Let me just summarize what we've talked about in topic three of this overall module. First, for purposes of this course, as I said before, the organization of the visual system concerns the primary visual pathway. We're not going to talk about the details, we're not going to talk more about many of the details of the primary visual pathway. But we're not going to talk about the pupillary light reflex, the head and eye movements and that for major issues in vision generally, but not about perception. The primary visual pathway is, by definition, the pathway from eye to cortex, primary visual cortex and higher order cortices that are concerned with, ultimately, what we end up seeing, which is the focus of the course. The main elements of the primary pathway are the eye and the retina, of course, the lateral geniculate nucleus in the thalamus and the visual cortex in the occipital lobe. The focus is very often, experimentally and is going to be in this course, on V 1 because much more is known about that than the higher [INAUDIBLE] areas that we discussed a minute ago. The pathway begins as a hierarchy from retina, to thalamus, to visual cortex, going up anatomically in the brain. You can think of this as an abstraction of information in this hierarchy going from retina to thalamus to visual cortex because there are many more receptor cells in the retina than there are cells in the thalamus and in the primary visual cortex. In the sense that there is at every level the focusing of information and abstraction of information that is basically a conceptual convergence, at least, and to some degree, anatomical as well, of information from the retina on through the cortex. The last point I made was that a guiding principle of the whole shebang is going to be the concept of receptive field. That entails not just the locus of where a stimulus that activates a particular neuron is but also what it's properties are, as we'll see, the idea of the receptive field gets complicated. It's not entirely as straightforward as one would like but it's an operating principle that's characteristic of the visual system and other sensory systems.
