so the upper banks of v1 the upper banks of the chelocrine sulcus map the lower visual field the lower bank of this sulcus maps your upper visual field so the visual world is flipped there and we'll take a more detailed look on monday on how this looks and also go into electrophysiology so last couple of slides here we'll skip for now because we're out of time and we will start with those on monday