Inferior parietal lobule – superior view animation Researchers have been trying to find the neurobiological basis of<unk>  since the condition was first identified in 1881. For example, some have tried to associate the common problem among people with<unk>  of not being able to see letters clearly to abnormal development of their visual nerve cells. Neuroanatomy Neuroimaging techniques, such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), have shown a correlation between both functional and structural differences in the brains of children with reading difficulties. Some people with<unk>  show less electrical activation in parts of the left hemisphere of the brain involved with reading, such as the inferior frontal gyrus, inferior parietal lobule, and the middle and ventral temporal cortex. Over the past decade, brain activation studies using PET to study language have produced a breakthrough in the understanding of the neural basis of language. Neural bases for the visual lexicon and for auditory verbal short-term memory components have been proposed, with some implication that the observed neural manifestation of developmental<unk>  is task-specific (i.e., functional rather than structural). fMRIs of people with<unk>  indicate an interactive role of the cerebellum and cerebral cortex as well as other brain structures in reading. The cerebellar theory of<unk>  proposes that impairment of cerebellum-controlled muscle movement affects the formation of words by the tongue and facial muscles, resulting in the fluency problems that some people with<unk>  experience. The cerebellum is also involved in the automatization of some tasks, such as reading. The fact that some children with<unk>  have motor task and balance impairments could be consistent with a cerebellar role in their reading difficulties. However, the cerebellar theory has not been supported by controlled research studies. Genetics Research into potential genetic causes of<unk>  has its roots in post-autopsy examination of the brains of people with<unk> . Observed anatomical differences in the language centers of such brains include microscopic cortical malformations known as ectopias, and more rarely, vascular micro-malformations, and microgyrus—a smaller than usual size for the gyrus. The previously cited studies and others suggest that abnormal cortical development, presumed to occur before or during the sixth month of fetal brain development, may have caused the abnormalities. Abnormal cell formations in people with<unk>  have also been reported in non-language cerebral and subcortical brain structures. Several genes have been associated with<unk> , including DCDC2 and KIAA0319 on chromosome 6, and DYX1C1 on chromosome 15. Gene–environment interaction The contribution of gene–environment interaction to reading disability has been intensely studied using twin studies, which estimate the proportion of variance associated with a person's environment and the proportion associated with their genes. Both environmental and genetic factors appear to contribute to reading development. Studies examining the influence of environmental factors such as parental education and teaching quality have determined that genetics have greater influence in supportive, rather than less optimal, environments. However, more optimal conditions may just allow those genetic risk factors to account for more of the variance in outcome because the environmental risk factors have been minimized. As environment plays a large role in learning and memory, it is likely that epigenetic modifications play an important role in reading ability. Measures of gene expression, histone modifications, and methylation in the human periphery are used to study epigenetic processes; however, all of these have limitations in the extrapolation of results for application to the human brain. Language The orthographic complexity of a language directly affects how difficult it is to learn to read it. English and French have comparatively "deep" phonemic orthographies within the Latin alphabet writing system, with complex structures employing spelling patterns on several levels: letter-sound correspondence, syllables, and morphemes. Languages such as Spanish, Italian and Finnish primarily employ letter-sound correspondence—so-called "shallow" orthographies—which makes them easier to learn for people with<unk> . Logographic writing systems, such as Chinese characters, have extensive symbol use; and these also pose problems for<unk>  learners.
Dyslexia
Hearing loss
Diplophonia
Visual impairment
Dysgraphia
Reading
Social-emotional agnosia
Misophonia