Beside the behavioural experiences mentioned befor this, the number of evidences to a direct connection between problems with reading and specific patterns of braindisturbances is growing. These are coming from the neurology. The have comparised the symptoms of somebody with dyslexia with the symptoms of somebody with alexia. People with alexia lost their faculty to read by a change at the left half of their brains. This connections led to neuropsychological explanations.
It's normal that the left half of the brains is specialized in at the processing of verbal information, while the right half of the brains is specialized at the processing of visual-spatial information. By dyslectics you see an other pattern. Orton concluded in 1928 tha dyslexia develops because none of the halfs of the brains is dominant. Both halfs of the brains compete with each other by the processing of information. That's why characters are changed during reading. Learning to read turns well when one of the halfs of the brains is dominant.
An other model based on deviating dominance is the balance-model, developed by Bakker. The right half of the brains checks preponderating at the beginning of the process of learning to read. The visual-spatial aspects are important. At the aspects of the meaning, which are important later, the left half of the brains is important. At the first type of dyslexia (see: What is dyslexia) the right of the brains stays checking to long. At the second type (see: What is dyslexia) the left half starts to early to check on the reading.
This model is based on the fact that children have a developing of the functions in phases. Feeling, motion and language develop after each other. This model says that dyslexia develops by a slowing-down in the developing of the brains and the differentiation of their functions. However, it's not like normal ratardation. The reading retardation can't be made up. The most of the times a reading disturbance stays forever.
Since the end of the 19th century there is the suspect that dyslexia has something to do with structural damages in the brains. From recently inquiryof Geschwind and Galaburda it appears that especially the left half of the brains of dyslectics shows anatomical damages. The origin of this is to be found in the neuronal migration process. This is the phenomenon that nerve-cells (neurons), which develop in the central areas of the brains, during the developing of the foetus move to the cerebal cortex. According to Geschwind this develops by to much of the hormone of testotron. This also will explain the high rate of lefthandedness among read en language disturbed persons.
Source: 'Natuur en Techniek' no. 2 1991 (changed)
Last update: May 16, 1999
Margriet Zuidhof en Erna Oudman