As promised, starting at the "back" of the brain takes us to the Occipital Lobe of the brain. Unlike many of the "gross" or major identifications of the brain, this lobe is almost entirely devoted to processing one sense: vision. It makes sense, though (no pun intended) since the visual system is *the* major sense in the primates including humans.
It is also one of the "newer" structures of the brain.
What do I mean by that? Well, in developmental terms, comparative neurophysiologists "date" parts of the brain in terms of when they appear - both in evolution and during fetal development. No matter your *personal* opinion of evolution, as a fetus develops, complex organs appear and mature ("ontology") in a sequence very similar to the "phylogeny" of evolutionary development from lizard to higher order mammal. Thus the "ancient" lizard brain performs the most basic functions, and corresponds to the brainstem and subcortical structures. As more senses are added - smell, hearing, touch, vision - the appropriate cortical regions develop and expand. The most advanced functions are complex cognitive decisions centered in frontal lobe, hence that is the "newest" cortical regions. But aside from cognition, vision is the next most complex, and evolutionarily most recent development - hence the visual cortex occupies a disproportionately large area of the brain and it makes a certain sense that it occupies a large percentage of total brain area.
|From: Psychology Wiki - http://psychology.wikia.com/wiki/Visual_cortex|
Thus for each line, curve, pixel, light/dark spot, left/right - top/bottom portion of the visual field, there is a patch of visual cortex that represents it. Building a "picture" of what we see, is just a matter of recombining these visual elements into pictures. This is the role of the other regions in the occipital lobe. V2 - secondary visual cortex builds these elements into more complex images. V3 and V4 visual association cortex develop even more complexity - including templates for common features such as faces and familiar shapes.
One of the common *stories* told to neuroscience students is of the "Grandmother Cell" which is a neuron in the V4 (or other association) cortex that receives enough convergent connections from various shapes, colors, images that it only responds to your grandmother's face. While that is a bit of an exaggeration, there really are visual association cortex cells that respond to faces - and specific features of faces in fact. A personal example that I experienced was seeing a recording from V4 that responded to faces - when an experimenter peeked around the partition and was seen by the animal, the recording showed evidence of increased neural activity - but only if the person had a mustache! Clean shaven faces did not activate the neuron.
Real "Grandmother Cells" while rare, but can exist in the association cortices that combine visual (Grandma's face) with auditory (Grandma's voice) with olfactory (her fresh baked apple pie) - and memory. Yes, memory is a key component, and rather than just one neuron, there are many neurons that represent one or more portions of the total stimulus. But again, I get ahead of myself, and will return to this concept when we talk about memory later in the blog.
Tomorrow's blog will go into more detail on eyes, retinas and visual representation - before we finish the occipital lobe with another look at the association cortices. Tune in tomorrow and we will discuss how spots of red, green, green, blue and white light turn into a mental image of our surrounding environment.