Abbreviations: AD = Alzheimer's disease; FC = functional connectivity; HC = healthy controls; ROI = region of interest; SD = semantic dementia
AbstractThe severe semantic memory impairments in semantic dementia have been attributed to a pronounced atrophy and functional disruption of the anterior temporal lobes. In contrast, medial and posterior temporal lobe damage is predominantly found in patients with Alzheimer's disease, and has been associated with episodic memory disturbance. Despite these distinct neuropathological signatures, the hippocampal deterioration common in Alzheimer's disease and semantic dementia appears paradoxical. To gain more insight into the mutual and divergent functional alterations seen in Alzheimer's disease and semantic dementia, we assessed the differences in intrinsic functional connectivity between temporal lobe regions in patients with Alzheimer's disease (n = 16), semantic dementia (n = 23), and healthy controls (n = 17). We used a functional parcellation of the temporal cortex to extract time series. The Alzheimer's disease group showed only a single connection with reduced functional connectivity as compared to the controls. This connection was located between the right orbitofrontal cortex and the right anterior temporal lobe. In contrast, functional connectivity was decreased in the semantic dementia group in six connections, mainly involving the hippocampus, lingual gyrus, temporal pole, and orbitofrontal cortex. We identified a common pathway with semantic dementia, since the functional connectivity between the right anterior temporal lobe and the right orbitofrontal cortex was reduced in both types of dementia. This might be related to semantic impairments observed in both dementia types, and abnormal social behavior at more progressed disease stages. However, to substantiate such claims, longitudinal studies that more strongly relate behavioral outcomes to functional connectivity are needed.