Attention deficit hyperactivity disorder

An article published in the Times newspaper in January 2010, claimed that children who are ambidextrous are more likely to have learning and language difficulties.3 The article reported a recent study published in the American journal 'Pediatrics',4 to help identify children who are at risk from certain problems, such as attention-deficit-hyperactivity-disorder (ADHD). However, it is not clear from the Times article whether they are talking about true ambidexterity or mixed handedness. The article in 'Pediatrics' makes it clear that mixed handedness is the category being considered.

Handedness refers to a preference for one hand over the other.1 It is normally determined by the hand people choose to write with, however, most people are unaware that they are actually mixed handed. Mixed handedness is when one hand is more dominant than the other, but some tasks are still done with your less dominant hand.2 For example, you may write with your right hand, but throw a ball with your left hand. It is rare for people to be able to accomplish tasks at an equal standard with both hands, as one is normally better than the other. In cases where both hands are equally capable, this is referred to as true ambidexterity.

Only 13% of the population are left-handed, with it being more common in males than females; 85% are right-handed and the remaining 2% are truly ambidextrous.6 Research shows that males are more likely to be left-handed than females, as left-handedness may be linked to the level of testosterone in the womb before birth.14 Other research suggests that females are more likely to want to conform to social norms, so they may change hands to fit in with their peers.6 However, we now know that handedness is not our choice, but part of the physiology of our bodies. The proportion of left-handed people has increased slightly since the last study. This is believed to be because left-handedness is more socially acceptable now than it used to be a few decades ago.

Handedness is measured using a continuum as people normally prefer to use different hands for different tasks, and aren't confined to only being able to use one.7

Graph from book page 164

Various tests can be carried out to measure how mixed handed someone is. Most give a numerical score using a scale, to illustrate your hand preference.

Statistics show that there is likely to be a genetic link for handedness. There is a 9% chance of two right-handed parents having left-handed children, a 26% chance for two left-handed parents, and a 19% chance for one left and one right-handed parent.6

Handedness within a population is extremely asymmetrical as so many people are right-handed people, so it can be argued that there has to be more to it than pure chance.

There have been many theories of where left-handedness originates. An example of these is by Paul Bakan et al, which stated that left-handedness could be a result of birth stress. If the left hand side of the brain is damaged during birth, the right hand side will take over as the dominant one and the person is more likely to be left-handed due to cross laterality. This theory could also explain the apparent genetic determination of handedness. If the mother is left-handed due to birth stress from her mother having a small pelvis; then she is more likely to carry the gene for having a small pelvis and more likely to give birth to a left-hander.11

However, when data sets from pregnant women were collected, this theory was proven wrong as there was not a significant connection between birth stress and left-handedness.12

In another study, conducted by P. G. Hepper et al, foetuses were observed in the womb 10 weeks after gestation. It was observed that they showed some laterality at this early stage of development by moving one limb significantly more than the other. Motor function, at this stage, is believed to be controlled by the spinal cord and muscles, so the brain is not responsible for any signs of lateralisation. This tells us that lateralisation develops before the brain and could go on to determine neural development. This motor behaviour could result in the differences of the left and right hemispheres as they develop later on, but the causes of initial motor lateralisation are still unknown and could have some genetic factors. This study does, however, show sufficient evidence that lateralisation exists from as early on as it is possible to observe, and suggests that lateralisation is a prominent feature of development that can go on to affect behaviour, via future neural development.9

In order to explain where initial motor lateralisation could originate from and the statistics that suggest a genetic link for handedness, a model for co-dominant genes is hypothesised. 'RGHT' is a gene for right-handedness and 'r' is a gene for random handedness. An RGHT/RGHT genotype means the person has to be right-handed; an r/r genotype gives a 50% chance of left-handedness and an RGHT/r genotype gives a 25% chance of left-handedness.

By calculating the gene frequencies from right and left-handed families, the model explains the percentages of left-handed children observed in right-handed families and vice versa. In left-handed families, the children must have an r/r or RGHT/r genotype, to be born right-handed.

This model explains how, within a pair of monozygotic (identical) twins, one can be right-handed and one can be left-handed: this is because the r/r or RGHT/r genotype will result in random hand preferences.10, 13

The classical theory of laterality (i.e. asymmetries of the brain and body), 17 says that the dominant hemisphere of the brain, which represents speech and controls the preferred arm and leg, 16 is normally on the left hand side, as this controls the right side of our body and language is controlled by the left hemisphere of the brain, in right-handed people.15

It was originally believed that right-handed people always had their dominant hemisphere on the left hand side, and left-handed people were a mirror image of right-handers.

However, this is not the case as there are various combinations possible.

Table from page 189

People with right brain damage can still feel effects on their language. This proves that language is affected by both the left and right hemispheres, which make their own individual contribution.

The left hemisphere is responsible for grammar.19

Picture page 194

The right hemisphere provides speech with emotion and emphasis.18

They are both connected by a bundle of fibres called the corpus callosum. This allows the two hemispheres to communicate with each other and coordinate their actions. When cut, the hemispheres achieve what they can individually, but neither is complete without the other half.

For right-handed people, it is assumed that lateralisation for various brain functions is as follows.

