22. Visual systems, part 2

Key Takeaways

the following content is provided under a Creative Commons license your support will help MIT open courseware continue to offer highquality educational resources for free to make a donation or view additional materials from hundreds of MIT courses visit MIT open courseware at ocw.mit.edu the end of the last class I mentioned that uh this invasion of the end brain by visual Pathways uh we know that mostly it even though there was a direct pathway it was very small the one through the genicular body very small early on major Roots into the end braan came they began at the tectum also the pretectal area which projected into thalamic areas which then projected to the end brain and so I'm asking I asked there at the end of the class what what what were the Adaptive advantages of that why would it invade the inbre uh and first I I try to give an anatomical answer it's a it's a way to provide better information to this trium remember visual information could reach the striatum uh directly from the older phals but those structures weren't precisely topographic you know and the striatum wasn't a structure that made really good topography as easy to form in evolution now that's my interpretation obviously it that happened mostly in the cortex in the cortex you you you get this highly topographic mapping of the visual world and those Pathways came by way of the midbrain and directly from the geniculate body so that enabled a lot better Acuity uh for the learning that the end brain was capable of especially by means of its Pathways to where two different places to the striatum rehabit learning and to the hippocampal formation we're learning about where Lo locations of the animal in the environment not the locations of other animals the location of the animal whose brain we're talking about okay so the cognitive functions were that root to the hippocampus for Place information is a major part of our cognitive ability our memory formation but also another kind of memory it provided a visual root to the amydala which is really part of the striatum in the way I see it sort of a at least a sort of a codal output of a striatum it functions like a striatum for the learning of avoidance and approach okay and then also binocular vision which we won't talk very much about now but that was important too because the neocortex made made possible a lot better binocular vision by especially stereoptic stereopsis slight difference in the visual image from the two eyes because the information from right eye and left eye is kept separate all the way up to the cortex where then you can combine that information get depth at the end of that chapter 20 I mentioned I believe it's in chapter 20 I mentioned some of the expansions and specializations of the visual system mainly mid-brain tectum and the phm cortical projections of the visual system the midbrain Tech the these are the two major structures and we're going to look at that today uh just one second here uh and we'll be talking a lot more about transcortical Pathways in the following class and then I don't deal in the book with the tremendous differences among animals in retinas you develop many retinal specializations and animals you don't need larger eyes just to get a larger retina and get more retinal gangling cills and more Acuity that way what else you need large eyes for bring in more light why do you buy a camera with a big lens rather than a small lens brings in more light okay if you want to get a camera that can take pictures in very dim light get a camera with a very large lens opening like f1.8 or something like that it'll take in a lot of light okay question excellent question we we we know that animals with forward-looking eyes they're usually predators and primates and primates are usually Predators too but not all of them some of them have fruit Gathering primates but they also need good biner Vision so they have forward- facing eyes too uh but what about the animals with these eyes that like the the hamster it's got eyes that look 60° out and 30° up Horizon that's way their eyes look that's the natural position of the eyes I've measured it in the hamster but it's pretty similar in other little rodents they do have an area of binocular vision there's about at least 30° overlap okay and in that Central area which the center of it is 30° above okay they do have binocular vision okay and they but it's pretty rudimentary compared to the primates and also they don't have the High Acuity that primates have but it's still important for them but I've looked at hamster Behavior a lot and they they really things 5T or even 2 fet and 30 ft away they don't discriminate very well you know but for things near them they definitely can judge Vision they won't leap sometimes they do but usually they won't jump into places that are too dangerous because they're too high they can see the depth well enough it doesn't mean stereopsis cues though it could just be the the uh various other cues like when they move their head The Parallax that you get uh there's various cues to depth but if you study sensation perception class here at the MIT you'll learn about multiple different different various cues for depth okay that don't involve stereopsis all right so now we want to look at the retinal projections it's a pattern you find in all the mammals and I should add that it's really similar in all of the vertebrates okay okay this is the picture I showed in the last class the sort of a cartoon of the optic the main optic tract and the accessory optic tract here just showing so now we want to look at a little more detail of the layout of that system from supermatic nucleus to the two geniculate nuclei pretectal region where there's