Scientists once thought that dyslexia was mainly a visual impairment. A commonly held notion, for example, was that dyslexia was based primarily on the visual reversal of letters during reading. Nowadays, we know better. There are many subtypes of dyslexia, and although there is evidence that some dyslexic patients do suffer from visual processing difficulties, auditory processing difficulties are much more common.
The International Dyslexia Association defines dyslexia as a language-based learning disability that has neurological origins. Dyslexic people have trouble understanding words, sentences or paragraphs. These problems in language are not caused by difficulties in hearing, or vision. The IQ of the patient has to be within normal range, and he has to have received appropriate educational opportunities. Different dyslexics have differing symptoms. Patients suffering from auditory dyslexia have difficulties related to the auditory processing of language. These might include, for example, difficulties with remembering the sounds that individual letters represent, difficulties with processing rapidly changing sounds or problems with blending individual words into sounds.
There are numerous dyslexia theories. The evolutionary hypothesis states that reading is fundamentally unnatural to humans, and from the evolutionary point of view, humans have been engaged with reading for a short period of time. The phonological deficit theory states that people with dyslexia have a specific impairment in processing speech sounds. For a child to learn to read, he needs to learn that there is a connection between a certain sounds and letters. 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. The visual theory suggests that visual impairment causes reading difficulties in dyslexic individuals. The cerebellar theory states that dysfunctions in the cerebellum of dyslexics cause reading problems. The magnocellular theory postulates that the magnocellular dysfunction is not restricted to visual pathways but is generalized to all modalities.
Brain Structure and Functions
Dyslexia is a neurological disorder. Thus, an individual is born with dyslexic tendencies, and his brain structure and functions are different from birth. Thanks to such methods as magnetic resonance imaging, functional MRI and positron emission tomography, there is a substantial amount of information about the areas and functions of the brain that are not normal in patients suffering from dyslexia. In most people, the left hemisphere is involved in processing language. Some parts of this area, such as the inferior frontal gyrus, the parietal lobe, and the middle and ventral temporal cortex, show abnormalities in dyslexic patients. In healthy people, reading is almost an automatic process that requires little activation in the frontal areas of the brain. In dyslexics, the frontal areas are much more active during the reading process, signaling that reading requires more attention and focus from them.
Typically, dyslexia is not diagnosed until patients are school-aged. Ideally, experts of several different areas should join their knowledge about a patient before giving a diagnosis of auditory dyslexia. First, a medical doctor should rule out other conditions that might explain the language-related problems. Then, a patient is typically seen by a psycho-educational specialist, who employs a battery of assessment instruments to investigate the patient's cognitive skills. If auditory dyslexia is suspected, patients should also be seen by audiologist.
Numerous other problems might cause difficulties in learning to read or listen properly. Such problems might be because of attention deficit hyperactivity disorder or minor mental retardation, and it is possible that these problems are misdiagnosed as auditory dyslexia. That's why the intense collaboration between experts of different fields, such as psychology, speech therapy and audiology, is so crucial when diagnosing dyslexia.
- “Archives of Psychology”; Word Blindness in School-aged Children; Orton; 1925.
- “Developmental Neuropsychology”; Brain Responses to Changes in Speech Sound Durations Differ Between Infants With and Without Familial Risk for Dyslexia; P.H. Leppänen, et al; 2002.
- “Proceedings of the National Academy of Sciences”; The Science of Literacy: From the Laboratory to the Classroom.; Tallal; 1997