The Cerebral Cortex
The intricacy of the nervous system comes from the way axons connect to dendrites, between neurons, so that signals are passed from one to another in a complex electrochemical process occurring at the synapses.
It is thought that some mental conditions occur due to chemical imbalances which affect the action of these synapses, while others appear to be due to problems with the myelin sheath which insulates the axons.
It is the complexity of the cortex that distinguishes humans and provides human cognitive powers. In other mammals, such as mice and rats, it forms a relatively small part of the brain, appearing as a smooth covering sheet of tissue. In more complex species it becomes more complex, its relative area being increased as it becomes more convoluted.
Human skulls are not much larger, relatively speaking, than those of the present day great apes, but the area of the cortex, six cells thick and folded into innumerable convolutions, is far larger in area.
The growing cortex.
The fetus is not passive, it is alive and active. From as early as twelve weeks after conception the fetus can be seen moving, slow rhythmic flexing of its muscles. Its hands and feet respond to gentle stroking by clenching its fingers or toes. Even at this stage it can sometimes be seen with its thumb in its mouth, a complicated action for such a tiny organism. By the seventh month the tiny chest is moving.
There have been many studies about how much the fetus can sense of the outside world. It is believed that it responds to bright lights, and it seems certain that it responds to sounds, by the measurement of its heart rate. There has always been a feeling among mothers that their unborn baby responds to events on television programmes and, especially, music.
Opinions on this topic range from one extreme to the other, as to how much of this process is dictated by genetic inheritance.
Edelman(1) seems to be going further than anyone, in suggesting that very little of the neural organisation occurs directly through the genes, but is mediated by the interaction with the genetically determined fetus of which it is a part. He suggests that left to itself it would be random, but a special kind of randomness bounded by certain constraints, such as the physical body, which is already highly individual, and the things that happen to it and the things it does. He is proposing what is known as a stochastic process which begins at the moment of conception, and continues long after birth.
Geography of cortex.
The human cortex is not completely featureless in appearance. It is divided from front to back into the left and right cerebral hemispheres. Each hemisphere has four areas, the frontal, parietal, occipital and temporal lobes.
Researchers have, over the years identified a number of physical configurations which are associated with specific functions. It has been known for many years that the patterns of stimuli from light on the retina of the eyes are carried by nerves across the brain, via the optic chiasma, linking into the visual processing area at the rear, the visual cortex.
While there is considerable controversy about how specialised the cortex is, there is no doubt that there are areas are devoted to certain functions. Motor and sensory areas, along with those for hearing, are present in both hemispheres. The three areas for the uniquely human function of language appear only on the left. These are Broca's area, Wernicke's area, to do with producing and understanding language, and the angular gyrus, concerned with matching the visual representation of a word with its auditory form. Although there are these distinct areas, in terms of function, there seems little to distinguish them at the cellular level,
The axons from the various sensory nerves in the body extend from specific points in the spine, and in turn are connected to specific points in the cortex. Extending in a transverse band right over the brain, there is what amounts to a neural map of the body - the sensory cortex. Some areas, like the hands, the lips and the genitals, which have a high density of nerves, take up more room than others.
Next to this is the motor cortex, again a map of the muscles of the body. Again, structures that are involved in precise, fine movements, like the hands, take up larger areas.
Different areas have been found in different animals, according to their lifestyle. For instance animals which make great use of a particular motor function to explore their world have correspondingly large areas of their cortex devoted to that function, depending on whether they use mainly eyesight, say, or smell.
A rat, for instance, has an area of its cortex for each whisker. There is evidence that development of these areas depends on their usage. However, as one writer puts it, there are no muscles specifically devoted to fighting, thus there is no brain structure devoted specifically to aggressive motor output.
The quality of eyesight experienced by humans is probably the most highly organised and complex of functions. We take our eyesight for granted, yet it is very clear that what we see is, literally, what we think we see. During the 1980's, a review was prepared of all we know about human vision - it ran into sixteen volumes.
The retina of the eyes consists of thousands of light-sensitive cells, interconnected in a myriad ways to each other and to each optic nerve. The two nerve bundles pass through the optic chiasma near the centre of the brain to the visual cortex at the rear. All of these have developed in the individual way already described for the brain. The retinal cells do not detect a pattern of dots as in a television picture, but analyse the view in terms of edges and discontinuities, or relative differences in luminance between one point and the next. In the visual cortex, different cells analyse the response according to orientation of the edge stimulus, its length and rate of change in luminance. Other cell groups respond to temporal changes, appearance and disappearance of an edge in successive parts of the retina, in other words, movement.