several nuclei where they terminate Superior cicular so we look at the retinal projection in vertebrates the layout of the optic track and we'll look at them in several different ways of looking at it we'll talk about species differences in relative size of structures because qu qualitatively they all have the same basic layout but there are large species differences in relative sizes as well as in the eyes are different too so you have different different amount different levels of accuity Invision are possible in these different species we'll look a little bit at the at architectural differences especially in the genicular body and in the optic tectum so we'll look at lamination in the midbrain tectum we'll also look at lamination in the geniculate body and finally we'll look at topography and I know when you first get all this you get a little overwhelmed by all the structures and everything so I'm not asking you to memorize the topography but I want you to see I will present it to you in a way that does make it pretty easy to learn okay and these are the questions I put in the list of questions I put online for you these are things we've dealt with before these two questions let's just see if any of you have an idea about this first of all what's the first structure reached by the accents from the retinal gang cells Gan cells project through the nerve we start calling at the tract when it joins with the rest of the dianon so what's the first structure exactly supermatic nucleus right above the crossing of the accent so it's got to be the first one it's part of the hypothalamus definitely okay the area in front of that region we call preoptic area of the hypothalamus because it's in front of the opticis actually we talk about the anterior hypothalamic nucleus and then the preoptic nucleus so I modify my first statement a little bit so the four brain subdivision you've just named it's the hypothalamus and the major terminal nucleus in that subdivision is the super I nucleus now remember there there's some spill over there too uh that I showed you last time it does project bit to immediately adjoining parts of the hypothalamus but we know most about that supermatic nucleus it's very little known about the functions of the sparse projections people tend to think that they don't play any major roles it's why they're so sparse uh I'm not convinced of that uh they could play important roles you uh but the one thing that makes me doubt it a little bit is just that there's variability when I study the retinal projections in a number of different hamsters I do get differences in these very sparse projections some animals I see more some I see less but they're always there okay okay the next question is what's the major difference in the nature of the projections of the dorsal Thalamus on the one hand and the ventral Thalamus or the epithalamus that is after you go past the supermatic inuse the accents go up the side of the dlon and they reach the sub Talus just above the hypothalamus then they reach the dorsal Thalamus then they reach the epithalamus the pretectal nuclei in the epithalamus so there's a big difference though in the nature of the projections of the dorsal Thalamus and those other groups what is that difference exactly biggest difference is what she just said the dorsal phalance and all the major cell GR the the at least the Neo Thalamus that more recent but we think we of the dorsal Thalamus like the lateral genicular body but the same thing holds true for the ventral nucleus mediodorsal nucleus uh medial genicular body IT projects to Cortex okay mainly to neocortex it goes to the embran some neurons some accents will have collaterals in the them on their way to the end braan but the major projections to the end braan those other structures subs and epip Palms don't project to the end braan I mean there were reports you know that they do but modern techniques have never been able to verify that they don't project to neocortex in fact besides some ascending projections they have a lot of descending projections okay so that's the major difference okay now I last time I mention the epithalamus as a codl most dianic segment or neuromere it includes cell groups where the optic track as dense terminations so again what do we call those areas of termination we call them by we call them certain nuclei depending on where they are they're in front of the tectum so we call them prector okay it doesn't they are rather different these different groups and they have different functions but we lump them together because they're all in that same region call them the pretectal nuclei okay so this is two ways to look at don't worry I'll blow these up but here's the crosssection of the level through the Dippin this is actually behind the optic kais but you see the optic tract covering most of the dianon or tween brain at this level the pretectum up here dorsal thamus with the lateral geniculate nucleus right there here's the subthalamus with the ventral nucleus of the genicular body there there's hypothalamus okay this is behind the opticis and the optic track C goes covers all these that's why that area is sometimes called optic balance just because the aons of the optic track cover it and then these are the subdivisions written as the names for their the neuromeres now we know hypothalamus is actually two or three different neurom mirors and then we call this the ventral Talus then in the adult we call it sub Talus then the dorsal pelis then the epis okay but now there's another way to look at the track that I use for teaching purposes but I think it's very useful I just take the track all the way from the supermatic nucleus all the way to the tectum just stretch it out okay so you see the retina would