Methods : the investigators plan to recruit 60 patients with ALS, 10 patients with ALS/FTD (the diagnosis of dementia will rest on clinical data and formal neuropsychological testing) and 20 normal control subjects. The ALS patients will be divided into 2 subsets on the basis of a preliminary neuropsychological work-up, according to the presence or absence of "subclinical cognitive impairment" as defined by abnormal scoring on tests not meeting the criteria for dementia. In a second testing session carried out at the same time, a comprehensive assessment of memory, behaviour and emotional changes will be done. All subjects will then undergo morphological magnetic resonance imaging (MRI), resting-state functional MRI and 18-fluorodeoxyglucose positron emission tomography (18FDG-PET). Whenever possible, a second testing session will be carried out 9 to 12 months later in order to quantify the cognitive deterioration, if any, and to find early predictors of the evolution towards dementia. In deceased patients, the location and extent of neuronal loss will be determined, as well as the location and number of UBIs.