Disorders of Writing Single Words

The acquired dyslexias have their counterparts in the acquired agraphias. Patients with phonological agra-phia are severely impaired in their ability to spell or write nonsense words but are capable of very good performance on legitimate words, even words that are low in frequency and contain unusual spelling patterns (e.g., "leopard"). The deficit is these cases appears to lie in the ability to convert sublexical phonological units to orthographic units (graphemes and letters). The converse impairment—an inability to access the written forms of whole words with preserved ability to convert sublexical phonological units to orthographic units, termed surface agraphia or lexical agraphia— has also been described. A third disturbance, known as "asemantic writing,'' consists of the inability to write spontaneously but the retained ability to write to dictation. This suggests that the contents of the visual word-form system can be addressed from spoken input but not from the meaning of a word.

These disorders of writing can be strikingly different from patients' reading performances. For instance, one patient was unable to write nonsense words to dictation but read these items aloud without difficulty; another produced numerous misspellings when attempting to write orthographically irregular or ambiguous words but accurately read the majority of legitimate words perfectly. These dissociations have led researchers to infer that the orthographic knowledge necessary for word recognition in reading is different from the orthographic knowledge necessary for correct spelling in writing.

The abstract graphemic representation of familiar and unfamiliar words is ultimately converted by the writing mechanism into a sequence of rapid movements that generate letters on the page. A specialized working memory device is needed to maintain the graphemic code in a buffer zone while the spatial identity of each letter is chosen at the next processing stage. One patient made the same kind of errors in writing, oral spelling, and typing, a result that justifies the conclusion that the disturbance arose while these items were in the planning buffer and before the programming and execution of a particular motor act began.

Written production takes place by generating the spatial form of each letter (allographs) in the correct order. A few agraphic cases have been documented in which the impairment is plainly confined to the retrieval of elements in the allographic code. In these patients, adequate knowledge of the word's orthography can be demonstrated because oral spelling is carried out extremely well. When tested, other methods of forming a printed word that do not require a written response (typing or use of block letters) may also yield a high degree of accuracy. Writing, however, is characterized by numerous errors of omission, substitution, reversals, and insertions. The agraphia is not merely a disturbance in the production of a graphic motor pattern because patients can write single letters to dictation, and their writing of words, although flawed, is clearly legible.

Finally, the motor schema for a letter appears to distinguish between the movements denoting the shape of a letter and the parameters that govern scale factors such as magnitude and orientation. Cases of apractic agraphia reveal a loss of the motor programs necessary for producing letters. Written characters are poorly formed and may be indecipherable, although even severely affected patients maintain the distinction between cursive and printed letters and between upper-and lowercase. Evidence indicates that the disturbance need not be associated with limb apraxia. Certain patients with right hemisphere damage have no difficulty constructing written letters or words, but they exceed the correct number of strokes on letters that require repetitive movements.

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