Theories of Dyslexia

The primary symptoms of dyslexia were first identified by Oswald Berkhan in 1881. The term 'dyslexia' was coined in 1887 by Rudolf Berlin, an ophthalmologist practicing in Stuttgart, Germany. Since then generations of researchers have been investigating what dyslexia is and trying to identify the biological causes. (See History section of article "Dyslexia".) 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.

The cerebellar theory asserts that a mildly dysfunctional cerebellum can cause dyslexia. The initial cerebellar hypothesis suggested that a signal scrambling impairment of cerebellar origin secondarily impaired brain processors. In attempting to explain all the many known reading and non-reading dyslexic symptoms, therapies and theories as well as the presence of only cerebellar and related vestibular neurophysiological signs in dyslexics, the cerebellum was postulated to fine-tune in time and space all signals (visual, auditory, tactile, proprioceptive, motion) entering and leaving the brain as well as signal interconnections. The quality and severity of the many symptoms characterizing each dyslexic was reasoned to depend on the diverse cerebral cortical and other brain processors receiving scrambled signals due to a cerebellar dysfunction, the degree of signal- scrambling as well as the compensatory descrambling capability of specific brain processors. Helpful therapies were reasoned to enhance cerebellar fine tuning (e.g., the use of cerebellar-vestibular stabilizing antimotion sickness medications) and/or improve descrambling and other compensatory cognitive capabilities (e.g., tutoring, biofeedback). Most other theories equate the dyslexia disorder with impaired reading comprehension and so attempt to only explain the latter. Another cerebellar proposal indicated that articulation problems can contribute to the phonological deficits that can cause dyslexia. The cerebellum also contributes to the automitisation of learned behaviors, which can include learning the grapheme-phoneme relationships when reading texts. However, some have suggested that cerebellar dysfunction alone may not be a primary cause of dyslexia and that dysarticulation and phonological deficits appear unrelated.

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