Alexia is an acquired disorder of reading subsequent to brain injury in a person who had previously been literate. There are several types of alexic disorders, which are characterized by the types of paralexias (incorrect production of words in oral reading) produced, and by the properties of words that tend to affect reading performance. These properties include letter length, orthographic regularity, part of speech, concreteness, and familiarity.
Pure alexia is the best-known type of acquired alexia. Patients with this form of alexia retain the ability to write and spell; and in most cases, language functions are normal, except that there is often a mild anomia (difficulty in retrieving words). Although patients with pure alexia have great difficulty recognizing written words, they can recognize words that are spelled aloud to them. The identification of individual letters may also be impaired, particularly early in the course of the alexia. However, many patients with pure alexia recover the ability to identify and name individual letters. The ability to name letters, combined with the ability to recognize strings of named letters as familiar words, has led many patients with pure alexia to adopt a pattern of compensatory reading known as letter-by-letter reading. Patients with pure alexia show a characteristic length effect when reading written words. That is, the time needed to read words increases as the number of letters in the word increase. This effect of length may be a consequence of the need to identify each letter of a word individually in a serial fashion, rather than the normal rapid, automatic, and parallel identification of letters within a word. Pure alexia is caused by a lesion or combination of lesions that cuts off input from visual cortex to the left angular gyrus, which itself remains intact.
Patients with surface alexia appear to rely upon the pronunciations of written words in order to ascertain their meanings. An obvious consequence of this disorder is an inability to distinguish between homophonic words, such as flue, flu, and flew. That is, the patient with surface alexia accesses the correct pronunciation of the written word flu, but does not know which of the three words sharing that pronunciation is on the page. Consequently, reading comprehension is impaired. The surface alexic patient's inability to directly access a word's meaning from its orthographic composition is actually part of a larger problem in recognizing written words. Not only is the patient frequently unable to determine a word's meaning prior to ascertaining its pronunciation, but often the patient does not know whether or not it is a real word until its pronunciation is obtained. Nonwords whose pronunciations are homophonous with real words (i.e. hoam) may thus be accepted as real words. Patients with surface alexia typically have a lesion is the temporoparietal region of the left hemisphere.
The reading of patients with phonological alexia may be seen as the flip side of surface alexic reading. While patients with surface alexia tend to depend upon a "sounding out" process for reading, patients with phonological alexia are unable to read via this mechanism. This deficit is manifest in a type of familiarity effect, in which words that are known may be read well, while an unknown word or a pronounceable nonword (pseudoword) cannot be read. Some patients with phonological alexia also show a relative disadvantage for reading functor words (prepositions, conjunctions, etc.) compared with nouns. In addition to a part-of-speech effect, the reading of phonological alexic patients may exhibit a concreteness effect. Here, a word that is concrete (i.e. table) is more likely to be read correctly than a word that is abstract (i.e. idealism). The lesions that cause phonological alexia are quite variable, within the distribution of the left middle cerebral artery. Superior temporal lobe is frequently involved.
The defining feature of deep alexia is the production of semantic paralexias when reading aloud. A semantic paralexia is a type of reading error in which the word produced is related in meaning to the written target word. The semantic relationship may take many forms: synonyms (lawyer--attorney); antonyms (hot--cold); subordinates (bird--robin); superordinates (celery--vegetable); attributes (grass--green); associates (house--garden). All patients with deep alexia have a profound disturbance in pseudoword (pronounceable nonword) reading. They all show a part-of-speech effect. An effect of concreteness is always present. There is difficulty in reading words with affixes, and derivational paralexias are produced, in which word endings are added, deleted, or substituted for one another. It appears, then, that deep alexia and phonological alexia are part of the same continuum; with deep alexia representing the most severe form of phonological alexia, in which there is an impaired direct route from orthography to phonology, plus a significant impairment in the semantic reading route. Lesions associated with deep alexia are typically quite extensive, including much of the left frontal lobe, and extending posteriorly.