So consequently if we follow the lines of projection back from this illustration, we see that objects that are out here to the right side of our fixation point are seen by the nasal portion of the right eye and the temporal portion of the left eye. Consequently, those portions of the two retinas need to send their information together in order to develop a coherent, and congruous representation of that region of the visual world. That's where the optic chiasm comes in. Now notice what happens in the optic chiasm. There is a crossing of the projections derived from the nasal retina. In human beings, roughly about 55% or so of the axons in the optic nerve are dervied from the nasal retina and they cross in the optic chiasm, and terminate on the contra lateral side of the brain relative to their origin. The remaining 45% or so come from the temporal portion of the retina, and those axons remain on the same side of the nervous system. So by the time you are posterior to the optic chiasm in this visual pathway, these axons are now derived from the two eyes but they are seeing the same part of the visual world. So in the illustration that I've tried to make for you here, if we consider the left optic tract here, the left optic tract is now representing the right side of visual space. Specifically that region of right visual space that's seen by the right nasal retina and the left temperal retina. Conversely, the left side of the visual world Would be seen by the nasal retina of the left eye and the temporal retina of the right eye. And those projections with nasal being crossed, send signals back to the right optic chiasm, such that the right optic chiasm is now representing the left side of the visual world. So the optic chisam really solves the challenge of having two eyes in the front part of our faith. One might have thought that it would be easier to just have a single cyclopean eye but we are bilaterally symmetrical creatures. So we have two visual organs and we make great use of that fact in the construction of depth perception based on the slightly disparate views of the two eyes of near objects. But that's a story for a later session. For now let me just emphasize to you that the optic chaism sorts out the fact that the two eyes see portions of the same part of the visual hemifield and with the principle of contralateral representation at stake, it's necessary to sort out those central projections to bring what the two eyes see of the same region of the visual world together in the appropriate hemisphere. But once the axons of the optic tract terminate in their appropriate diencephalic targets with respect to the image forming pathway anyway, these signals reached the lateral geniculate nucleus. So here now with a lateral view of the brain, we see the lateral geniculate nucleus, which[UNKNOWN] the visual thalamus, the part of the thalamus that is receiving input from the optic tract. And it's the part that gives rise to projections to the part of the cortex that we call the primary visual cortex. Well, that's quite a distance. That primary visual cortex is in the banks of the calcarine solcus on the medial aspect of the occipital lobe and in order for axons to grow from the Lateral geniculate nucleus to the posterior medial temporal lobe, these axons have to grow around the lateral ventricle. So these axons swing first laterally, and then they run posterior in the white matter towards the occipital lobe. Now it's possible to recognize essential two parts to this so called optic radiation. There are the axons that are emerging and sweeping in the more dorsal and posterior direction, around the atrium of the lateral ventricle. These axons are destined to terminate on the Posterior back of the calcarine sulcus in the cuneus gyrus. Meanwhile there are axons that are emerging and sweeping down into the temporal lobe, and then around the lateral aspect of the temporal horn of the lateral ventricle. These axons perhaps have a slightly different challenge to try to get around the portion of the temporal lobe that otherwise is providing an impediment to the growth of these axons. But they do so, of course, and they make their path back to the lower bank of the calcarine sulcus, where they innervate cortex in the lingual gyrus. Now this more temporal sweep of the optic radiation has a special name, we call this Meyer's loop. So when you hear the term Meyer's loop think about this temoral portion of the optic radiation. Now lets look a the organization of these inputs along the banks of the Calcarine sulcus. So, as I just mentioned, the Calcarine sulcus is formed by the folding of the lingual gyrus and the cunear gyrus. So we have the cuneus which is the, the wedge, cuneus means wedge, on the upper back of the calcarine sulcus ,and then we have this tongue like structure down below . So this is the lingual gyrus, lingual means time. Now notice how the color code is indicating the topography of the projections, from the optic radiation into this cortex. Now the cuneus gyrus is representing the lower part of the visual field, and that is because the region of the lateral geniculate nucleus that was supplied by retinal fibers is coming from the upper part of the retina. The upper parts of the retina are viewing the lower portion of the visual world. So once we get into the thalamus