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.
Similarities of the Two Hemispheres
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 (called the 'primary motor cortex'), for the senses (called the 'primary somatosensory cortex'), for hearing (called the 'primary auditory cortex'), and for vision (called the 'primary visual cortex').
These also show some kind of point-to-point representation, but not as pronounced a manner as in the 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 very 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.
This is the case because 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 has been shown by babies who have undergone major surgery to remove one half of their brain. They recovered, grew up, and still live today. They 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. It is this very plasticity of the brain that makes it such a pain to investigate.
The brain can also be divided into six layers, named 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 more complex tasks, the higher functions of the brain. The integration and processing of thought and signals from various areas of the body takes place here. Layer I, the least complex area, deals with less complex functions. These go more the way of reflexes that require no real thought processing or integration.
Structurally, the layers are arranged from top to bottom. Layer VI is nearest to the top of the brain, and layer I rests nearest the bottom of the brain.
The Differences between Hemispheres
As said previously, the brain is divided into two halves; the left and the right. The left side of the brain generally controls the right side of the body and vice versa. These hemispheres do have similarities, but they also have many marked differences too.
This information is known due to '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. This could be caused by a stroke or an accident that left the persons brain starved for 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.
The left side of the brain is concerned with the expression of language and its comprehension.
Those who have lesions in the area called Broca's Area - which is responsible for the expression of language - 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 Wernicke's Area - which is responsible for the comprehension of language - can speak with great fluency, and indeed, in grammatically correct sentences. The content of the sentences on the other hand 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 sid. 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. When they hurt themselves, they manage to swear in a completely fluent and comprehensible manner, with excellent expression of emotion.
Here, the brain is primarily concerned with linear problems ie, arithmetic. Lesions in this area lead to 'acalculia' - an inability to carry out linear-type mathematics. Even so, you may recognise the numbers, and what the mathematical symbols mean.
This side concerns itself with mathematics which have a spatial component ie, geometry or 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; for example, 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 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.
This side of the brain concerns itself with verbal memorisation. The recognition of words comes from this area. In addition, so does 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 a 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.
This is the optimistic half. It 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 lead 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 right side is the pessimistic 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' (literally, 'the beautiful indifference'), 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. The reason for this is that 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, their symptoms were incredibly severe. These people would have devastating fits which could not be controlled by drugs. They couldn't go out of their own homes, such was the unpredictability of when the fits would occur.
The surgery worked, and their symptoms went away. Their quality of life improved to no end. They also started to comment on strange things happening, which pricked the curiosity of psychologists. They had long suspected that the two halves of the brain were associated with different things.
It was during this time that 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 was, and then write what it was down. This may seem easy, but these people reacted quite differently. They could recognise the object, but not say what it was. They knew what it was used for and could describe that with great fluency in speech and writing, yet they were not able to say or write down that it was a cup. From tests such as this, it was first established that the two halves, though being one brain, coordinated different aspects of thought.
The next step in our knowledge of the brain was more recent. The advancements made in scanning technology enabled psychologists to scan the brain. In fact, one of the hangovers of the Cold War1, was the number of 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. Of course, these physicists were no longer wanted by the military after the end of the Cold War. So they offered their services to medicine2.
The detection and localisation of small electrical currents in the brain would have been impossible without this system. It was then demonstrated that the primary visual cortex is in the back of the brain, an area known as the 'occipital lobe'. This experiment involved overlaying an image showing the pattern of electrical activity in the brain in response to a flash of light, with a plain MRI (Magnetic Resonance Image) of the brain.
Consequential studies on language comprehension and memory have shown the pattern of association which has already been outlined above. There may be one brain, but two very different halves.
A significant difference has been found, which helped in some way of 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 while 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. 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. The right side, still is associated with the holistic side of things. Neither is it the case that the right half is completely in control.
What happens in righties, is that they show on the electrical activity maps a 'left-dominance'. This is 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. This allows the more objective and linear side to have some executive control over our emotions and thoughts.
What the case is in lefties on the electrical activity maps is that they tend to show less of a disproportionate dominance. The activity is spread more evenly between the two halves of the brain. Hence the common train of thought in lefties seems to be more artistic, or emotional. Right-handers seem to be more thoughtful, less emotional, even more clinical. A really extreme example of this can be seen in left-handed John McEnroe and right-handed Björn Borg - two legends in the sport of tennis. McEnroe was, and still is, renowned for his complete inability to keep his cool in a match, making full use of the area of his brain associated with 'emotionally potent language'. Borg, was the complete opposite, renowned for his clinical deliverance of shots, and a complete indifference to the high emotions of the situation on the tennis court.
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 Possibilities are Infinite
This is not everything that could be known 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.