Look on any model of the brain, and you will see that is it divided quite symmetrically into two halves, the left hemisphere, and the right hemisphere. Not much is understood about the brain, even less about where the so-called 'seat of the soul' lies. However, what has been discerned, is that the left and right halves are similar in looks, but very different in function.
Although they are different in some aspects, the left and right hemispheres are similar in some aspects too. The whole brain can be divided into three areas, Primary, Secondary and Tertiary.
These tend to show what is technically known as 'point to point' representation, in other words, direct stimulation of an area elicits a response in a certain area. Stimulation of the same area elicits the same response. For example, if you stimulate an area which is responsible for the movement of the hand, then the hand will move. Do it again, and the hand will move in exactly the same way. This will happen on both sides, equally.
It might be important to say here that the left hemisphere of the brain controls the right side of the body, and vice-versa.
There are primary areas for movement (Primary Motor Cortex), for the senses (Primary Somatosensory Cortex), hearing (Primary Auditory Cortex), and for vision (Primary Visual Cortex).
These also show some kind of point to point representation, but in not as a pronounced manner as in primary areas. They are also not as symmetrical. Stimulation in these regions leads to more complex movements, or perception of senses. For instance, hearing music, or smelling a flower. Stimulating the same area provokes a similar response.
The secondary areas are also organised in a similar way to the primary areas.
Tertiary Areas/The Association Cortex
This is the nasty one. There is no point to point representation and this area is asymmetric too. So why is it under 'symmetry'? Simply because these areas exist in both hemispheres.
Stimulation of this area produces complex responses. For instance, stimulation may cause the person to remember walking down a beach in their youth, or playing tennis last week with their boss. However, stimulate the same area in a week's time, and they will describe something different.
Why is this the case? The brain is a constantly changing organ. Memories are shifted, and stored in one area, and then may be moved to another; constantly adapting to the changing and ever varied experiences that we have. This is shown by babies who have had to undergo major surgery to remove one half of their brain. They recovered, grew up, and still live today, have normal intelligence, and some have gone on to get degrees and so forth. It seems that the brain can adapt to such dramatic changes as these, and delegate tasks to the other side or different areas to some degree, after damage or quite extensive surgery such as that outlined before. It is this, the very plasticity of the brain that makes it such a pain to investigate.
In addition, the brain can be divided into six layers, named, unsurprisingly, layers I-VI. These layers are the same on both sides of the brain, and divide it into different areas of complexity. In other words, different layers are concerned with different difficulties of tasks.
Layer VI deals with the more complex tasks, the higher functions of the brain; integration and processing of thought and signals from various areas of the body.
Descending to layer I, the least complex area; predictably, this layer deals with less complex functions, more in the way of reflexes with no real thought processing or integration.
Structually, they are arranged from top to bottom, with layer VI at the top, and layer I being nearest the bottom of the brain.
So those are the similarities - so what are the differences?
The Different Hemispheres
As said previously, the brain is divided into two halves; left and right, with the left side generally controlling the right side of the body and vice versa. These do have similarities, but they also have many marked differences too.
This is known through 'lesion studies'. Most studies of areas of the brain come about when a person has had an injury, or lesion to that area of the brain, through a stroke or an accident left their brain starved of oxygen.
As a result, it has been discerned that the left side is the more technical, the more linear side of the brain. Its main functions are in speech and writing, along with balance and the organisation of movement. The right side is more holistic, more concerned with the emotional and visual aspect of things.
Here, it is concerned with the expression of language and its comprehension.
Those who have lesions in the area which is responsible for the expression of language called Broca's area, will have some difficulty in the expression of language. Even though they may know what to say, they cannot put it into a recognisable sentence.
Those who have lesions in the area which is responsible for the comprehension of language called Wernicke's Area, can speak with great fluency, and indeed, in gramatically correct sentences, but the content is pure gibberish.
In both cases, the difficulties resulting from these lesions leads to aphasia; a difficulty in the expression and/or the comprehension of language.