be way off here to the left off the screen there to the left there's the super chiasmatic nucleus it's more commonly abbreviated scn here we call it just but could be done either way and then I just follow the accents straighten out the tract okay and I show what happens to them the longest ones make it to the superior cicular where they no longer travel mainly at the surface the rest of the tract is mainly at the surface there are some that travel internally we call that the internal optic track but there's smaller numbers of accents that do that the main ones travel to the surface except when they get the sphere calculus actually they do travel at the surface of the culus early in development when they first get there so then how do they end up down below those superficial layers what has to happen they're on the surface and then they're not with further development either the axons that were on the surface have to die off or the cells in the calculus migrate up through them they're still migrating when this tract is farming and the latter interpretation appears to be true because you do see cell migrations going up to the superficial gray occurring after the first accents have reached there and when you look with stains that are capable of seeing degeneration occurring you don't see a lot of degeneration okay so we think it's it's actually doe with the cell migration okay now here you see them it looks like they're branching off let let's do it this way here's that first picture again where I blow it up but it's the same thing and I point out here ventral Thalamus is not the ventral nucleus of the dorsal pamus it's really subthalamus okay the vental nucleus is this nucleus right up in here part of the dorsal pal so here I just show the places the optic crack terminates you see the dorsal part of the lateral genicular body now remember this is a road the lp is not huge but it's the rest of the lateral phalam besides the genicular bodies you have lateral thalamic nuclei the the medial geniculate is another lateral thalamic nucleus uh here is a human 12 weeks post conception okay okay so 3mon human fetus there's the rodent adult here's the human Fus the adult human FS doesn't look like this at all okay I just want to point out that it's really the same as the rod in fact the embryo looks very similar okay but then it changes why anybody figure that out what happens to the human fals development after that first 3 months of Life the brain of a 3 month human fetus is pretty similar to a hamster just after birth but then something continues developing in the hamster in both the hamster and human but mostly in human after that first three months that means by the way the hamster at the human at 3 months postconception is like an early postnatal hamster and the hamster is born in the 16th day of gestation just to give you some times but anyway what happens this nucleus n labeled NL here the lateral nucleus nucleus lateralis this is the posterior part of nucleus lateralis okay that grows and grows and grows and in primates it really gets huge and we change its name we call only part of it the lateral posterior nucleus like we call the whole thing in rodents the rest of it we call the pulvinar which means pillow the pillow because it's bulging out so much because the cortex IT projects to even though it's there real early it expands so much and Thalamus you know a lot of people that talk about cortex just think the sensory input comes to it then you go from one region to the other in the cortex then you go out through the motor cortex not a very good accurate picture even though the majority of people in this building think like that that's not really a good picture connections of the cortex so we'll be dealing with that including some next next class but we'll continue dealing with it for the other systems all right so that's what happens otherwise this this at this stage these pictures are pretty similar okay and remember those divisions here happen because here an even earli very early embryon embryo where I show a side view with the segments remember the seven ROM mirors here then a little segment we call the ismos and then the midbrain which is a pretty big segment and then the epipal with the pretectal farming area farming here the vanela farming here and then dorsal thalamus ventral Thalamus or subthalamus hypothalamus which is actually turns out to be several subdivisions okay so then these are the questions we want to answer now I want you to be able to name the five main optic track termination areas in the order they're reached by the optic track so here they are they're already pass the supermatic then you have the two geniculate bodies then they get to the pretectal area here I show them on this picture and then finally they they turn codly and they get to the spir click so those are the areas one supermatic two three four and then five Superior clickers and then I ask what addition areas receive sparse retinal projections well I already talked about some near the supermatic nucleus but there's some other sparse projections in the phalance that are somewhat variable from animal to animal at least among the hamsters that I've studied I suspect it's true of other species too and that would be projections to the lp nucleus here okay so and there are other little projections near the main projections that are also a little bit variable and then I say inputs from the right and left eyes terminate in different areas a separation that's especially important for creating binocular disparity cues for depth for perceiving depth visually visual object so describe the appearance of the distinct areas in the dianon of a small rodent and of a monkey so here we're talking about the layers in the