Functional anatomy of verbal and visuospatial span tasks in Alzheimer's disease

The aim of the study was to emphasize cerebral regions which subserve the performance of short-term memory tasks in patients with Alzheimer's disease. We correlated scores obtained on span tasks with cerebral metabolism measured at rest with positron emission tomography. Scores obtained on the digit span task correlated with glucose metabolism in a brain area centered on the premotor cortex and extending to the adjacent motor and parietal gyri. There exists some evidence suggesting that this area may subserve the sequential organization of material stored in short-term memory. In a secondary analysis, we also observed significant interregional correlations between left-sided brain areas which are part of the neural network subserving verbal working memory processes in healthy controls. These data suggest that individual performance on verbal span tasks in AD patients may essentially depend on the preservation of their ordination processing capacity. The absence of correlation with prefrontal regions suggests that AD patients might not spontaneously engage central executive resources to reach their maximal span score. For simultaneous visuospatial span task, the performance of patients correlated with posterior brain regions, and not with prefrontal cortices.