Due to the complex nature of the problem, and the many different ways in which it manifests itself, it is impossible to find a definition upon which everyone may agree. Nevertheless, several definitions exist which may serve as a starting point. A definition by Critchley and Critchley in 1978 is: "children with specific learning difficulties are those who in the

It may be that dyslexics have general difficulties in storing visual information in memory. However, Bryant and Bradley (1985) are emphatic in stating that backward readers do not suffer an overall visual deficit, and suggest that the evidence which does point to it is flawed, because it uses a mental-age match on subjects, rather than a reading-age match, this means that any apparent problems may be a result rather than a cause of reading difficulties. Another suggestion made by Ellis (1981) is that dyslexics have problems when visual stimuli have to be named. These results also rule out the possibility of a basic visual perceptual problem, and suggest that the problem emerges when name coding is required. Hornsby (1992) mentions two further possibilities involving visual perception. One is that some backward readers do not have a fixed reference eye (30%), and so have problems with convergence. For example, the images of the two eyes do not match on the retina, making perception difficult, although the test has received some criticism because subjects respond verbally to a visual test. Secondly, Hornsby noted that dyslexics' make larger, jerkier eye movements when reading than do normal readers, which may partly account for reversals. Recent research is supporting the view of a visual problem being at least partially responsible for dyslexia. It is suggested that visual perceptual difficulties may be one of the causes of dyslexia.

To try and clear up this confusion, we turn to twin studies, of which many have been conducted. Twin studies are important because they allow comparison between monozygotic twins (who share 100% genes) and dizygotic twins (who share 50%), but where both have been brought up in the same environment, so that any difference between dizygotic twins can be reliably concluded to be based on genes rather than environment. Using this method, Herman (1959) found a 100% concordance for monozygotic twins, and 33% for dizygotic twins: this is a summary of three studies undertaken in Scandinavia in the 1950s. Jorm (1983) states that all twin studies show greater similarity between MZ than DZ twins, suggesting a role for genetic factors. However, as Young and Tyre (1983) point out, this is not absolutely conclusive, unless we can be sure that these MZ twins have not been subject to prenatal disturbances, which are more likely than in single births.

There are two possible descriptions of the brains of dyslexics: either they do not have a dominant hemisphere for language, or the language center in the left hemisphere is smaller. An experiment was conducted where results suggested that in dyslexic brains, language is shared more equally between the two hemispheres, and that this means that more messages have to be passed from one hemisphere to the other, resulting in a confusing 'traffic jam' of nerve signals in the corpus callosum

Some topics in this essay:
Secondly Hornsby, MZ DZ, Morgan Kerr, Bryant Bradley, Critchley Critchley, , Byrne Shea, normal readers, left hemisphere, dizygotic twins, visual perceptual, hornsby 1992, twin studies, family history dyslexia, visual perceptual difficulties, perceptual difficulties, hemisphere specialized, 'specific learning difficulties', organic damage, recent research, words normal readers, bryant bradley 1985,