genicular body so in this picture that stretched out optic tract I show that so there's the supermatic then they reach the sub pamus this is the lateral genicular body ventral portion often abbreviated just LG V and it's also abbreviated lgn B lateral genicular nucleus vental part but notice here these are accents from the controlateral eye that I'm drawing I show a little area separated here by the dash lines in both the vental and dorsal genicular bodies where those accents don't go they don't go there because they're getting input from the outer eye okay it's a laminated structure not so obvious in the rodent but when you go to other animals like here's a four layer geniculate body which you find in some species then you find that the controlateral eye is projecting to the outermost the layer nearest the optic tract then it skips a layer and then it projects to the next layer and then not to the last l so it projects to one and three not to two and four and then in primates like the monkeys not all primates but at least monkeys and the Apes and humans you have six layers sometimes you see seven in some parts of the genic body usually we number six number them as six so we'll see that then the axons go through and over the lp lateral posterior nucleus that is the rest of the lateral Palance that part that grows so big in humans and I show a few terminations there it's somewhat variable but they are there they're sparse determinations okay again we don't know what those do maybe we don't need to know because those same neurons where those sparse projections occur get very heavy visual input but coming from the calculus not directly from the rtin one of the ideas about how the geniculate body appeared was just that these sparse scattered projections that if some reduction of the calculus occurred they just sprouted more there and that was adapted to have a shorter root to the cortex so the genicular body evolved but be that as it may be the next structure then is the pretectal area and then finally spiculus or Optic tectum but for mammals we usually use the term spirit cicular all right so let's look at the geniculate body now we'll look at animals you know with this kind of layer but even more the here's the geniculate body of a monkey it's similar to a picture a color picture I took from Hubble David Hubble uh and from a book that he has online that anybody can download very nice book unfortunately David died just recently uh but he was a very productive visual neuroscientist who did many studies of both cat and monkey uh genicular stride system working mostly in visual cortex he did the work with ton weasel who's retired now but has been had a position at Rockefeller University for the last quite a few years after he left Harvard from his work with hu but here it shows you how they number these uh oh yeah how where did this horseshoe shape come from that's not what I'm showing here can you figure that out let's go back to the embryo it looked like this monkey the same this structure here grew and grew and grew so this structure got pushed this way okay so and the pulvinar in growing so big uh the thalamus in front of it also grew and so this thing that's on the side of the thalamus here do right here side of the phalus got pushed over like that and it also get got rotated so the genicular body ends up on the back of the phal so it goes that way and gets so this becomes like that and the optic crack which was out here is now under here okay so that means this is the optic track surface those are the axon the tissue is torn off here but that's where the optic tract is but it's exactly top topologically it's the same as as this this picture here and then you have you see the way they number them they always start numbering them nearest the optic tract so one two notice the first two layers are larger cells and then the smaller cell layers or paral cellular layers 3 four five and six and again you have half of them they getting ipol in contr leral input and the other half are getting exal almost always the contralateral ey is projecting to the surface layer and to the last layer here and to one of these middle layers okay it varies a little bit among species here's a human and this is a human that uh picture that was donated to me by a former student who developed a silver stain that stained cell bodies gives you very high contrast picture this happens to be from a human case who had a pathology of one eye so the cells in the of the ipsilat ey that gets ipsilateral projections the cells are have shriveled a bit okay so this is one of the magnos cellular layers here and then here's the other controlateral layer and the last controlateral layer these are the layers getting IAL projections most of the other magnos layer we don't see here okay all of these cells here are all parts of that lateral Thalamus the pulvinar nucleus and there's a small part of it they call the the part that's the inferior pul pulvinar is similar to LP of the loaden and often they do name an LP too but it's the homology is not not always that clear okay so this is a picture from a very famous neuroanatomist in the early part of the 20th century lro Clark who didn't have access to beautiful optics for taking low power pictures and all that so he drew them and he drew these beautiful pictures in this case of many different uh primates and at that time they thought the Treo might be the most primitive primate it turns out to be similar to very primitive primates but it's actually an Inc work but you see the different patterns of lamination that occur in these different animals okay they don't all have identical structures okay binocular separation the anatomy underlying that binocular separation in the brain has evolved