NOTICE: Posting schedule is irregular. I hope to get back to a regular schedule as the day-job allows.

Wednesday, February 23, 2011

The Vision to See

Occipital Lobe.  Visual Cortex.

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 -
At the most "rostral" (anterior or rear-most) extent of the occipital lobe, the part referred to as the occipital pole, and in the fold between the left and right hemispheres of the brain, is the primary visual cortex.  This is the region that directly represents the shapes, shades and colors that we see.  In stained sections, this cortex looks striped, thus one of the names is "striate" cortex. 

To better understand the structure of this brain area, it is necessary to understand the *signals* that come to this area of the brain from the eyes.  This really deserves it's own blog, and I'll talk more about it in tomorrow's blog, but for now, the important point is that by the time visual information reaches the visual cortex, it consists of short lines - bars or edges of light (or dark).  These bars are then organized by visual orientation (angle), as shown at left.  In the visual cortex, these representations of visual angle are alternated with the identical image from the left and right eye. In scientific terms, these are refered to as "ocular dominance columns," and research has shown that these columns develop with days of birth (or eye opening in animals) when the brain first begins to receive input from the eyes.  Rows of these representations are stacked to form the primary visual area, with each row representing different areas of the entire field of vision. 

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.

1 comment:

  1. I like the memory as a Hologram, argument. It neatly handles the ability to use partial matches, and also speaks to the efficiency that the Brain has to use, to be able to store all the info available! Expecially since I have a memory that is partially picture-perfect - mostly for pages of a book, but also works for scenes that I was especially intent upon, at the time. An early fishing memory, in a narrow mountain rivulet, in Coleman, Alberta, is one such, and the sounds, smells, and even the colours of the algae on and around the rivult rocks are clear. So I have driving-in-a-Blizzard memories that bring up a cold sweat.
    Neil Frandsen
    aka Bigfootneil


Please add comment - no links, spammers will be banned.