Thalamic radiodensity and cognitive performance in mild and moderate dementia of the Alzheimer type

Eighteen patients with mild to moderate dementia of the Alzheimer type underwent cranial computed tomography (CT) and tests of visual attention, recognition and learning. Two subgroups emerged. Subgroup 1 was made up of ten patients who showed impaired visual recognition learning and memory, but intact attention in marked contrast to subgroup 2, which was made up of eight patients, in whom all of these functions were impaired. Planimetric and densitometric CT measurements yielded one significant difference between the two subgroups: a decreased radiodensity in the dorsomedial thalamus of the patients from subgroup 2. Lower radiodensity in the right dorsomedial thalamic area was significantly correlated with impaired performance on the test of attentional set shifting, more specifically, with deficits at the reversal learning stage. These results are interpreted in the context of recent evidence linking reversal learning to a neural network comprising the cholinergic basal forebrain, the amygdala and the orbitofrontal cortex, as well as the mediodorsal nucleus, and recent evidence of cholinergic deficits in this structure in patients with Alzheimer's disease.