Impairment of homonymous processing in Alzheimer's disease.

An important issue in research on language is how concepts are represented and associated with each other in the brain. Many investigations have focused on language ambiguity and the phenomenon of homonymy in which a single lexical item, presenting the same form, is related to different meanings. Our study aims to test the hypothesis that weak association of meaning characterizing homonyms may be especially prone to brain damage. To verify this hypothesis a test of attribution of the meaning of homonymous words, the Humpty Dumpty (HD) test, was applied to 50 patients with Alzheimer's disease (AD) and 50 healthy subjects. Results show that AD patients are impaired in the HD test in an early phase of disease and that performance correlates with naming ability and phonological fluency. The data are in keeping with a growing body of literature that supports dual impairment to the semantic system in AD, i.e., to semantic knowledge and active processing and access to the semantic field. The evaluation of the ability to resolve homonymous ambiguity, using the HD test, may provide a useful and quick clinical tool to detect the anomalies of the semantic network linked to either a loss of the core system where meaning of words is stored or an impairment of the access to an intact semantic representation.

Axonal injury within language network in primary progressive aphasia.

Primary progressive aphasia (PPA) is characterized by an isolated progressive impairment of word use and comprehension reflecting the distribution of pathological processes within the left hemisphere. We used proton magnetic resonance spectroscopy (1H-MRS) to study in vivo the integrity of axonal fibers connecting perisylvian language areas in 11 patients with PPA, 11 subjects with Alzheimer's disease, and 22 controls. Brain metabolites (N-acetylaspartate, myoinositol, choline, creatine) were measured bilaterally within a volume of interest located in the central portion of the superior longitudinal fasciculus, a long associative bundle connecting Broca's area with Wernicke's area, and other language regions of the temporal lobe. In the PPA group, there was an asymmetrical N-acetylaspartate to creatine ratio reduction compared with Alzheimer's disease and controls, with greater changes on the left side. The myoinositol to creatine ratio was increased in the PPA group bilaterally compared with controls. The choline to creatine ratio did not differ among the three groups. These results indicate an asymmetrical focal axonal injury within the language network in PPA. The marked difference in the distribution of N-acetylaspartate to creatine between PPA and Alzheimer's disease suggests that proton magnetic resonance spectroscopy may help to differentiate between these two conditions.