cerebral cortex

The cerebral cortex is the phylogenetically most advanced part of the central nervous system in mammals. Meaning? It is the "newest" part of the brain in an evolutionary sense. The cerebral cortex is divided into various parts, but before getting into what they do, lets clear up one point about the anatomy of the brain:

The cerebral cortex is not the entire brain, only a large and very prominent structure. When you look a the brain from almost any angle, you see the complex and convoluted folds (sulci and gyri) of the cortical surface. However, the cortical surface itself is only a few millimeters thick -- in fact what looks like a very dense, noddle-y mass is actually a thin sheet of grey matter (neuron cell bodies) which completely covers and conceals from view most of the "lower" structures of the brain* such as the basal ganglia, thalamus, midbrain and spinal chord. Thus, there is much of the brain that is not the cerebral cortex. That being established, lets move on . . .

I must correct ophie's classification of brain parts on two small points above: 1) The insula is anatomically and functionally really a small part of the parietal lobe -- though it is located in a very out-of-the-way spot. 2) The "limbic lobe" is not a well defined set of structures. It would be more precise to say that the brain parts that make up the limbic system are somewhere "in between" the cerebral cortex and lower brain structures. Some limbic structures, such as the entorhinal cortex, are located near to and have functions similar to cerebral cortex proper, while others, like the putamen, are classified as diencephalic (lower) brain centers.

So lets talk about the lobes:

• Occipital lobe: The most well understood part of the cortex. Located at the posterior most area of the cortex, the occipital lobe is responsible for all aspects of visual information processing; detecting edges and borders, depth perception, the processing of color, the processing of motion and the beginnings of object recognition. I will try to treat these in more detail in another WU, but suffice it to say that many of these different activities occur in anatomically distinct areas, and that these areas are organized in a hierarchical fashion. There are two main visual processing "output" pathways, the "dorsal stream", which is concerned with the location of objects in the visual world, and the "ventral stream", which is concerned with the identification of objects in the visual world.
• Parietal lobe: Located in front of the occipital lobe but behind the frontal lobe, the parietal lobe has two main functions that we know of: 1) processing somatasensory (touch) information in much the same way the occipital lobe processes visual information. and 2) forming a 3-D representation of the outside world using somatasensory information as well as the information contained in the dorsal stream (object location) mentioned above.
• Temporal lobe: Located below the frontal and parietal lobes, the temporal lobe has several functions, including auditory information processing, language processing and execution, object recognition and memory encoding. These latter two functions are subserved in great part by the dorsal stream (object recognition) information from the occipital lobe, as the structures of the temporal lobe feed this information into the hippocampus and associated structures.
• Frontal lobe: Located at the front-most part of the cortex, the frontal lobe is the most evolutionarily advanced, highly developed, and complex part of the brain, responsible for personality, decision making, reasoning and higher thought in general. The frontal lobe receives no primary sensory information, but instead receives most of its information from the other three lobes and is responsible for the primary output of the brain -- that is it prepares and executes the neural commands which control voluntary muscle movement. The frontal lobe is the least well understood part of the brain, though some aspects of its function have been mapped out: for instance, it has been shown that specific parts of the frontal lobes communicate heavily with the parts of the parietal lobe that represent the 3-D outer world. It is thought that these lines of communication help update the location of objects in the brain's working memory.

So how, exactly do all of these parts work? The first person to figure that out gets a nobel prize = ) However, in general we are able say that the occipital, parietal and temporal lobes receive primary sensory information and use that information to discern the nature of the outside world. This information is fed into the frontal lobes, where it is integrated, processed and responded to. Of course, this is a gross oversimplification: this description ignores the heavy interconnectivity within and between lobes and the critical role of the many, many, many lower brain centers.

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* That is, the diencephalon, the mesencephalon, the metencephalon and the myencephalon. The cerebral cortex itself is considered part of the telencephalon.

Besides being divided into separate lobes based on function, the material of the cortex has distinct features that are much smaller and more specialized. Two millimeters of cortical (gray) matter sits on top of the white matter, and is actually responsible for all cortical functions. The much more voluminous white matter below does no processing at all -- it's more responsible for transporting oxygen, mollifying damage, and maintaining the blood-brain barrier. Each square millimeter of that cortical surface, however, contains around 148,000 neurons; each is involved with the others in the intense communication that eventually exhibits itself as action, response, and thought.

The cerebral cortex itself, that crucial layer of gray matter, is subdivided into six layers. While these layers are not perfectly defined, there is usually much more crosstalk between neurons in any given layer and neurons in other cortical layers or brain areas. From the six layers, three general subdivisions can be made:

• Layers I, II, and III are the three outermost layers, often called the superficial layers. They read information from one another (and to some extent the thalamus), and send information to all the other layers. Being the outermost layers, they are the most recently developed on the evolutionary time scale. Collecting and collating all of the cortical input and influencing all of its output, these layers might well be considered the seat of thought itself.

• Layer IV is the primary terminus of all information coming from the thalamus, which processes all of the body's state and perceptions for the cortex. This layer also is partially responsible for sending information back to the thalamus, as a check against cortical overstimulation. This layer's contact with layers II and III, along with the direct thalamus input noted above, gives the cerebral cortex all of its knowledge about its surroundings.

• Layer V picks up information from layers I and II and sends it to sub-cortical brain areas, as well as the and body. Some layer V neurons take information from layers II and III and send it to the corpus callosum so it can reach the other half of the cortex. Others collate information from layer VI and some from layer I, and send it to the pons, superior colliculus, and even the spinal cord itself. If layer IV is the inbox, then layer V can be thought of as the outbox, sending the results of cortical processing wherever they need to go.

• Layer VI, finally, takes the information available in itself and layer IV, and sends it to the thalamus. Interactions between these two areas probably allow for conscious control, and to a lesser extent automatic, of what stimuli we are paying attention to, like glancing toward your cat from the computer monitor.

Surprisingly, this variegation is constant over the whole cortical surface, and even more surprisingly has the same configuration in all mammals. What varies between mammals is the cortical surface area and ratio of layers II and III to the entire cortical depth. That is, the surface area of the human cerebral cortex is around 2200 cm², whereas chimpanzees have around 600 cm², cats have 220 cm², and rats have less than 7 cm². More importantly, since percentages don't corelate with animal size, rodents have only around 25% of their cortex depth taken up by level II/III neurons, while carnivores have about 36% and primates 48%. These differences probably arose as evolution created opportunities for animals with ever greater cortical processing power, which is to say, ever more complex thought.

After single neurons, the first level of cortical organization found (in research thus far, at least) is the minicolumn. These are groups of cells between 20 and 65 micrometers in diameter, which cross all six cortical layers in sort of a cylinder shape. Composed of roughly 100 neurons, these are generally separated from one another by 30 micrometers or so. While it is tempting to think of these as "modules," interchangeable on some level, they are each wired differently and each have axons and dendrites that differ both spatially and functionally.

A group of minicolumns together makes up a macrocolumn, which is generally between .4 and 1 millimeters in diameter. Macrocolumns are probably the result of a large neural input to that particular area of the cortex, which during development causes neurons to grow around it. Again, these structures are not functionally interchangeable -- each one is different from all the others. While macrocolumns usually penetrate all six cortical layers, in the visual cortex they only exist in layers I-III, and are known when found in that area as color blobs.