and now into the occipital cortex, we have a sort of inversion of the world, relative to the way that its projected into the brain, such that the cuneus is representing the lower part of visual space. Now, that leaves it for then the lingural gyrus to represent the upper part of the visual field. And again that's simply because the part of the lateral geniculate nucleus that grows the axons that forms the Myer's loop, is receiving input from the inferior parts of the retina, that see the upper part of the visual world. If we look at the way the visual world projects onto the retina, we see that the central part of visual space is relatively limited. Just a few degrees, in fact with the fovea sees is about the width of your thumb at arms lenght, so that's a bit of a rule of thumb about what the field of view is of the Fovea. So that thumbs with amount of visual space is actually perhaps the inner region here of what the macula might see, and yet its given over a very large representation in the visual cortex. So what the fovea is seeing is represented in the posterior pole of the hemisphere, and increasingly peripheral regions of the visual world are representing an increasing anterior regions of the visual cortex along the banks of calcarine sulcus. So far what I've been describing for you is how the visual world is represented in the primary visual cortex. Well, there is a considerable amount of the cerebral mantle devoted to the processing of vision. Perhaps as much as 25% or so in the human cerebral cortex is representing vision. So there is a series of visual cortical areas beyond the primary visual cortex that have now been mapped in the human brain. For reasons that we'll discuss when we look in more detail at the cellular structure of the visual cortex. The primary visual cortex is also known as the striate cortex. So these regions beyond the primary visual cortex are called extrastriate visual cortex. And we see a number of them here represented in blue and in the orange colors, and in the purple and in the yellow. And what we discover is that beyond the banks of the calcarine sulcus, which is back in this region, we have a series of basically concentric rings of visual areas that emanate around the occipital lobe. And if it were possible to unfold, flatten out the cerebral cortex, as can be done computationally in a representation of the human brain obtained in a magnetic resonance imaging experiment. We see the concentric rings now cut open and laid out. And there's ongoing study trying to define in exactly how many areas we have here, and that's still a matter of some discussion. But what we find is an increasingly more abstract representation of particular aspects of our visual world. One particular strategy that our visual system employs is the process of parallel pathways. And we see that elaborated in our extrastriate visual cortex. Broadly speaking, there are pathways that are mainly concerned with what we are looking at. And so the answer to the question, what we are looking at is often provided in terms of the detailed shapes, and patterns and texture, and color of the objects and the faces that we see in the visual world. All that information is supplied in a ventral pathway that runs through the inferior aspect of the occipital lobe into the inferior temporal lobe. Well, in addition to what we're looking at, one fundamental question in vision is, where are we looking? And so the answer to the question where is essentially a spacial answer. And it has to do with the location of visual objects in the environment and perhaps where they happen to be moving about in that environment. And that sort of information is provided by a pathway, that runs through an important structure in the brain called MT. Mt stands for middle temporal visual area, which is where it was discovered in the nonhuman primate brain. In our brains it seems to be near the junction of the temporal lobe and the occipital lobe. And from area MT, this sort of spatial and motion selective information tends to arise and travel up into the parietal lobe. So, area MT as we'll see in a different session on visual cortical processing is enriched in cells, that developed for us a notion of motion. As my chairman in the Department of Neurobiology here at Duke likes to say. So our notion of motion, is derived by neural circuits, that interact between our primary visual cortex in area MT. And from there the signals are further elaborated, into a broader scheme of our visual environment that's elaborated in the parietal lobe. So remember, in the temporal lobe we have answers to the question, what am I looking at, and in the parietal lobe we have answers to the questions about, where am I looking or where are objects moving in the environment. Well, I hope this tutorial has helped lay the foundation for you, in your understanding of the visual pathways and the concept of visuotopy in particular. This will be a very important topic as we consider the impact of lesions or disease that might afflict some component of the visual pathway. It will be important for you to understand, what kind of visual field deficits might you expect to find in patients with damage to particular visual components or particular visual pathways. Well, we will have an entire tutorial on that topic, and I'll see you then.