In order to achieve binocular vision, the stimuli detected by each eye are compared, by combining them within the visual as a three dimensional structure. This is achieved by the way the fibres cross over in the optic chiasma. Briefly the fibres from each eye that sense left half of the view connect to the right side of the cortex and vice versa.
The development of the brain continues by the growth of axons until at least six years, and perhaps longer. Many people consider that a newborn baby's brain is about three months premature. They suggest this is nature's trade off due to the difficulties of delivering a baby with such a large skull. By comparison with other species, calculations of length of gestation versus brain size, suggests that birth should be at twenty one months.
The first two years are what is what Piaget called the sensorimotor period as the baby develops physical and perceptual abilities that are not mature at birth. Walking and talking are the obvious examples, but even eyesight follows a process of continuous development through early life, as the eyes grow and the number of cells changes. Although the newborn baby has almost its full complement of neurons, its brain roughly doubles in weight during the first three years, almost entirely due to the development of synapses.
Edelman described the learning process of an infant reaching out to grasp an object. A six week-old baby will almost always reach out to try and touch an object in front of it, but at that age it does not know how to co-ordinate its movements. At one time, it was thought that this was somehow genetically programmed in the brain. All that was needed was the necessary muscular development. It turns out that every baby has to learn how to do it. For many weeks its will wave its arms about aimlessly, trying to focus its eyes on the object. Occasionally by luck they will make contact with the toy. Edelman's theory suggests that there are many thousands of possible connections competing with each other. In other words as the brain develops, it lays down a huge diversity of possible firing patterns, and huge range of possible actions, most of which will be of no possible use whatever. Every time that a sequence of movements, and therefore a pattern of synaptic activity, produces a successful outcome, it strengthens the associated connections, especially since the behaviour is likely to be repeated, while unsuccessful connections fade away.
Almost nothing is known about the effects of the fetal environment, except in terms of obvious damage or deformity of the resulting infant. Gross effects, due to alcoholism, smoking and other poisons, can be distinguished fairly readily. More subtle effects are a matter of individual interpretation, particularly where behaviour is the study, especially if it is the object of social stereotyping.
It may be that differing hormone environments produce different behaviour patterns, which in turn, influence neural organisation, but it would seem likely that they are even more subject to social environmental influence, especially social ones. Nevertheless, the result is that no two humans have exactly the same neural organisation, not even identical twins.
Edelman's theory of 'Neural Darwinism' in its 'strong' form has, not unsurprisingly, received some criticism. His three books are not an easy read for the uninitiated, but they are reviewed in Rose's book The Making of Memory.(2)
There are, for instance, clearly defined maturational stages. What makes the baby reach out for an object in the first place? Crawling and walking are learnt as the muscles and limbs develop. Adults are so much larger and so different to infants, it is by no means clear how the latter recognise that the former are role models to be imitated.
There is also a built-in urge to communicate, later expressed by language. Language develops so quickly that there must be some predetermined disposition, one for which there seems to be a distinct period of maximum ability.(3) Moreover, just as hearing children learn spoken language, deaf children rapidly learn sign language as quickly.
Some writers even suggest the newborn baby has the necessary mental template to enable it to swim but, not being exercised, in the absence of a water environment, it disappears.
The cortex therefore seems to be composed of generally similar general purpose cells, with a subtle innate code which takes the form of guidelines, rather than rules.
What modern biologists are saying is that the idea of nature versus nurture is out of date. An organism must show specificity, that is, it must grow in accordance with its genes, which have developed over thousands of years of natural selection. It becomes a clearly defined species, specialised for a certain lifestyle, developing in a stable way to resist influences in the environment. But it must also show plasticity, the ability to adapt to rapid changes in the environment. The real challenge for biologists is to understand the relationship between specificity and plasticity, and to acknowledge that there is not one archetypal human brain.
Our genes give us a brain which is wired up to become a specifically human brain, but equally they give us the plasticity to enable our memories and our ways of retrieving them to survive whatever befalls us.
What applies to the cortex in general, applies especially to the association cortex - simplistically the thinking area. It is safe to say that we know virtually nothing about it.
Different parts, it is true, seem to be used in different ways. The forward areas seem to be involved in problem solving and strategies, whereas towards the rear different areas seem to involved in making inferences from the senses.
