Dyslexia is a developmental disorder of READING that is based on abnormal brain development. The brain changes exist from before birth and persist throughout life, although they do not usually manifest themselves clinically until the early school years, and many sufferers of this disorder compensate significantly by the time they reach adult life. The etiology of dyslexia remains unknown, but it is clear that both genetic and environmental factors play a role in its clinical manifestations.
The term dyslexia is used in the United States to refer to a developmental disorder of reading, whereas in the United Kingdom acquired disorders of reading may also be called dyslexias. Whereas dyslexia appears as an entry in ICD-9-CM for Neurologists (Neurology 1994) to represent either developmental or acquired disorders of reading, DSM-IV (Association 1994) does not have an entry for dyslexia altogether and instead has one for reading disorder. In this article only the developmental form is considered.
Some researchers have been unhappy with the term dyslexia and prefer to use developmental reading disorder instead, even though the term dyslexia means reading disorder. In DSM-IV the definition for reading disorder includes reading achievement (accuracy, speed, and/or comprehension as measured by individually administered standardized tests) that falls substantially below that expected given the individual's chronological age, measured intelligence, and age-appropriate education. Sensory-perceptual, cognitive, psychiatric, or neurological problems may coexist with the reading disorder, but should not be sufficient to explain the reading underachievement.
Other researchers (see Shaywitz et al. 1992) do not consider that intelligence should be a factor in the diagnosis, and prefer to include all individuals with reading difficulties, even those who are frankly mentally retarded. Still others insist that the reading disorder should be the consequence of disturbances of language function (see Vellutino 1987), specifically phonological processing (see PHONOLOGY), and that a reading disorder resulting from other mechanisms not be included.
It is generally accepted that the reading disorder is often accompanied by problems with writing, arithmetic, verbal memory, and subtle motor dysfunction. Sometimes there is also coexistence of anomalous HEMISPHERIC SPECIALIZATION, ATTENTION deficits, and emotional and personality disorders. Subtle problems with oral language are also commonly seen, but the presence of more severe disturbances in oral communication gives rise to the diagnosis of developmental language impairment, albeit with associated disturbances in reading and writing.
Dyslexia is the most commonly recognized form of learning disorder. The prevalence of dyslexia is accepted by most to be in the order of 4-5 percent of the school-age population. Depending on how the condition is defined, the prevalence figures range between 1 percent and 35 percent. Although problems may persist into adulthood, no clear figures about clinical prevalence in adult age groups exist. Most studies have shown a male prevalence in excess of that for females in the range of three to four males to one female. Some of this discrepancy may relate to reporting bias (the argument is that dyslexic girls are better behaved and go unnoticed). However, even when this is taken into consideration, a significant male preponderance still remains. As with other complex behaviors, normal and abnormal, that have their origin in a genetic background with added environmental influences, the prevalence of dyslexia depends in part on its definition, severity, and sociocultural attitudes.
The most common form of dyslexia is associated with deficits in phonological processing (Morais, Luytens, and Alegria 1984; Shankweiler et al. 1995), but other varieties, some based on disturbances affecting the visual system, have also been identified (Lovegrove 1991; Stein 1994). A difficulty with processing rapidly changing stimuli, affecting at least visual and auditory functions has also been implicated, which blurs the distinction between perceptual and cognitive mechanisms in the etiology of dyslexia (Merzenich et al. 1996a, 1996b; Tallal et al. 1995). The temporal processing hypothesis states that sensory-perceptual temporal processing deficits impede the development of normal phonological representations in the brain, which in turn produce the reading disorder. The main idea is that reading requires knowledge of the normal sound structure of the language before appropriate sound-sight associations can be made. Sensory-perceptual temporal processing deficits may lead to difficulties representing some language sounds that require rapid processing, which in turn leads to an incomplete or ambiguous sound repertoire and consequently difficulty with reading. The main objection to this theory is based on observations such as the presence of normal reading in many deaf people, and the counterargument is that the reading disorder rather reflects a deficit in semiconscious parsing (metalinguistic) of the sound stream into phonemes, as a prerequisite for mapping the parsed elements onto visual words.
