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Aquired dyslexia

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Aquired dyslexia


The use of language is one of the most complex tasks the human brain must carry out. The way in which children acquire language is studied very carefully. This acquisition is enhanced by teaching from skilled language users, but in itself acquired by the child's own observation and learning. For this reason the acquisition of spoken language is perhaps more well documented then the taught acquisition of reading skills.

It is hard to determine how the human brain deals with the task of reading the written word.

It can not be determined by introspection or studying the human brain in action as this is not possible. The way Psychologists and Neuroscientists have developed to determine hoe humans read is to observe people who have suffered some form of brain damage and have thereby incurred some form of reading disability. This disability is known as acquired dyslexia.

From the study of such patients several variations of a basic model have been developed to highlight the way in which the written word is processed in the brain. The model is subdivided in to two main processing routes, the Non Lexical Route and the Lexical route.

A model produced based on theories from Coltheart (1981) shows that there are several routes to speech production in the brain. The eye first identifies the printed word. In the adult skilled reader the eye does not move in a smooth pattern but actually jumps to certain focus points in a sentence the brain itself actually fills in the missing words. It does this by top down processing in which the brain applies words to the sentence to make the overall meaning correct. The eye does not move smoothly but actually jumps from point to point. The term given to the period when the eye is stationary and focused on a particular point is called a fixation. It is at these fixations when the eye takes up the information. The jumping movements are known as saccades. Javal (1887) first used this term. This jumping in saccades allows information to be processed faster. However it is believed that little or no information is taken up during a saccade. This is known as saccadic suppression. Dodge (1900) and Holt (1903).

In addition to this saccadic motion it has been discovered that in readers of English the perceptual span of the visual field is greater to the right of the fixation point. Conversely in readers of Hebrew it has been found that the perceptual span of the visual field is located to the left of the fixation point. This indicates that the perceptual span in reading is learned as reading is learned and it is not preconditioned from birth. Rayner, Well and Pollotsek. (1980)

Once the information is correctly converted to electrical impulses by the retina it is sent via the optical nerve to the brain. Many theories suggest that some of the component letters of a written word must be identified before the word is recognised. The early theory of letter identification was based on the fact that it was believed that templates were present in the brain to which the letters in written words could be compared. The main problem with this theory is that if letters do not appear to be the same we can still recognise them as the same letter. (e.g. A and a.) An alternative theory to this has therefore been proposed. This theory is based on the belief that humans contain a unit in the brain, which instead of containing letter templates contains sets of distinctive features which letters can be identified by. The written information is believed to enter the first processing unit of the reading model known as 'The Abstract Letter Identification ' unit. The information then has a choice of two routes from here. It can pass directly to the 'Letter Naming' unit from which spoken letter names can be produced directly. On the other hand the information can pass to the 'Word Recognition' unit (identified by McClelland and Rumelhart (1981) and Morron (1979)) or the 'Non Lexical Phonological' unit. If the information passes to the 'Word Recognition' unit it can either pass to the 'Word Comprehension' unit and then be produced as speech or it can pass to the 'Word Pronunciation' unit and from there it is produced as speech. If the information passes to the 'Non Lexical Phonological Recoding ' unit it can be processed directly to speech or alternatively it can pass to the 'Word Comprehension' unit and from there to speech. The direct route from sound to meaning is known as phonic mediation. This model assumes that letter identification and word identification are independent. (See Flow Chart) An alternative theory is that letter identification and word identification would be accessed at the same time. McClelland and Rumelhart (1981). As humans hear sounds as they read silently it was assumed that meaning is accessed through sound. An alternative theory is that in order for the meaning of what is read to be maintained in the brain then sound is needed.













The studies carried out to determine the routes taken within the brain as it processed the written word were undertaken by a variety of people over a wide timescale. Different research teams have identified several forms reading disability relating to specific parts of the brain.

The ability and disability of brain ...

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