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There is ample proof that the cerebrum’s thin gray covering layer, called the cortex, is the major site for human intelligence. Beneath this cortex is the bulk of the cerebrum. This is the white matter whose white appearance is caused by the presence of fatty sheaths protecting nerve-cell fibers—much like insulation on electrical wire.
The white matter is primarily a space thru which an abundance of nerve pathways, called tracts, pass. Hundreds of millions of neurons are bundled into different tracts, just as wires are sometimes bundled into larger cables. Tracts are often composed of long axons that stretch the entire length covered by the tract.
As an example of a tract, consider the optic nerve, which leaves the back of the eye as a bundle of roughly a million axons. The supporting cell bodies of these axons are buried in the retina of the eye. The optic tract passes into the base of a thalamus, which is primarily a relay station for incoming sensory signals. There, a new set of neurons—one outgoing neuron for each incoming neuron—comprises a second optic tract, called the optic radiation. This optic radiation connects from the base of the thalamus to a wide area of cerebral cortex in the lower back of the brain.
There are three main categories of white-matter tracts, corresponding to those parts of the brain the tracts are connecting. Projection tracts connect areas of cortex with the brainstem and the thalami. Association tracts connect, on the same cerebral hemisphere, one area of cortex with a different area of cortex. Commissural tracts connect, on opposite cerebral hemispheres, one area of cortex with a different area of cortex. Altogether, there are many thousands of different tracts. It seems that all tracts in the white matter have either their origin, destination, or both, in the cortex.
The detailed structure of the cortex shows general uniformity across its surface. In any square millimeter of cortex, there are roughly 100,000 neurons. This gives a total count of roughly fifteen billion neurons for the entire human cortex. To contain this many neurons in the cortex, the typical cortex neuron is very small, and does not have a long axon. Many neurons whose cell bodies are in the cortex do have long axons, but these axons pass into the white matter as fibers in tracts. Although fairly uniform across its surface, the cortex is not uniform thru its thickness. Instead, when seen under a microscope, there are six distinct layers. The main visible difference between these layers is the shape and density of the neurons in each layer.
There is only very limited sideways communication thru the cortex. When a signal enters the cortex thru an axon, the signal is largely confined to an imaginary column of no more than a millimeter across. Different areas of widely spaced cortex do communicate with each other, but by means of tracts passing thru the white matter.
The primary motor cortex is one example of cortex function. This cortex area is in the shape of a strip that wraps over the middle of the cerebrum. As the name suggests, the primary motor cortex plays a major part in voluntary movement. This cortex area is a map of the body, and the map was determined by neurologists touching electrodes to different points on the cortex surface, and observing which muscles contracted. This map represents the parts of the body in the order that they occur on the body. In other words, any two adjacent parts of the body are motor-controlled by adjacent areas of primary motor cortex. However, the map does not draw a good picture of the body, because the body parts that are under fine control get more cortex. The hand, for example, gets about as much cortex area as the whole leg and foot. This is similar to the primary visual cortex, in which more cortex is devoted to the center-of-view than to peripheral vision.
There are many tracts carrying signals into the primary motor cortex, including tracts coming from other cortex areas, sensory tracts from the thalami, and tracts thru the thalami that originated in other parts of the brain. The incoming tracts are spread across the motor cortex strip, and the axons of those tracts terminate in cortex layers 1, 2, 3, and 4. For example, sensory-signal axons terminate primarily in layer 4. Similarly, the optic-radiation axons terminate primarily in layer 4 of the primary visual cortex.
Regarding the outgoing signals of the primary motor cortex, the giant Betz cells are big neurons with thick myelinated axons, which pass down thru the brainstem into the spinal cord. Muscles are activated from signals passed thru these Betz cells. The Betz cells originate in layer 5 of the primary motor cortex. Besides the Betz cells, there are smaller outgoing axons that originate in layers 5 and 6. These outgoing axons, in tracts, connect to other areas of cortex, and elsewhere.
Besides the primary motor cortex and the primary visual cortex, there are many other areas of cortex for which definite functions are known. This knowledge of the functional areas of the cortex did not come about from studying the actual structure of the cortex, but instead from two other methods: by electrically stimulating different points on the cortex and observing the results, and by observing individuals who have specific cortex damage.
The study of cortex damage has been the best source of knowledge about the functional areas of the cortex. Among the possible causes of localized cortex damage are head wounds, strokes, and brain tumors. The basic picture that emerges from studies of cortex damage, is that mental processing is divided into many different functional parts, and these functional parts exist at different areas of cortex.
Clustered around the primary visual cortex, and associated with it, are other cortex areas, known as association cortex. In general, association cortex borders each primary cortex area. The primary area receives the sense-signals first, and from the primary area the same sense-signals are transmitted thru tracts to the association areas.
Each association area attacks a specific part of the total problem. Thus, an association area is a specialist. For example, for the primary visual cortex there is a specific association area for the recognition of faces. If this area is destroyed, the person suffering this loss can still see and recognize other objects, but cannot recognize a face.
Some other examples of cortex areas are Wernicke’s area, Broca’s area, and the prefrontal area. When Wernicke’s area is destroyed, there is a general loss of language comprehension. The person suffering this loss can no longer make any sense of what is read or heard, and any attempt to speak produces gibberish. Broca’s area is an association area of the primary motor cortex. When Broca’s area is destroyed, the person suffering this loss can no longer speak, producing only noises. The prefrontal area is beneath the forehead. When this area is destroyed, there is a general loss of foresight, concentration, and the ability to form and carry out plans of action.