This deals with different aspects of language. Prosody; the variation in the tone of the voice, is integrated on the right hand side, and lesions here would give you a monotonous tone of voice, no matter how emotional you are feeling. Some language comprehension also occurs here.
It also seems to be the location for the area which deals with 'emotionally potent language' - or in other words, swearing. This is evident in those who have lesions in both the comprehension and expression of language areas on the left side, and thus be unable to speak, yet when they hurt themselves, they manage to swear in a completely fluent and comprehensible manner, with excellent expression of emotion.
Here, it is primarily concerned with linear problems, ie arithmetic, more of the 1+1=? problems. Lesions in this area lead to acalculia - an inability to carry out linear-type mathematics, even though you may recognise the numbers, and what the mathematical symbols mean.
This side concerns itself with maths which has a spatial component, ie geometry and algebra.
Praxis - Coordination of Movement
Both the left and right side are concerned with constructional praxia, or the coordination of movement. However, the left side is more on the ordering and programming of movements - ie, writing, or making a cup of tea. The right side is more visual, and spatial - ie building something out of bricks.
The right side also has an additional feature, and that is what is known as 'Dressing' praxia, or the coordination of putting your clothes on. This phenomenon is relatively unexplained, and was only discovered when those who had lesions in the right parietal lobe (here is a diagram of where the lobes all are in the brain for a guide to the location of different lobes of the brain), have what is known as 'dressing' apraxia; in other words, they couldn't put their clothes on, yet could walk, and move about quite normally.
Predictably, this side concerns itself with verbal memorisation. The recognition of words comes from this area.
In addition, the short term memory of word sequences - or the articulatory loop. For instance, you can remember the short phrase, 'I am a fish' so long as you keep repeating it. If you stop repeating it, and do something else, then you will quickly forget the phrase.
This does something similar to the left side, only with the more visual aspect. It is involved with the recognition of faces and your spatial location.
It has a type of articulatory loop called the visuospatial sketchpad. For instance, look at the four corners of your computer monitor in this order; top right, bottom left, bottom right and top left. As long as you keep repeating that sequence, you will remember it. As with the articulatory loop, as soon as you stop and do something else, you will forget it.
One of the most marked differences between the two halves is the emotional response. Whereas previous features have resulted in similar functions for different aspects of understanding the world around you, this is where the two halves become diametrically opposite.
The optimist's half. This is associated with positive emotions, ie happiness, joy, pleasure, and general ebullience. It is also associated with the control of emotions. Lesions in this area leads to what is known as a 'catastrophic reaction', where even the slightest of emotional situations leads to what can only be described as an inappropriately large emotional response. For instance, you may tell them that you've spilt a small drop of tea on the floor, and they will burst into floods of tears.
The pessimist's half. This is associated with negative emotions, such as anger, sorrow and generally being moody and bad-tempered. Whereas the left side deals with emotional control, the right side deals with emotional perception ie, the ability to read the emotions of others. It is also associated with the expression of emotion. Lesions in this area lead to what is known as 'la belle indifférence', where the person, no matter how stressful or traumatic the situation is, couldn't give a proverbial. They react calmly to even the worst of news, simply because, even though their control of emotion is intact, their emotional understanding and the appropriate expression of emotion is impaired.
How is this Known?
In the mid-20th century, the only cure for severe epilepsy, was surgery to divide the brain into two, in the hope that their symptoms would disappear. It has to be understood that though this may seem brutal, that their symptoms were incredibly severe - these people would have devastating fits which were not controlled by drugs. They couldn't go out of their own homes, such were the unpredictability of when the fits would occur.
The surgery worked, and their symptoms went. Their quality of lives improved no end. They also started to comment on strange things happening, which pricked the curiosity of psychologists, who had long suspected that the two halves of the brain were associated with different things.
Several tests were devised. One was showing the subject a picture of an everyday household object, say, a cup. They were first asked to say what it is, and then write what it was down. This seems easy, but these people reacted quite differently. They could recognise the object, but not say what it was. They knew what it was for and could describe that with great fluency in speech and writing, yet not be able to say or write down that it is a cup. From tests such as this, it was first established that the two halves, though being in one brain, coordinated different aspects.
