A Synthesis of Functional Neuroimaging in the Frontal Variant of Frontotemporal Dementia

The frontal variant of frontotemporal dementia (fvFTD) is clinically characterized by a disruption of the social behaviour. Frontotemporal decrease of activity on functional imaging is an independent marker of the disease. Principal component analysis of cerebral functional images reveals one ensemble comprising both frontal lobes, and two lateralized clusters comprising temporal and subcallosal frontal regions. Executive dysfunction and verbal difficulties are shown to interact in fvFTD, and they are related to large scale frontal and temporal neural networks in the disease. The neural substrate of the memory impairments in patients with fvFTD is also heterogeneous, since correlations were observed with frontal activity, but also with medial temporal atrophy. Behavioural changes are the hallmarks of fvFTD. Apathy is consistently related to frontal involvement, while disinhibition occurs in patients with predominant posterior orbitofrontal and temporal involvement. The consequences of lateralized brain lesions remain a matter of debate, since impulsive and compulsive behaviour was evenly related to right or left temporal involvement in fvFTD. Those patients know social rules, but they are impaired in assessing the importance of social transgressions and in recognizing emotions. Processing transgressions of social norms results in a complex activation of medial prefrontal, orbitofrontal and temporal regions in control populations. The variable decrease in these regional activities observed in different fvFTD patients would explain their complex, individual social disturbance.