differently in different animals in these pictures the the old world monkey the greenon is is most similar to the human this particular this picture here is fairly close and level to that picture the the photograph okay so now I want to go to the rodent which they're usually using in laboratory work uh for lot of the studies of many systems and uh I want to I use side views and I show the op here here's a very simplified picture of it the outline here is a fairly accurate view a side view of the the upper brain stem of a hamster so you see the inferior calculus and spheri culus of the midbrain here this is all palus the dorsal palus would be all through here but the geniculate body here way out at the edge can here's the two parts dorsal and dental nuclei now remember I showed you I said that the lateral gen the ventral nucleus lateral geniculate is really subus well most of the sub remember it's a curv structure so this is at the lateral Edge so it's much higher up but then as we went deeper into the thalamus it would be way down here okay and what I show here is a little schematic of one little bundle of optic tract ACC coming from the kism up the side I don't show the terminations in the superism itic nucleus because those are different axons but I show that axons that go to the genicular body are usually branches of optic trct accents that go further that go to the tectum and often the pretectum as well I also show where the major input to the lp that provides the lp with visual information doesn't come from the retina even though there are a few projections there they come from the superficial or visual layers of the spiculus and IT projects both LP and to the outer layer of the lateral the ventral nucleus of the lateral genicular body so then uh I here's the question that I want to answer next the there are accents that leave the main optic cact and terminate in these small cell groups there up to three of them they're described as what kind of optic track the word is accessory optic track and this is a anatomical reconstruction of the hamster uh where I had labeled the axons all of the axons that I could which is most of them so I get the entire optic TR and I I put together Ser this is done from serial sections so you know the sections were actually like this but pretty close together okay and I then reconstructed the entire optic track from km to culas and this particular brain with the I was using degenerating X stains for degeneration I didn't get a good picture of the super kais nucleus so like most scientists I never lie with the data I present I don't show it okay but in fact with other techniques I could see it and it's right here all right and what I'm showing here is the accessory optic tract axons and you can see why they're called accessory optic tract they leave that main track that goes all the way from km to culas here here's some that leave just just below the cereal peduncle and they travel codly and they terminate medial to the peduncle right there to a nucleus that sort of hugs the medial side of the of the substantia and the coped unle and others do just travel codly below the phus here they just sort of peel off at the level of the ventrolateral genicular body and they terminate in a little nucleus here and some accents go down this way and get to this other way there's even some accents going in the other direction that's called the trans peduncular tract and then finally here's a little tract that goes from the pretectal area it leaves the main tract seems to go down to the lateral termin nucleus but it has another little nucleus there way out at the lateral edge of the pretectal so that's the accessory optic track very interesting system because all the cells in that tract respond to movement of the whole scene across the retina work no matter where you are in the retina you can the cells will detect movement in the same direction so when does that happen when the whole animal starts to fall it happens when the animal's locomoting and so there's streaming of the visual World Past his eyes and it functions as a kind of it does the same kind of functions that the distributor system does okay but it gives visual signals to indicate changes in head Direction and eye Direction okay very important for uh locomotion and very important if you need to keep track of your of head position all right so then I just want to show you that that what that reconstruction uh is doing by taking photographs of a similar brain now here I didn't label it here I did so here you can see because of the way I've shaped the light you can see major tracks cuz they're wider that's the lateral Factory track there's the optic track there's a pathway carrying auditory input from H brain up to the inferior calculus there's the bundle one of you see these two bundles here we cut those when we remove the cerebellum those are the cerebella pedes okay there's hypothal down below and there's where I uh when I remove the brain I tore the pituitary off right there okay that would be the mammary bodies at the codal end of the hypothal so here I've labeled them and I've labeled a few other structures so this structure here it's not bamus it's part of the end brain it's Corpus triot then I show you exactly where the geniculate body is is I know you say well how what do you mean exactly because I can see the Shadows there I can see them there I know that that's exactly the edge of the dorsal nucleus and this is the vental nucleus right below it there's the optic tract hypothalamus this big bundle here that emerges from behind the optic track those accents are course ing through the Corpus trium here but in the rodent they're all they're all separated a whole bunch of little separate bundles that collect in