Understanding The Human Brain
July 21, 2008: We know that people differ in the way they solve problems. We call this their intellectual, or cognitive, ability. We may differ from our friends not only in overall ability, sometimes referred to as IQ, or "smarts", but also in the pattern of our abilities. One person may be especially good at problems involving words, whereas another may be better at dealing with problems relating to real-world objects. Since the brain is the centre for all such activities, there must be some way in which brains differ slightly from one person to another.
For most of the history of brain research, until about 20 years ago, our information about the human brain was obtained primarily from post-mortem cases. This is information from people who have died, and whose brains are studied after being fixed in solutions to preserve them. But in the past few years, new ways of looking at the brain safely in living persons have become possible, so now we are better able to link the abilities of a particular person with his or her brain structure.
Our problem-solving abilities depend mainly on a part of the brain called the cerebrum. The cerebrum is divided into two parts called the left and right cerebral hemispheres. These hemispheres don't have exactly the same functions, and this difference in the way the two hemispheres work is called functional asymmetry.
Differences between the hemispheres
The left hemisphere is important for all forms of communication. We know this because when it is damaged, perhaps as a result of an accident or a stroke, there can be serious problems in speaking (this is known as aphasia). After left-hemisphere damage there can also be difficulties with other complicated movements of the mouth, or of the hands and arms -- demonstrating a pout, or miming how to salute or to hammer a nail, for example. It seems that the left hemisphere specializes in controlling certain movements, including the movements we use to communicate. In people who are born deaf and who communicate using hand movements (manual sign language), damage to the left hemisphere can badly affect their signing ability.
The right hemisphere, by comparison, doesn't appear to be involved much in communication, although it can help us understand words to some extent. Instead, it specializes in receiving and analysing information from the outside world. Therefore, damage to the right hemisphere may result in our being unable to tell the difference between melodies, or having difficulty in identifying a face or in locating an object accurately in space. Some parts of the right hemisphere are mainly concerned with helping us understand what we hear (auditory), while other parts help us make sense of things that we see. The temporal lobe (in the lower part of the hemisphere) analyses much of the auditory input, while the occipital and parietal lobes (in the rear and upper regions) provide information about where objects are. The frontal lobes in each hemisphere seem to be important in planning our actions.
Seeing the differences
Although we have known for over a hundred years that the two cerebral hemispheres have these different functions, or properties, until recently people thought that they looked the same. Now we know that there are also small differences in the appearance, that is, in the actual anatomy or visible structure of the left and right sides. These differences are called anatomical asymmetries. Let's look, for example, at the temporal lobe. Part of this lobe is hidden in the Sylvian fissure. This part, important for understanding speech, is larger in area on the left side than the same region on the right side. Babies are born with this difference, so we know that it doesn't develop as a result of children learning to speak. There is also a difference in the appearance of the Sylvian fissure on the two sides, with the left side arching up more gradually than the right. Other anatomical asymmetries between the left and right sides have also been observed.
Right left, left right?
We have seen that the two sides of the brain are different, but how much can one person's brain differ from another's? Research shows that all human brains are very much alike, but there are minor differences between one brain and another. The question is whether these are just chance variations which don't mean anything, or whether it can be shown that they are associated with different traits in different people. One characteristic that might be expected to relate to brain asymmetry is hand preference, that is, which hand a person prefers for most everyday activities, including writing. Over 90 per cent of people are right-handed. We know that fine movements of the hands depend on activity in areas of the opposite hemisphere of the brain. Right-hand movements depend critically on the left hemisphere. This led scientists to believe that the left hemisphere was somehow "dominant" in most people, because it controlled the preferred hand and was also essential for speaking.
At first it was taken for granted that left-handers would show an opposite, or mirror-image, pattern to right-handers, that is, their speech would rely mostly on the right hemisphere. But over time it has become clear that this is not quite the case. Although it is true that left-handers more often rely on the right hemisphere than do right-handers, well over half of them are like right-handers in so far as they rely mainly on the left hemisphere for speech. So how do left-handers' brains differ from right-handers? Research is still going on to explore this question. So far it seems that the temporal-lobe and Sylvian-fissure anatomical asymmetries we described above are less marked in left-handers than in right-handers.
Male and female brains
Whether you are a boy or a girl also determines how your brain looks and works. We know from animal research that substances called sex hormones, produced by the sex glands, are needed to develop the differences between males and females. Sex hormones are necessary both for forming the genitals and for the behavioural and brain differences between the sexes. The hypothalamus, which is a tiny structure at the base of the brain, regulates many basic functions, such as eating, sleeping, temperature control, and reproduction. One part of the hypothalamus responsible for sexual behaviour is larger in male brains than in female brains, in human and non-human animals. In rats the enlargement is known to depend on male sex hormones, called androgens.
