For example, affected children may have difficulty determining root words or word stems and determining which letters in words follow others and form specific sound-symbol associations, such as vowel patterns, affixes, syllables, and word endings. Dyslexia is considered to be due to intrinsic brain factors, but the underlying cause is unknown. Cerebrovascular accidents, prematurity, and intrauterine complications have been linked to dyslexia. Alexia, in which there is almost a complete inability to read, may also stem from direct insults or trauma to the brain. Dyslexia results predominantly from specific cortical dysfunctions stemming from congenital neurodevelopmental abnormalities.
Lesions affecting the integration or interactions of specific brain functions are suspect.
Asymmetries of the left and right hemispheres, fewer neurons, and a smaller left planum temporale have also been cited. Dysfunctions or defects in the angular gyrus, the medial occipital area of the brain, and the right hemisphere appear to produce word recognition problems. Dysfunctions in or between the cerebellum and the vestibular system have also been implicated but have not been substantiated. Nonlinguistic processing deficits such as visuoperceptual problems are no longer considered primary factors in dyslexia. However, abnormalities in visual processing can interfere with word learning.