One favored definition of Dyslexia tells us that: 

“Dyslexia is characterized by an unexpected difficulty in reading in children and adults who otherwise possess the intelligence, motivation, and schooling necessary for accurate and fluent reading,” (Shaywitz, 2001).

What are the causes of the reading difficulties that prevent some people from gaining the fluent reading skills that most of us take for granted?

5 Causal Theories

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After many years of research on the topic, scientists have settled on five different theories of the causes of dyslexia:

    1. Magnocellular Impairment
    2. Cerebellar Impairment
    3. Phonological Disorder
    4. Auditory Processing Disorder
    5. Visual Disorder 

1. Magnocellular Impairment
Of late, the magnocellular theory of dyslexia has gained much attention. This theory holds that the visual, learning, and processing issues found in dyslexics may be due to a deficit in the magnocellular pathway. This is an area of the brain that is sensitive to visual motion and helps focus the direction of our gaze in response to movement. This system helps us to orientate to the world and objects around us and their meaning, but does not register details such as color. The magnocellular pathway also contributes to the control of eye movement. 
For more more info about the flow of visual and motor information and the magnocellular pathway: 

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2. Cerebellar Impairment
In the cerebellar impairment theory, there is a belief that there is a mild dysfunction of the cerebellum. The cerebellum is needed for timing motor control. Impairment of the cerebellum may lead to an inability to effect the timed eye movements necessary for reading or the phonological processing deficits that are responsible for causing dyslexia. The cerebellum is also necessary for the fluid interpretation of letters to sounds (grapheme-phoneme relationships) that are crucial to reading text. 
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3. Phonological Disorder
Proponents of the phonological-deficit theory believe that dyslexia is the direct result of impairment in the ability to process and represent speech sounds.

Most scientists in the field of dyslexia favor this theory which asserts that in order to learn a writing system based on an alphabet, the brain must be able to map letters to speech sounds (grapheme-phoneme conversion). This school of thought suggests that dyslexics have a difficulty in finding a way to represent and/or recall the basic sound units. Many dyslexics find it hard to retain speech within their short-term memories or to break up this speech into shorter segments of sound (phonemes). According to this theory, dyslexia doesn’t have a direct impact on reading per se, but slows down the development of spoken language that is crucial to attaining reading skills. 
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4. Auditory Processing Disorder
Auditory Processing Disorder (APD) is a term used to describe a number of disorders that affect the brain’s ability to process auditory information. This is unrelated to hearing impairments. People with APD tend to have normal hearing ability. But they do find it difficult to process the information they hear. This makes it hard for them to recognize what they hear. Some think of APD as a ‘listening disorder’. This theory holds that the dyslexic finds it hard to process and interpret short segments of sounds within a short span of time.
Learn more about the auditory processing approach and theory:

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5. Visual Processing Disorder
Dyslexia as a visual processing disorder is the most traditional way of viewing reading difficulties. This theory believes dyslexia to be the result of a visual impairment that makes it difficult for the dyslexic to process information from the letters and words of a written text. Visual processing issues include visual crowding (the inability to recognize a single object in a crowded environment), problems with binocular vision (coordinating both eyes), and poor vergence control (both eyes must rotate in opposite directions at the same time to arrive at binocular vision).
Learn more about the visual processing approach and theory:

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