Brain Connectivity Marker for Alzheimer's Disease

Inclusion Criteria: 1. Inclusion criteria for subjects with subjective cognitive complaints: * MMSE (Mini-mental test) or MoCA (Montreal Cognitive Assessment) points between 26 and 30. * Absence of cognitive impairment. * Memory problems reported by the participant/family member. * Do not fulfill criteria for mild cognitive impairment or dementia. * Must speak and understand Swedish. 2. Inclusion criteria for patients with mild cognitive impairment: * MMSE (Mini-mental test) or MoCA (Montreal Cognitive Assessment) points between 24 and 30. * Impaired memory function. * Do not fulfill criteria for dementia. * Must speak and understand Swedish. * Must have abnormal cerebrospinal fluid amyloid-β 42/40 ratio levels, which is a biomarker of Alzheimer's disease. 3. Specific inclusion criteria for patients with Alzheimer's disease: * MMSE (Mini-mental test) or MoCA (Montreal Cognitive Assessment) points between 18 and 28. * Impaired memory function in addition to impaired executive abilities, language function, visuospatial ability and/or attention/psychomotor speed. * Meet NINCDS-ADRDA and DSM-IV criteria for probable Alzheimer's disease. * Must speak and understand Swedish. * Must have abnormal spinal fluid amyloid-β 42/40 ratio levels, which is a biomarker of Alzheimer's disease. Exclusion Criteria: * Alcohol or drug abuse. * Unstable somatic disease or organ failure. * Refuse to cerebrospinal fluid testing and/or blood sampling, neuropsychological testing, brain imaging, electroencephalogram or magnetoencephalogram. In addition, participants who have claustrophobia or some form of metal implant in their body that may interfere with the brain imaging scan will be excluded from the study.