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Theory of cerebral dominance failure

This ‘dominance failure’ theory proposes that specific learning difficulties are due to failure of one side of the brain to become dominant over the other. This is a theory that is often too readily accepted by those who use terms such as ‘cerebral dominance’ and ‘laterality’ without realizing how difficult these are to ascertain, and how complex their interrelationship is.

The major part of the brain consists of two halves: the right and left cerebral hemispheres. They are connected by a bundle of nerve fibres called the corpus callosum. The two cerebral hemispheres have similar appearances and complementary functions. Each controls the movement of the opposite side of the body: the right hemisphere controls movement on the left side of the body and the left hemisphere controls movement on the right side. Usually one hemisphere has the area that controls most of the language function, while the other is more important for other functions such as spatial, complex visual, and musical skills. By convention we refer to the cerebral hemisphere that controls language as the ‘dominant’ hemisphere. This is because language is such an important function. Sometimes, but not always, this hemisphere is slightly larger than the non-dominant, or minor hemisphere.

Most people have a preference for their one hand that is stronger and more skilful than the other. This preference for one or other side is known as ‘laterality’. In most right-handers, laterality and dominance correspond, i.e. their left cerebral hemisphere controls both language and the preferred hand. In some, though, their language may be controlled by their right hemisphere. This dominant hemisphere does not, in such cases, control the preferred hand.

By contrast, in most left-handers, dominance and laterality do not correspond; their language is usually controlled by their left cerebral hemisphere, while their preferred hand is controlled by their right. Some left-handers have language controlled by both areas.

The dominance failure theory states that one cerebral hemisphere has to dominate over the other, that is, become specialized in certain functions, notably language, in order for a child to be able to learn. If this does not occur, it will give rise to confusion and delay: the so-called ‘strephosymbolis’ (twisted signs), postulated by Dr Samuel T Orton in 1925.

Two findings are usually quoted in support of the theory. First, there is evidence that children with specific learning difficulties are often late in developing hand preference, and second, there are reports of ‘crossed laterality’ in children with specific learning difficulties. ‘Crossed laterality’ is the term used for children who use limbs on different sides of their body for different actions: for example, their right hand to write, and their left to catch a ball. It is also used to describe other situations: for example, the preference for the left eye in a right-handed person, or the preference for the right foot in a left-hander.

There are important objections to the use of these data to support this theory. Tests for laterality, particularly foot, eye, and ear preference, are not reliable. Much of the information on laterality and learning is contradictory and there is a great variation in the relationship between laterality and dominance. In addition, laterality may have nothing to do with hemisphere dominance in many children. A child may prefer one eye, or one ear, because of some minor localized problem in the other, not because of cerebral hemispheric dominance.

The cerebral hemisphere dominance theory, therefore, remains unproved. Unfortunately, it has already given rise to much dubious testing of laterality and the implementation of teaching practices that are of questionable value. These will be discussed in Chapter 13.

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