A PET STUDY OF THE FUNCTIONAL NEUROANATOMY OF WRITING IMPAIRMENT IN ALZHEIMERS-DISEASE - THE ROLE OF THE LEFT SUPRAMARGINAL AND LEFT ANGULAR GYRI

A dissociation in the central processes of spelling, with preferential ly lexical over phonological impairment, frequently affects patients w ith early Alzheimer's disease. The aim of this work was to test whethe r dissociations in the language domain in Alzheimer's disease can be e xploited with PET to assess the neural basis of cognition. To this end , we studied the functional neuroanatomy of writing impairment in Alzh eimer's disease by means of PET measurements of the local cerebral glu cose utilization and neuropsychological tests specially designed to as sess the phonological and lexical components of writing. We analysed t he performance in written spelling of irregular words and non-words of 11 right-handed patients with mild-to-moderate Alzheimer's disease. F or each patient we calculated a residual phonological score and a resi dual lexical score, based on a cognitive interpretation of the errors according to the item category. In each of these 11 patients, using PE T, we measured the resting-state utilization of glucose in the left su pramarginal gyrus and the left angular gyrus, two cortical regions sel ected a priori because of their presumed role in the central processes for spelling, and identified on CT scans obtained according to stereo toxic references and coregistered with PET. To assess the relationship s between the neuropsychological scores and the metabolic data, we use d the 'ratio paradigm', the sensitivity of which has been previously d ocumented in cognitive-metabolic correlative PET studies of Alzheimer' s disease that were less focused than the present study in both cognit ive and anatomical terms. We found a highly significant positive corre lation between phonological score:lexical score neuropsychological rat ios and corresponding supramarginal gyrus:angular gyrus metabolic rati os. These findings further support the role of these two left-sided te mporo-parietal regions in the central processes of writing and show th at the neuropsychological dissociations in early Alzheimer's diseases can be exploited to further our understanding of the functional neuroa natomy of cognitive operations. The role of focal, as compared with mo re diffuse, brain damage in the development of impaired written langua ge of central origin in Alzheimer's disease is also discussed.