The next step was more recent. The advances in scanning technology enables psychologists to scan the brain. In fact, one of the hangovers of the Cold War*, was the number of redundant physicists; wildly talented, but jobless. A few of them had worked on a system which was designed to detect small changes in electric current. These people had worked on a system so sensitive, that when put on a satellite, high up in orbit, they could detect a submarine cruising through the oceans, far below them. Of course, they were no longer wanted by the military. So they offered their services to medicine*.
The detection and localisation of small electrical currents in the brain would have been impossible without this system. It was then, demonstrated, that indeed, the primary visual cortex is in the back of the brain, an area known as the occipital lobe, by overlaying an image showing the pattern of electrical activity in the brain in response to a flash of light, with a plain MRI* image of the brain.
Consequential studies on language comprehension and memory have shown the pattern of association which has already been outlined above; that indeed, there may be one brain, but two very different halves.
Indeed, a significant difference has been found, which went to some way into explaining the differences between the thought processes of males and females.
La Belle Différence*
There is a well known assumption that although women supposedly cannot read maps, men supposedly cannot cope with concentrating on more than one thing at a time. In short, women have no sense of direction, and men have a one track mind.
Joking aside, this was thought to be just chauvinistic or feministic opinion, oft made fun of, but with no scientific evidence to back it up. That was, until someone decided to carry out a study on the differences between the brains of the genders.
Using the electrical activity technique (see above) whilst subjects were carrying out various tasks, different patterns of brain activity were shown. Men performed better in tasks associated with the left side; ie mathematics, and the linear order of events. Women were shown to have less of a disproportionate map of activity, ie, spread more equally on both sides.
The map also showed that male brains tend to be more active in the areas of visuospatial understanding, and women tended to show more activity in the verbal regions of the brain.
Hence why women apparently tend to read emotions better, yet allegedly have some difficulty with reading maps or parking, and also seem to be able to do several tasks at once, and men can allegedly read maps, yet apparently have an inability to express their emotions in words.
So this may go some; but not all the way to explaining the clear differences between the cognitive strengths of the genders.
The left half controls the right hand side of the body, and vice-versa. So what happens in left handed people?
Contrary to popular belief, the brain areas are not swapped, and the left is still associated with the technical, linear side of things, and the right, the holisitic side of things. Neither is it the case that the right half is completely in control.
What is shown in righties, is that they show, on the electrical activity maps, a 'left-dominance', where the left half of the brain is more dominant over the right. Hence, the more empathic, holistic and overly emotional side is suppressed slightly, allowing the more objective and linear side to have some executive control over our emotions and thoughts.
What is the case is that lefties, on the electrical activity maps, tend to show less of a disproportionate dominance, and the activity is spread more evenly between the two halves. Hence the common train of thought where lefties seem to apparently be more artistic, or emotional. Right handers seem to apparently be more thoughtful, less emotional, even more clinical. A really extreme example of this is of left handed John McEnroe and right handed Björn Borg*.
It may also partially explain the disproportionate number of left-handed people who are diagnosed with mental illness. For even though left handers make up only 10% of the UK population, they make up 20% of those who are diagnosed with a mental illness, and there are more lefties who suffer from epilepsy, autism and learning disorders. This may be because of the even distribution of activity between the two sides; they may have less executive control over their emotions and thoughts than right handers.
However, it must be stressed that this is only a theory, rather than out and out fact, simply because we know very little about the associations between different sides and the control and expression of emotion.
The Possiblities are Infinte
However, this is not to say that here is everything to know about the brain. Painfully little is known about the higher functions of the brain, and the 'seat of the soul' has yet to be found.
However, our understanding of the brain, and its mysterious tale of two halves, is gradually starting to unravel.
BBC Links and Further Reading
Want a basic overview of the nervous system? Here is a nice overview from BBC Science: Brain and Spinal Cord.
For some very nice pictures and clear, no-fuss diagrams, then click on over to BrainConnection.com.
For the basics of neuroscience, why not have a look at 'Neuroscience for Kids'?