Neurophysiological and psychophysical studies have shown abnormalities in visual perception and eye movements in dyslexics (Cornelissen et al. 1991). These findings are consistent with dysfunction of the magnocellular pathway of the visual system, which among other functions deals with rapid temporal processing (Greatrex and Drasdo 1995; Livingstone et al. 1991). Psychophysical evidence indicated that language-impaired children exhibit slow processing in the auditory system, too (Tallal and Piercy 1975). More recently studies employing functional MAGNETIC RESONANCE IMAGING showed that the area involved in motion perception, MT, does not activate normally in dyslexics, an area that forms part of the magnocellular system (Eden et al. 1996). In sum, therefore, there is increasing evidence to suggest that rapid processing is impaired in dyslexics, which may help account for the phonological disorder and hence the reading disorder. The ongoing debate has to do with the question of whether the type and degree of temporal processing perceptual difficulty seen in dyslexics is sufficient for explaining their language problems (for instance, see Paulesu et al. 1996).
Dyslexia is associated with anatomic changes in the brain. The normal human brain often shows asymmetry in the planum temporale, a region concerned with language function. Normal brains that are not asymmetric in the planum temporale show two large planums, rather than two small ones or two medium-sized ones. Dyslexic brains fail to show the standard asymmetric or symmetric pattern in the planum temporale (Galaburda 1993), presumably indicating a disturbance in the development of hemispheric specialization for language (see Annett, Eglinton, and Smythe 1996).
There are also subtle changes in the lamination of the CEREBRAL CORTEX, which are focal in nature and affect the left cerebral hemisphere more than the right in most cases (Galaburda 1994). They consist mostly of displaced nests of neurons and glia in the frontal, parietal, and temporal cortex. These anomalies, called ectopias (see figure 1), reflect disturbances in neuronal migration to the cerebral cortex during fetal brain development. The fundamental cause of the migration disturbance, though suspected to be genetic, is not known (for a review of recent work on the genetics of dyslexia, see Pennington 1995).
Figure 1 Example of an ectopia found in the brain of a dyslexic. In the upper part of the photomicrograph there is an extrusion of neurons and glia into the molecular layer (uppermost layer of the cortex). This is one example of a neuronal migration anomaly.
Associated with the ectopias in the cerebral cortex are changes in the sizes of neurons of some thalamic sensory nuclei, including the visually linked lateral geniculate nu-cleus (LGN) (Livingstone et al. 1991) and the auditory medial geniculate nucleus (MGN; Galaburda, Menard, and Rosen 1994). These are structures close to the input channels for visual and auditory experience and are not involved in cognitive functions but rather in sensory perceptual functions. In the LGN, the neurons comprising the magnocellular layers are smaller in dyslexic than in control brains, and in the MGN there is a shift toward an excess of small neurons and a paucity of large neurons in the left hemisphere.
Ectopias have been induced in newborn rats, and the displaced neurons exhibit abnormal connections with neurons in the THALAMUS as well as with other cortical areas in the ipsilateral and contralateral cerebral hemispheres (Rosen and Galaburda 1996). This provides a possible conduit for the propagation of changes from the ectopias to the thalamus and/or vice versa. Additional research has shown that induction of cortical malformations related to ectopias lead to secondary changes in the thalamus, namely the appearance of excessive numbers of small neurons and a paucity of large neurons (Herman et al. 1997). The animals with the induced malformations also exhibit slow temporal processing involving rapidly changing sounds. There are sex differences in these findings, such that induction of cortical malformations produce both behavioral changes and changes in thalamic neuronal sizes only in treated males. Females demonstrate the anatomic changes in the cortex, but no changes in the thalamus and no abnormal slowing in auditory processing. Moreover, administration of testosterone to pregnant rat mothers in the perinatal period produces masculinization of the female offspring complete with thalamic neuronal changes (Rosen, Herman, and Galaburda 1997) .
In summary, animal models for the brain changes seen in association with developmental dyslexia indicate that abnormal cortical development can lead to abnormal development of the thalamus, and that it is likely that brain areas that deal with cognitive tasks and brain areas that deal with sensory-perceptual tasks are both affected in dyslexia. Moreover, the research indicates that multiple modalities, as well as multiple stages of processing, are involved, which may limit the ability of the developing brain to compensate. On the other hand, because of the relative discreteness of the neural connections even during development, not all cortical and thalamic areas are affected, setting up the possibility for a relatively delimited form of learning disorder.
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