The primary symptoms of dyslexia were first identified by Oswald Berkhan in 1881.[1] The term 'dyslexia' was coined in 1887 by Rudolf Berlin,[2] an ophthalmologist practicing in Stuttgart, Germany.[3] Since then generations of researchers have been investigating what dyslexia is and trying to identify the biological causes. (See History section of the Dyslexia article.) The theories of the etiology of dyslexia have and are evolving with each new generation of dyslexia researchers, and the more recent theories of dyslexia tend to enhance one or more of the older theories as understanding of the nature of dyslexia evolves.
Theories should not be viewed as competing, but as attempting to explain the underlying causes of a similar set of symptoms from a variety of research perspectives and background.[4][5]
Contents |
The Cerebellar Theory asserts that a mildly dysfunctional cerebellum can cause dyslexia. The cerebellum contributes to motor control during the articulation of speech, and the Cerebellar Theory proposes that articulation problems can contribute to the phonological processing deficits that can cause dyslexia. The Cerebellum also contributes to the automatisation of learnt behaviors, which includes learning the grapheme–phoneme relationships when reading text.[4][6]
This theory considers that reading is an unnatural act carried out for a very brief period in human evolutionary history. It has only been in the last hundred years that reading a visual form of speech has been promoted as a major form of communication, and subsequently a lack of time for reading behaviors to evolve. In many societies around the world the majority of the population do not use the visual notation of speech as a form of communication, and do not use reading skills, and therefore have no dyslexia.[7]
The Magnocellular theory attempts to unify the Cerebellar Theory, the Phonological Theory, the Rapid Auditory Processing Theory, and the Visual Theory. The Magnocellular theory proposes that the magnocellular dysfunction is not only restricted to the visual pathways but also includes auditory and tactile modalities.[4][8]
The speed with which an individual can engage in the rapid automatized naming of familiar objects or letters is a strong predictor of dyslexia.[9] Slow naming speed can be identified as early as kindergarten and persists in adults with dyslexia.
A deficit in naming speed is hypothesized to represent a deficit that is separate from phonological processing deficit. Wolf identified four types of readers: readers with no deficits, readers with phonological processing deficit, readers with naming speed deficit, and readers with double deficit (that is, problems both with phonological processing and naming speed). Students with double deficits are most likely to have some sort of severe reading impairment.
Distinguishing among these deficits has important implications for instructional intervention. If students with double deficits receive instruction only in phonological processing, they are only receiving part of what they need.[10]
The concept of a perceptual noise exclusion deficit (impaired filtering of behaviorally irrelevant visual information in dyslexia or visual-noise) is an emerging hypothesis, supported by research showing that subjects with dyslexia experience difficulty in performing visual tasks (such as motion detection in the presence of perceptual distractions) but do not show the same impairment when the distracting factors are removed in an experimental setting.[11][12] The researchers have analogized their findings concerning visual discrimination tasks to findings in other research related to auditory discrimination tasks. They assert that dyslexic symptoms arise because of an impaired ability to filter out both visual and auditory distractions, and to categorize information so as to distinguish the important sensory data from the irrelevant.[13]
The phonological deficit theory proposes that people with dyslexia have a specific sound manipulation impairment, which affects their auditory memory, word recall, and sound association skills when processing speech. The phonological theory explains a reading impairment when using an alphabetic writing system which requires learning the grapheme/phoneme correspondence, the relationship between the graphic letter symbols and speech sounds which they represent.[4]
The rapid auditory processing theory is an alternative to the phonological deficit theory, which specifies that the primary deficit lies in the perception of short or rapidly varying sounds. Support for this theory arises from evidence that people with dyslexia show poor performance on a number of auditory tasks, including frequency discrimination and temporal order judgment.[4]
The visual theory represents a traditional perspective of dyslexia, as being the result of a visual impairment creating problems when processing information from letters and words from a written text. This includes visual processing problems such as binocular, poor vergence, and visual crowding. The Visual Theory does not deny the possibility of alternative causes of dyslexia[4]
In his book "The Gift of Dyslexia"[14] author and researcher Ronald D. Davis discusses his research into dyslexia and even suggests methods to correcting it. He goes on to explain that dyslexia is a functional problem and not a structural problem and can be corrected through excercises that he developed at the Reading Research Council's Dyslexia Correction Center, in Burlingame, California. He explains that the dyslexic is a visual thinker and is born with the ability to access part of their brain that most people cannot. This ability allows the dyslexic to distort their perception which if it goes uncontrolled interferes with their ability to learn and read. Because the dyslexic is a visual thinker they also have trouble with words that do not have visual representations. Half of the book goes into detail the methods and techniques he developed to teach the dyslexic to turn off this ability and thus correct the dyslexia.
Davis goes on to explain that the condition and ability manifests itself differently in different people. Some people are able to make the letters jump right off the page and fall off the the desk. The common attribute is the ability for the dyslexic to be able to move their so called minds eye. For most people its above the center of their head. But a dyslexic can move that eye to where ever they want.
|
|||||||||||||||||||||||||||