Hemispheric asymmetries of hypometabolism associated with semantic memory impairment in Alzheimer's disease: a study using positron emission tomography with fluorodeoxyglucose-F18

Considerable disagreement exists about the neuroanatomical basis of conceptual–semantic impairments observed in asubgroup of patients with Alzheimer’s disease (AD) at mild to moderate stages of the disease. Several studies of groups ofpatients have shown correlations between focal hypometabolism or hypoperfusion in left hemispheric areas and measures ofverbal semantic memory impairment in AD patients. The question remains, however, whether left hemispheric hypometabolismis sufficient to produce such impairment in the single case and whether nonverbal semantic knowledge is also affected. We usedpositron emission tomography (PET) with fluorodeoxyglucose-F18 (FDG), statistical parametric mapping (SPM), and tests ofverbal and nonverbal semantic memory in 11 AD patients with a mean score on the Mini-Mental State Examination of 22.6(F2.8). Naming impairment was significantly associated with left hemispheric asymmetry of hypometabolism on a single-casebasis. Our correlation analysis showed that metabolism in left anterior temporal, posterior inferior temporal, inferior parietal andmedial occipital areas (Brodmann areas: 21/38, 37, 40 and 19) correlated with both verbal and nonverbal semantic performance.We conclude that left hemispheric synaptic dysfunction, as measured by regional glucose hypometabolism, was sufficient toproduce semantic impairments in our patients. The majority of areas affected in our patients with semantic impairments wereinvolved in multimodal or supramodal (verbal and nonverbal) semantic knowledge.