The left hemisphere controls grammar of language, but the right hemisphere is responsible for emotion, emphasis, metaphor, sarcasm and humour.18

When drawing, the right hemisphere will form 3D concepts even though it can't control the left hand very well, but the left hemisphere has no understanding of perspective even though it is very controlled.20

In problem solving, the left hemisphere can handle logic, but the right hemisphere reflects on personal experiences.21

The parietal lobe of the right hemisphere is also particularly involved in attention. It allows us to focus on tasks that are important so we aren't flooded with pointless information that would overwhelm our brains, such as the face of everybody we pass on the street. We are unaware of many things until they are pointed out to us.22

The dominant hemisphere for swallowing is on the right in half the population, and the left for the rest.23

The right hemisphere is more sensitive to smells and flavours, but the left is better at naming and identifying them. The right is more important for music and singing, whereas rhythm and pitch are appreciated by the left hemisphere. Sexual arousal activates two areas in the right, associated with motivation, and the left for involuntary responses.24

It is mainly the right hemisphere that is involved in recognising emotions.25

However, this idealisation of modules of brain functioning being split into these hemispheric divisions is only a simplistic overview. The brain is much more complicated than this, as we have seen for language. Also, not everyone's brain is the same; left-handed people have their dominant hand controlled by the right hemisphere, but their brains aren't just mirror images of those in right-handers.

Broca became aware that speech wasn't always on the left hand side of people's brains soon after his discovery of the speech area. However, modern literature still emphasises that language is found on the left hemisphere in right-handed people, and on the right hemisphere in left-handed people. This idea seems to have carried through all these years because it is psychologically satisfying to have an answer to something that is otherwise unknown.26

In terms of brain functioning, left-handers aren't the mirror-image of right-handers, but there is a relationship between handedness and language dominance in the hemispheres.

Table from page 215

This shows that the majority of left and right-handers have language dominance in the left hemisphere, but left-handers are six times more likely to have language in the right hemisphere than a right-hander. Since there are many more right-handed people in society, the actual number of right-handers with language in the right hemisphere is more than the number of left-handers with language in the right hemisphere.27

Language dominance is not a single thing; it is responsible for many different aspects such as speech, reading, writing, spelling and many more. Evidence from patients who have experienced brain damage suggests that some of these functions can be present in one hemisphere, whilst other aspects are on the opposite side.28

One hypothesis that can be used to explain how language dominance is decided is by using a genotype model. Using the same principles as before, an RGHT genotype pushes language dominance to the left hemisphere and an r genotype has no effect on pushing it to one side, so it is left to chance.

Individuals with RGHT/RGHT genotypes are right-handed with language in the left hemisphere. An r/r genotype gives individuals an equal chance of language being in either hemisphere. However, as we found earlier, motor control is pushed to one side and language control is also moved to one side, but these processes happen independently of each other. Therefore, a quarter of people with an r/r genotype will be right-handed with language in the right; a quarter will be right-handed with language in the left; a quarter will be left-handed with language in the right; a quarter will be left-handed with language in the left.

An RGHT/r genotype has the same effects as an r/r genotype, but 3/4 of people are right-handed and ?4 are left-handed. Therefore, the calculations result in a good approximation to the numbers in the earlier table.29

However, it is difficult to know which hemisphere language is located in, for certain.

One way of measuring where language is situated in each individual is by using Transcranial Doppler ultrasonography. This uses the Doppler Effect to measure an increase in the ultrasound pitch, which allows us to measure the speed of blood travelling in the middle cerebral artery. Generating words is a hard task for the brain to accomplish, so the blood supply increases on the side where language is situated, when the individual speaks. By measuring the left and right arteries, we can work out which side language is situated on.

Research done by Knecht et al, in 2000, provided results that were close to the proportions in the table, so we know that their techniques were accurate.30

The left hemisphere may be mainly responsible for language because this side works faster. It allows us to distinguish between phonetics such as 'big' and 'pig', due to the timing of vibrations in the larynx.19

Since foetal research has shown that lateralisation is present even before development of the brain, the neuronal interconnections that develop later on could be responsible for language lateralisation. Studies show that the left hemisphere is more complex in humans than the right, so this may explain why it works faster in most cases.

The brain has always been in two parts, called the right and left hemispheres. They are joined by the corpus callosum, a giant bundle of nerve fibres. The corpus callosum sends all the information that needs crossing between the two hemispheres; this makes it slow and of limited capacity. A solution to this is for each hemisphere to specialise in certain tasks and only cross information to the other side when required.

Table page 243

The ability of each hemisphere to be able to complete certain tasks can explain why the brain becomes lateralised and one side becomes dominant.

Brain functions can become lateralised in the brain due to genes that rely on an aspect of chance, such as the 'r' gene.

Right-handers with RGHT/RGHT genotypes will be the same as everyone else with that genotype, so their brains will be arranged typically.

Each brain function of individuals with an RGHT/r genotype has a 25% chance of being arranged randomly, whereas people with an r/r genotype can have half their functions arranged randomly across the two hemispheres. This may not be beneficial and can explain why conditions such as dyslexia, stuttering, autism and schizophrenia are more common in mixed handedness individuals.

The various components of language probably work better when they are arranged close together, rather than being spread over the whole brain due to chance arrangements. This prevents the various modules from having to continuously communicate across the corpus callosum.31

There can be both benefits and drawbacks of having various modules of brain function in different places. This is more common in left-handers due to their r/r or RGHT/r genotype which leaves room for chance to play a part in brain structure.

For example, if the module involved in symbolic processing of language is moved to the right hemisphere, from the left, alongside modules to process three-dimensional space, the individual may find it easier to carry out some forms of maths, but find it more difficult to understand concepts of emotion.

If you can find something about structural differences in the hemispheres to tie in with your foetal research it would be good. - Basal ganglia in the paediatrics article, but not found anything else (ADHD)

References

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