One possible cause of dyslexia is that it emanates from neurological (brain) factors. This is a biological explanation. There is some evidence that dyslexic people share a group of genes on part of chromosome 6 which is close to the MHC genes that control the immune system. This idea is supported by the evidence that dyslexics often tend to suffer from disorders of the immune system such as asthma and rhemutoid arthritis.One theory which attempts to explain how dyslexia may be caused by problems in the brain is Gerschwinds theory. He claims that high testosterone in the foetus could lead to damage of the left hemisphere of the brain, which ultimately may cause dyslexia to arise. There ...view middle of the document...
Another theory, which attempts to explain how dyslexia may be caused by problems in the brain, is the timing theory. Galburda '94 provides some evidence for this theory. The thalamus in the brain receives and processes auditory information. Galburda claims that there are fewer cells in the thalamus of dyslexics compared to non-dyslexics. He says that this may explain why dyslexic children are unable to distinguish fast consonants - this supports Tallal's work. The evidence for this theory is taken from the post mortems of five dyslexic people that were compared to five post mortems of five non-dyslexic people. Similar problems that arise with Gerschwinds theory also occur with the timing theory. Although 5 post mortems is a better sample size than the two (that Gershwind used), it is still very small and cannot be generalised to all dyslexic people. Again, the issue of whether a dead and living brain would show the same problem (in this case the lack of cells in the thalamus) means that the results of the post mortems can't be applied or generalised to living dyslexic peoples brains. There is more evidence, by Stein, which also supports the timing theory. Stein says that the mother's immune system attacks the developing foetus causing damage to nerve cells that relay information about fast changing events, for example reading. He says that specific damage occurs in the Magno-Pathway from the retina to the cortex, which is meant to match up images from each eye into a single image. Stein says that dyslexia is a widespread neurological problem. The evidence is from Stein and Talcott which demonstrates that visual processing is an important factor (in contrast to Tallal's focus on sound). They asked dyslexics to watch...
