Peripheral Dyslexias

A useful starting point in the discussion of acquired dyslexia is the distinction made by Shallice and Warrington between "peripheral" and "central" dys-lexias. The former are conditions characterized by a deficit in the processing of visual aspects of the stimulus that prevents the patient from reliably matching a familiar word to its stored visual form or visual word form. In contrast, central dyslexias reflect impairment to the "deeper" or "higher" reading functions by which visual word forms mediate access to meaning to speech production mechanisms. In the

Figure 1 The relationship between the procedures involved in reading.

Speech

Figure 1 The relationship between the procedures involved in reading.

following sections, I discuss the major types of peripheral dyslexia.

A. Alexia without Agraphia (Pure Alexia; Letter-by-Letter Reading)

This disorder is among the most common of the peripheral reading disturbances. It is associated with a left hemisphere lesion affecting left occipital cortex (responsible for the analysis of visual stimuli on the right side of space) and/or the structures (left lateral geniculate nucleus of the thalamus and white matter, including callosal fibers from the intact right visual cortex) that provide input to this region of the brain. It is likely that the lesion either blocks direct visual input to the mechanisms that process printed words in the left hemisphere or disrupts the visual word form system. Some of these patients seem to be unable to read at all, whereas others do so slowly and laboriously by means of a process that involves serial letter identification (often termed "letter-by-letter" reading). At first, letter-by-letter readers often pronounce the letter names aloud; in some cases, they misidentify letters, usually on the basis of visual similarity, as in the case of N —M. Their reading is also abnormally slow.

It was long thought that patients with pure alexia were unable to read except letter by letter. There is now evidence that some of them do retain the ability to recognize letter strings, although this does not guarantee that they will be able to read aloud. Several different paradigms have demonstrated the preservation of word recognition. Some patients demonstrate a word superiority effect of superior letter recognition when the letter is part of a word (e.g., the "R" in "WORD") than when it occurs in a string of unrelated letters (e.g., "WKRD"). Second, some of them have been able to perform lexical decision tasks (determining whether a letter string constitutes a real word or not) and semantic categorization tasks (indicating whether or not a word belongs to a category, such as foods or animals) at above chance levels when words are presented too rapidly to support letter-by-letter reading. Brevity of presentation is critical in that longer exposure to the letter string seems to engage the letter-by-letter strategy, which appears to interfere with the ability to perform the covert reading task. In fact, the patient may show better performance on lexical decision at shorter (e.g., 250 msec) than at longer presentations (e.g., 2 sec) that engage the letter-by-letter strategy but do not allow it to proceed to completion. A compelling example comes from a previously reported patient who was given 2 sec to scan the card containing the stimulus. The patient did not take advantage of the full inspection time when he was performing lexical decision and categorization tasks; instead, he glanced at the card briefly and looked away, perhaps to avoid letter-by-letter reading. The capacity for covert reading has also been demonstrated in two pure alexics who were completely unable to employ the letter-by-letter reading strategy. These patients appeared to recognize words but were rarely able to report them, although they sometimes generated descriptions that were related to the word's meaning (e.g., "cookies"-"candy, a cake"). In some cases, patients have shown some recovery of oral reading over time, although this capacity appears to be limited to concrete words.

The mechanisms that underlie implicit or covert reading remain controversial. Dejerine, who provided the first description of pure alexia, suggested that the analysis of visual input in these patients is performed by the right hemisphere as a result of the damage to the visual cortex on the left. It should be noted, however, that not all lesions to the left visual cortex give rise to alexia. A critical feature that supports continued left hemisphere processing is the preservation of callosal input from the visual processing on the right. One possible account is that covert reading reflects printed word recognition on the part of the right hemisphere, which is unable either to articulate the word or (in most cases) to adequately communicate its identity to the language area of the left hemisphere. By this account, letter-by-letter reading is carried out by the left hemisphere using letter information transferred serially and inefficiently from the right. Furthermore, this assumes that when the letter-by-letter strategy is implemented, it may be difficult for the patient to attend to the products of word processing in the right hemisphere. Consequently, performance on lexical decision and categorization tasks declines. Additional evidence supporting the right hemisphere account of reading in pure alexia is presented later.

Alternative accounts of pure alexia have also been proposed. Behrmann and colleagues, for example, proposed that the disorder is attributable to impaired activation of orthographic representations. By this account, reading is assumed to reflect the residual functioning of the same interactive system that supported normal reading premorbidly.

Other investigators have attributed pure dyslexia to a visual impairment that precludes activation of orthographic representations. Chialant and Caramaz-za, for example, reported a patient, MJ, who processed single, visually presented letters normally and performed well on a variety of tasks assessing the orthographic lexicon with auditorily presented stimuli. In contrast, MJ exhibited significant impairments in the processing of letter strings. The investigators suggest that MJ was unable to transfer information from the intact visual processing system in the right hemisphere to the intact language processing mechanisms of the left hemisphere.

B. Neglect Dyslexia

Parietal lobe lesions can result in a deficit that involves neglect of stimuli on the side of space contralateral to the lesion, a disorder referred to as hemispatial. In most cases, this disturbance arises with damage to the right parietal lobe; therefore, attention to the left side of space is most often affected. The severity of neglect is generally greater when there are stimuli on the right as well as on the left; attention is drawn to the right-sided stimuli at the expense of those on the left—a phenomenon known as "extinction." Typical clinical manifestations include bumping into objects on the left, failure to dress the left side of the body, drawing objects that are incomplete on the left, and reading problems that involve neglect of the left portions of words (i.e., neglect dyslexia).

With respect to neglect dyslexia, it has been found that such patients are more likely to ignore letters in nonwords (e.g., the first two letters in "bruggle") than letters in real words (compare with "snuggle"). This suggests that the problem does not reflect a total failure to process letter information but, rather, an attentional impairment that affects conscious recognition of the letters. Performance often improves when words are presented vertically or spelled aloud. In addition, there is evidence that semantic information can be processed in neglect dyslexia, and that the ability to read words aloud improves when oral reading follows a semantic task.

Neglect dyslexia has also been reported in patients with left hemisphere lesions. In these patients, the deficiency involves the right side of words. Here, visual neglect is usually confined to words and is not ameliorated by presenting words vertically or spelling them aloud. This disorder has therefore been termed a ''positional dyslexia,'' whereas the right hemisphere deficit has been termed a ''spatial neglect dyslexia.''

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Adult Dyslexia

Adult Dyslexia

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