A strong clue that biological factors are involved, is if particular specialisations are present at birth, not always an easy thing to determine. One writer,(4) for instance, suggests that there is considerable lateralisation at birth and it seems that these differences are apparent quite early in life. Another(5) suggests that the two hemispheres are fairly equal up to five years old. Possibly the difference between the two writers is a matter of degree, rather than outright contradiction.
What of a boy who spends his early childhood with books, rather than construction sets? Or the girl who is free to roam the countryside, rather than being closeted within the home? What about a child who grows up in an academic or intellectual family? Would it benefit boys' later language abilities, if fathers, with whom they identify, were more articulate and demonstrated a love of books? What about more subtle social stimuli. Does a child have an innate predisposition to react differently to different sex others? Or is it a learned reaction to different personalities regardless of their sex? Is a toddler, growing up in Northern Ireland forever doomed to an unreasoning hatred of those who don't share its family's religious loyalties?
Left brain, right brain.
The discovery that, between the two hemispheres there have been found marked, and fairly controversial, differences in function, came from a group of operated epileptic patients. Normally, the two halves are in continuous communication through the connecting nerves of the corpus callosum.
In the patients concerned, the very severe seizure they experienced in one hemisphere would cross over and trigger a massive effect in the other. To prevent the crossing over, their corpus callosum was surgically severed, with the result that their symptoms diminished considerably, and they appeared to function perfectly well in daily life.
What had actually happened was fairly subtle. If you gave a such people, say a bunch of keys, they could both name it and undo a lock with it. However, if a word was presented only to their left field of view, and they were asked to select from a group of objects behind a screen, they could do so but not say what the word was.
The two halves of the subject's brains were unable to communicate with each other, and it was suggested that the two halves were specialised for dealing with different kinds of information. It will be noted that naming and recognition are functions of the association cortex, not of the visual system which remained complete. The subjects would be asked to fix their eyes on a spot in the centre of their visual field. An image would be flashed briefly on either the left or right.
Later, goggles were devised that would occlude either the right or the left half of the visual field of each eye. To show that the effect was not a result of incidental brain damage in the patients, similar studies were conducted with people whose corpus callosa were intact.
There are two hypotheses concerning the way this works. One suggests that the information is processed by the hemisphere that first receives it, but there may be a difference in ability. The other view is that the information is transferred to the other hemisphere, incurring a slight delay.
The general hypothesis is that there is a left hemisphere specialisation for language functions, and a right hemisphere specialisation for visuospatial stimuli. It has been claimed that subjects recognise faces presented to the left visual field, and therefore the right hemisphere, more quickly. Generally, although the processing of words by the left has been supported, it has been more difficult to find material which is preferentially processed by the right.
It should be emphasised that the two halves of the cortex work in conjunction with each other, as an integrated system, by communication through the corpus callosum. However, studies using EEG (electro encephalograph) measurements show differences in activity according to the task. There is proportionally higher activity in the left side for verbal tasks, such as reading, and in the right side for spacial tasks, non verbal tasks, such as making complex designs, or interpreting facial expressions.
There is some evidence from other neurological patients. People with right side damage often lose their sense of direction. Post mortem examinations show that the left hemisphere is usually larger and, while the right hemisphere has many long neural fibres connecting widely separated areas, the left hemisphere has shorter fibres providing rich connections over smaller areas.
There are several studies connecting lateralisation with whether one is right or left-handed. Until recently it was thought that only humans were lateralised but it has been shown that other animals show handedness. They will, for instance, use one paw preferentially for handling food or beginning to walk. However the proportion of different individuals is fifty- fifty, whereas around 90% of humans are right-handed. It seems likely that animals begin life using a particular paw, which then acquires more dexterity, so that the habit continues.
Among primates there has been controversy, with arguments that it depended on the tasks observed. It would seem that there is greater handedness on more complex tasks, a handedness that has been magnified in humans. In other words the distinction seems to be between handedness and manual specialisation. However, some humans may, for instance, use the right hand exclusively for writing, but either hand for reaching out. Others may be exclusively right handed.
Bibliography and good reading.
Bland, J., (2003) About Gender: The Cerebral Cortex
Book graphics courtesy of Amazon.co.uk
Web page copyright Derby TV/TS Group. Text copyright Jed Bland.
08.04.98 Reorganised 19.08.03 Last amended 08.04.98