Varieties of Acquired Dyslexia: A Linguistic Approach

More than a century ago, Wernicke' indicated the proper objective of a neuroscientist's approach to disorders of cognition in his seminal monograph. This was one of the first (and best) attempts to study the breakdown of linguistic skills via an information-processing model of normal language. This model was envisaged in terms of computational centers and their connectivity and can be directly translated into the "routes" and "representations" of contemporary terminolo y A no less distinguished successor, Coldstein! neatly summarized the issue of functional localization for the present-day student: "the question of the relationship between the symptom-complex and a definitely localized lesion again becomes a problem, no longer, however, in the form: where is a definite function or symptom localized? but: how does a definite lesion modzfy the function of the brain so that a definite symptom comes to the fore? " This question points explicitly to two of the main areas of interest in the study of the dyslexias: the definite lesion and the definite symptom. The definite lesion in the case of the pure, modalit specific dyslexias has been eloquently described. & The patient cannot read visually presented words, but he can identify tactually presented letter strings. He can write to dictation but is unable to read what he himself has written. The dyslexia is directly attributable to anatomic di~connection. ~ In some patients with right hemianopsia and splenial involvement, visuoverbal information from the right occipital cortex cannot reach the language-processing structures of the left hemisphere. "n patients with left occipital lesions sparing the optic radiations (such as the classic case of Dejerine"), information available in both visual cortices is disconnected within the left hemisphere from these language processing structures. Far more common in clinical practice are cases in which reading and writing problems coexist. When these problems are disproportionately severe (compared with the breakdown of speech, comprehension of spoken language, and other language skills), the syndrome is labeled "alexia with agraphia" and is associated with lesions in the region of the angular gyrus.7 Many more cases are found in which the impairment of reading occurs in a setting of more general disruptionVof language. It is then more difficult to trace the anatomy of the dyslexia than that of the severe repetition problem characteristic of conduction aphasia or the symptom-complex of the transcortical aphasia^. ^ Moreover, reading is a comparatively recent acquisition in the cognitive/linguistic equipment of Homo sapiens. It is perhaps not necessary to go to the length of describing reading as an unnatural act to propose that its neural circuitry may be less rigidly or consistently wired than that sub. serving speech and the understanding of spoken language. Moreover, the effects of left occipital lesions "vary very much in different individuals, in accordance with their different sensorial aptitudes and different degrees of education. "" Our presen<concern is with the definite symptom. In other words, what is the overt nature of the reading problem? In discussing speech errors, Jackson" stressed both positive and negative aspects. The substitution of "worm powder" for "cough medicine," or "parasol" for "castor oil" was attributed by him to "the activity of healthy (except perhaps slightly unstable) nervous elements. " We suggest that comparable errors in reading (semantic, and visual and phonological, respectively) carry the same interpretation and, moreover, reflect alternative processing modes. Our eventual objective, then, is to delineate the functional architecture of readine disorders. To that u end, we and our colleagues elsewhere have studied the error patterns of dyslexic patients and the linguistic elements that are differentially impaired, in order to specify preserved processing capacity and demonstrable processing failure. In so doing, we are following directions envisaged by earlier students, among them Franz, Goldstein, and Low (reviewed by Marshall and Newcombe"). The contemporary emphasis is to obtain more systematic