Sex hormones also affect other parts of the brain. For example, the outer layer of the cerebrum, called the cortex, is thicker on the right hemisphere than on the left in male rats, but not in female rats. Another recent discovery is that male and female brains in some ways work differently. When set the same task, females may use both hemispheres, while male brain activity is restricted to one side. For example, if the task is to define words, men appear to use only their left hemisphere, while women use both. For many other problem-solving activities however, men's and women's brains work in the same way.
The left and right cerebral hemispheres are connected by fibres running crosswise between them called commissures. The largest and most important commissure is called the corpus callosum; another important connection is the anterior commissure. One way the commissures are useful is in exchanging information between the two hemispheres.
These connections between the hemispheres may also be somewhat different in men and women. The area of the anterior commissure seems to be larger in women, and some researchers have found that the back part of the corpus callosum is larger in women. If the larger area of the commissures results in better communication between hemispheres, this could make some difference to the way men's and women's brains work.
Finally, there is probably also a difference between men and women as to which part of the left hemisphere is responsible for speech and hand movements. There are two major areas devoted to speech, one in the frontal lobe, and the other at the back, where the temporal and parietal lobes meet. In women, the frontal region is more important than the area at the back, so problems with speaking are more likely to happen if the front part of the left hemisphere is damaged. In men, the areas contribute more equally, but if anything the back part, especially the parietal region, is more important.
Evolutionary change
Some of the differences between the ways that men's and women's brains work must have evolved over time. We know that the average man and woman have slightly different intellectual strengths. Some of these differences appear to be the result of a division of labour between men and women going back to our hunter-gatherer past. For example, men are better at spatial-navigational skills such as map reading and judging distances and at targeting skills (dart throwing for instance). These skills probably developed through hunting. Women have a better memory for words and objects, and are better at fine motor skills. These abilities probably developed through food gathering near the home and through making clothes and preparing food.
We know from animal research that sex hormones help determine such patterns, because if the brains of young female rats are exposed to androgens right after birth, their spatial abilities as adults are different from normal females', and more like males'. Similarly, in humans, girls exposed to excessive androgens early in life have better spatial skills than other females. Exactly how sex hormones cause changes in the brain to make one person intellectually different from another is not yet understood in detail, but it is a fascinating subject which is the focus of much current research.
Monday, July 21, 2008
Understanding The Human Brain
Understanding The Human Brain
July 21, 2008: We know that people differ in the way they solve problems. We call this their intellectual, or cognitive, ability. We may differ from our friends not only in overall ability, sometimes referred to as IQ, or "smarts", but also in the pattern of our abilities. One person may be especially good at problems involving words, whereas another may be better at dealing with problems relating to real-world objects. Since the brain is the centre for all such activities, there must be some way in which brains differ slightly from one person to another.
For most of the history of brain research, until about 20 years ago, our information about the human brain was obtained primarily from post-mortem cases. This is information from people who have died, and whose brains are studied after being fixed in solutions to preserve them. But in the past few years, new ways of looking at the brain safely in living persons have become possible, so now we are better able to link the abilities of a particular person with his or her brain structure.
Our problem-solving abilities depend mainly on a part of the brain called the cerebrum. The cerebrum is divided into two parts called the left and right cerebral hemispheres. These hemispheres don't have exactly the same functions, and this difference in the way the two hemispheres work is called functional asymmetry.
Differences between the hemispheres
The left hemisphere is important for all forms of communication. We know this because when it is damaged, perhaps as a result of an accident or a stroke, there can be serious problems in speaking (this is known as aphasia). After left-hemisphere damage there can also be difficulties with other complicated movements of the mouth, or of the hands and arms -- demonstrating a pout, or miming how to salute or to hammer a nail, for example. It seems that the left hemisphere specializes in controlling certain movements, including the movements we use to communicate. In people who are born deaf and who communicate using hand movements (manual sign language), damage to the left hemisphere can badly affect their signing ability.
The right hemisphere, by comparison, doesn't appear to be involved much in communication, although it can help us understand words to some extent. Instead, it specializes in receiving and analysing information from the outside world. Therefore, damage to the right hemisphere may result in our being unable to tell the difference between melodies, or having difficulty in identifying a face or in locating an object accurately in space. Some parts of the right hemisphere are mainly concerned with helping us understand what we hear (auditory), while other parts help us make sense of things that we see. The temporal lobe (in the lower part of the hemisphere) analyses much of the auditory input, while the occipital and parietal lobes (in the rear and upper regions) provide information about where objects are. The frontal lobes in each hemisphere seem to be important in planning our actions.
