Semantic decision making in early probable AD: a PET activation study.

We studied the neural correlates of semantic decision making in early probable Alzheimer disease patients and age-matched controls by measuring changes in relative cerebral blood flow by PET (oxygen-15) while they performed a category-specific word-matching task with either animal or artefact names. In comparison to a baseline (matching words by physical identity), semantic decision making elicited increased blood flow in the left frontal lobe (BA 6, 11, 47) and in the contralateral cerebellum in the controls. The only category-specific difference in controls was activation of the left supplementary motor area (BA 6) when making decisions on artefacts. In contrast, the patients recruited more extensive brain regions, encompassing areas in the left frontal lobe (BA 44, 45, 47), right occipital cortex (BA 18), midbrain, and the cerebellum bilaterally. Behaviorally, the patients performed the tasks with equal success albeit more slowly. No animal-artefact differences emerged in the patients' activation patterns, indicating that their activation increases reflected a generalized, presumably compensatory reaction to higher task demands in the face of a progressive neurological disease.