form the peduncle comes along the side of the dlon and then passes right on into the pontine region here and then when we study auditory system we'll learn that this is called the lateral liscus and that bump right there behind the Celler pet unes that's the clear nucleus and there's the stump of the eighth nerve right there so you one way is to to learn this stuff is to you know study reconstructions like this look at these photographs and try to learn to pick out the structures right now you should be able this is midbrain you should know what the calculi are you should know the optic tract and the hypothalamus here if I've already told you I've removed the hemispheres and the cerebellum then you know this has got to be below the hemispheres and there's only one big structure like that it's go the stum okay so one way is to cover those up and uh in the book you can it's done so you can just put a piece of paper and still see the line for the labels okay I want you to do that in your book cover up the labels and see if you can learn them learn what these structures are on this kind of picture uh this kind of picture and this kind of reconstruction were very important to me from a lot for a lot of the experiments I did because I needed to do neurosurgery where I would open up the brain of course I would only see a small part but I learned the landmark so well that I could open it up and also using blood vessels I could see these structures and see the boundaries that way I could make injections or even lesions in just small regions all right this is another somewhat easier view where I have the adult brain on the left and the brain of a newborn on the right and here I've just labeled major parts of the brain Factory B NE cortex Superior calculi inferior calculi cerebellum and the Medela blada the codal part of the hindbrain the Ral part of the hind brain is underneath the cerebellum cerebellum is part of it but what I want you to note here look at the cerebellum here in the baby it's just this tiny little collar behind the inferior cus it's mainly developing with huge numbers of cell migrations from the romic lip to the hbr just developing in the newborn hamster it's born remember at a stage it's like a 2 and 1/2 to 3mon human fetus which means that a 2 and 1/2 month human fetus looks just like this too cerebellum grows mostly after that time okay so one way to study that just use the book and make yourself a card and cover those up and make sure you know what all those pointers are pointing towards this one should be the easiest this one will be a little harder all right can somebody tell me what e is optic crack and what about what about D hypothalamus uh can anybody do c p right cereal cerebral pedon p means cereal it's always it's it's always the abbreviation for Cal pet unle is PE and I didn't label the PS but this would be the ponts right there this is the trapezoid body which I don't name Co nucleus cerebel peduncle and latal liscus but I just labeled the ones here that you know we might put a a diagram like this we wouldn't put all those things but we might ask you to do a give you a bunch of terms and say well which one goes with each of these letters you know so if you're at least familiar with it you would be able to do that okay and then here the same thing here I've covered up just name them off for me what's that what's this okay or Cal hemisphere either one would be proper what's that okay notice most of it exposed here here only part of it's exposed and in most animals with an either bigger bigger hemospheres you don't even see the membrane from a dors of view like that okay and this one cerebellum this CLE hind brain or medel BL okay oh I'm asking you another question here about naming the structures I said what would make this more difficult during a Neurosurgical procedure think about it in a Neurosurgical procedure you don't expose such a huge amount of the brain you only can you know you want to make a smaller window as possible so you don't damage so much tissue so that's the first problem and the second thing is there's a lot of blood vessels in fact you have to be very careful not to go through any really big ones because you'll get so much bleeding that you'll be spending all your time stopping the bleeding and you do spend a lot of time doing that neuros surgery although now we have a method to apply that does stop the bleeding of all the smaller vessels just not the huge ones okay look at these other pictures for next time and just see if you can figure out what's exposed okay I use different lights lighting just so you this is that same kind of brain stem that you see here with the hemispheres removed and you can see various structures here I've done a little more removal up front here let see if you can figure out what those things are now I do name a lot of them in the book I don't think I show all these pictures in the book but you I wanted you to see how by if you adjust the light a little bit you can actually see boundaries like look at this one there's the spiculus there's the pretective if you look at this one even clearer there's the boundary between spiculus and pretectum and look at how clear this boundary is and this one pretectal areia LP geniculate body that is the bundle carrying information from the amydala which we will be studying soon okay and this just shows what it looks like in a the embryo when it just the accents have first Grown back to the tectum very straight pathway back to detecton and we'll next time we will start in this area and talk about the midbrain a little more just to look at these pictures but they're in the book uh they'll give you a chance to ask about and uh we'll go through some of these before we go on to the Endra okay