Seeing the differences
Although we have known for over a hundred years that the two cerebral hemispheres have these different functions, or properties, until recently people thought that they looked the same. Now we know that there are also small differences in the appearance, that is, in the actual anatomy or visible structure of the left and right sides. These differences are called anatomical asymmetries. Let's look, for example, at the temporal lobe. Part of this lobe is hidden in the Sylvian fissure. This part, important for understanding speech, is larger in area on the left side than the same region on the right side. Babies are born with this difference, so we know that it doesn't develop as a result of children learning to speak. There is also a difference in the appearance of the Sylvian fissure on the two sides, with the left side arching up more gradually than the right. Other anatomical asymmetries between the left and right sides have also been observed.
Right left, left right?
We have seen that the two sides of the brain are different, but how much can one person's brain differ from another's? Research shows that all human brains are very much alike, but there are minor differences between one brain and another. The question is whether these are just chance variations which don't mean anything, or whether it can be shown that they are associated with different traits in different people. One characteristic that might be expected to relate to brain asymmetry is hand preference, that is, which hand a person prefers for most everyday activities, including writing. Over 90 per cent of people are right-handed. We know that fine movements of the hands depend on activity in areas of the opposite hemisphere of the brain. Right-hand movements depend critically on the left hemisphere. This led scientists to believe that the left hemisphere was somehow "dominant" in most people, because it controlled the preferred hand and was also essential for speaking.
At first it was taken for granted that left-handers would show an opposite, or mirror-image, pattern to right-handers, that is, their speech would rely mostly on the right hemisphere. But over time it has become clear that this is not quite the case. Although it is true that left-handers more often rely on the right hemisphere than do right-handers, well over half of them are like right-handers in so far as they rely mainly on the left hemisphere for speech. So how do left-handers' brains differ from right-handers? Research is still going on to explore this question. So far it seems that the temporal-lobe and Sylvian-fissure anatomical asymmetries we described above are less marked in left-handers than in right-handers.
Male and female brains
Whether you are a boy or a girl also determines how your brain looks and works. We know from animal research that substances called sex hormones, produced by the sex glands, are needed to develop the differences between males and females. Sex hormones are necessary both for forming the genitals and for the behavioural and brain differences between the sexes. The hypothalamus, which is a tiny structure at the base of the brain, regulates many basic functions, such as eating, sleeping, temperature control, and reproduction. One part of the hypothalamus responsible for sexual behaviour is larger in male brains than in female brains, in human and non-human animals. In rats the enlargement is known to depend on male sex hormones, called androgens.
Sex hormones also affect other parts of the brain. For example, the outer layer of the cerebrum, called the cortex, is thicker on the right hemisphere than on the left in male rats, but not in female rats. Another recent discovery is that male and female brains in some ways work differently. When set the same task, females may use both hemispheres, while male brain activity is restricted to one side. For example, if the task is to define words, men appear to use only their left hemisphere, while women use both. For many other problem-solving activities however, men's and women's brains work in the same way.
The left and right cerebral hemispheres are connected by fibres running crosswise between them called commissures. The largest and most important commissure is called the corpus callosum; another important connection is the anterior commissure. One way the commissures are useful is in exchanging information between the two hemispheres.
These connections between the hemispheres may also be somewhat different in men and women. The area of the anterior commissure seems to be larger in women, and some researchers have found that the back part of the corpus callosum is larger in women. If the larger area of the commissures results in better communication between hemispheres, this could make some difference to the way men's and women's brains work.
Finally, there is probably also a difference between men and women as to which part of the left hemisphere is responsible for speech and hand movements. There are two major areas devoted to speech, one in the frontal lobe, and the other at the back, where the temporal and parietal lobes meet. In women, the frontal region is more important than the area at the back, so problems with speaking are more likely to happen if the front part of the left hemisphere is damaged. In men, the areas contribute more equally, but if anything the back part, especially the parietal region, is more important.
Evolutionary change
Some of the differences between the ways that men's and women's brains work must have evolved over time. We know that the average man and woman have slightly different intellectual strengths. Some of these differences appear to be the result of a division of labour between men and women going back to our hunter-gatherer past. For example, men are better at spatial-navigational skills such as map reading and judging distances and at targeting skills (dart throwing for instance). These skills probably developed through hunting. Women have a better memory for words and objects, and are better at fine motor skills. These abilities probably developed through food gathering near the home and through making clothes and preparing food.
We know from animal research that sex hormones help determine such patterns, because if the brains of young female rats are exposed to androgens right after birth, their spatial abilities as adults are different from normal females', and more like males'. Similarly, in humans, girls exposed to excessive androgens early in life have better spatial skills than other females. Exactly how sex hormones cause changes in the brain to make one person intellectually different from another is not yet understood in detail, but it is a fascinating subject which is the focus of much current research.
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