This article includes discussion of alexia, aphasic alexia, attentional alexia, deep alexia, frontal alexia, hemialexia, occipital alexia, parietal-temporal alexia, phonological alexia, spatial alexia, and surface alexia. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
Brain pathology is frequently associated with disturbance in reading ability (alexia). Since the 19th century, 2 major types of alexias have been recognized (alexia with and without a preserved ability to write). In the mid-20th century, 2 additional types of alexias were proposed (alexia due to spatial disturbances and alexia associated with frontal pathology). During the 1970s and 1980s, a new approach to the analysis of alexia was developed. This new approach (psycholinguistic or cognitive perspective) shifted the focus from the anatomical correlates of acquired reading disturbances to the functional mechanisms underlying alexias. Some special types of alexias such as alexia for Braille reading have also been reported. Contemporary neuroimaging studies have significantly contributed to a better understanding of brain organization of reading processes and reading disturbances.
• During the mid-twentieth century 2 additional types of alexia were proposed: spatial alexia and frontal alexia.
• Psycholinguistic models of alexia distinguish between central and peripheral alexias.
• Excepting spatial alexia, acquired reading disturbances are associated with left hemisphere pathology – usually strokes, tumors, or traumas.
• Alexia rehabilitation is usually carried out simultaneously with aphasia rehabilitation.
Historical note and terminology
Alexia (or acquired dyslexia) refers to an acquired disorder in reading caused by brain pathology (Benson and Ardila 1996). Alexia has been recognized for over a millennium, but only in the twentieth century did literacy become sufficiently widespread that alexia represented a significant medical problem. Two case reports published by Dejerine in 1891 and 1892 represent important milestones in the study of alexia (Dejerine 1891; Dejerine 1892). In the 1891 paper, he described a patient who suffered a cerebrovascular accident that produced some degree of right-sided visual field defect and mild difficulty in naming and in understanding spoken language together with a complete loss of the ability to read. The patient could write nothing but his signature. Spoken language improved, but the alexia and agraphia remained basically unchanged until his death. Postmortem examination showed an old infarct in the left parietal lobe involving three quarters of the angular gyrus and extending deep to the lateral ventricle (Dejerine 1891). One year later, Dejerine reported a second patient who noted an inability to read, but no other language disturbances. The only neurologic finding was a right hemianopia. Unlike the former case, this patient, although unable to read except for a few individual letters, could write adequately. Four years later, a second vascular accident led him to death. Postmortem examination revealed 2 different infarcts: 1 infarct was a large softening that involved the left angular gyrus and was obviously of recent origin, and the other infarct was an old gliotic infarct that involved the medial and inferior aspects of the left occipital lobe and the splenium of the corpus callosum. The old infarct was the source of the alexia without agraphia (Dejerine 1892).
Alexia without agraphia (also known as occipital alexia or pure alexia) and alexia with agraphia (parietal-temporal alexia or central alexia) were extensively corroborated during the following years. They represent the classic alexic syndromes. A third, clinically distinct alexia syndrome, frontal alexia, which is associated with pathology in the frontal language areas, has been proposed (Benson 1977). Reading difficulties in cases of right hemisphere pathology, on the other hand, have been noted since long ago. Some mentions of spatial alexia and visuospatial reading disorders associated with right-hemisphere damage are found in the neurology and neuropsychology literature (Hecaen and Marcie 1974). Only a few studies, however, have approached the visuospatial reading defects using large samples of patients with right hemisphere pathology (Hecaen 1972; Ardila and Rosselli 1994).
These 4 types of alexias (without agraphia, with agraphia, frontal, and spatial) represent the neurologic, classic, or neuroanatomically based classification of alexias. Significant variability, however, in the pattern of disturbances is observed particularly in parietal-temporal alexia (with agraphia). During the 1970s and 1980s, a new approach to the analysis of alexia was developed (Marshall and Newcombe 1973; Caramazza et al 1985). This approach to alexias is usually known as the psycholinguistic or cognitive perspective of alexias. Interest shifted from the anatomical correlates of acquired reading disturbances to the functional mechanisms underlying alexias. It should be noted that, in the psycholinguistic or cognitive interpretation of alexias, the name "acquired dyslexia" is preferred, rather than "alexia."
The linguistic and cognitive approaches to alexia required the development of models for normal reading. Several partially coincidental cognitive models of normal reading have been proposed (Coltheart 1978; Caramazza et al 1985). In general, most of these models propose that after the initial letter identification, reading proceeds along 2 linguistically different routes: (1) the direct route, wherein the written word is associated with a visual word in the lexicon memory; and (2) the indirect route, wherein the written word is transformed in a spoken word following a graphophonemic set of rules, and the meaning of the word is attained through its phonological mediation. If 1 or the other of these reading systems is altered, different error patterns can be observed. In some cases, both systems can be disrupted simultaneously.
Psycholinguistic models of alexias usually introduce a major distinction between central and peripheral alexias (Warrington and Shallice 1980). In central alexias, the patient can perceive a word correctly but has difficulties recognizing it with either semantic or phonological processing. Three different types of central alexias are distinguished: (1) phonological, (2) surface, and (3) deep. Each features a specific pattern of reading errors (paralexias). In the peripheral alexias, the reading impairment corresponds more exactly to a perceptual disturbance. The patient has difficulty attaining satisfactory visual word processing (Riddoch 1991). Usually, 3 different types of peripheral alexias are recognized: (1) letter-by-letter reading, (2) neglect alexia, and (3) attentional alexia.
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