Longitudinal study:  Inclusion criteria:  MCI due to AD, or mild or moderate AD dementia according to the National Institute on Aging and Alzheimer's Association (NIA-AA) diagnostic criteria Caregiver willing to participate as an informant MMSE >19 at inclusion Brain FDG-PET/MRI or FDG/PET-CT Able to cooperate to the investigations and give informed consent  Exclusion criteria:  Other neurological or psychiatric illness that may affect neurofilament light (NfL) levels (severe neuropathy, multiple sclerosis (MS), stroke within the last 3 months, Wernicke encephalopathy) Diagnosis of previous or current major psychiatric disorder (schizophrenia, bipolar disorder, psychosis) within last 2 years Excessive alcohol intake or substance abuse within the last 2 years Ophthalmological disorders that may affect pupillometry Participating in drug trials or other intervention trials  Short-term study:  Inclusion criteria:  Patients under investigation of a neurodegenerative disease MMSE >19 Scheduled lumbar puncture/lumbar puncture performed within the last week prior to inclusion Written consent form to the Danish Dementia Biobank Able to cooperate to the investigations  Exclusion criteria:  Other neurological or psychiatric illness that may affect NfL levels (severe neuropathy, MS, stroke within the last 3 months, Wernicke encephalopathy) Excessive alcohol intake or substance abuse within the last 2 years Ophthalmological disorders that may affect pupillometry Participating in drug trials or other intervention trials Cross-sectional study:  Inclusion criteria - Patients:  A diagnosis of a dementia disorder Caregiver willing to participate as an informant MMSE >15 at inclusion Able to cooperate to the investigations Able to give informed consent  Exclusion criteria - Patients:  Diagnosis of previous or current major psychiatric disorder (schizophrenia, bipolar disorder, psychosis) within last 2 years Excessive alcohol intake or substance abuse within the last 2 years Other known brain disorder Ophthalmological disorders that may affect pupillometry Participating in drug trials or other intervention trials  Inclusion criteria - Healthy Controls:  Able to cooperate to the investigations Normal cognition Age 50-90 year  Exclusion criteria - Healthy Controls:  Diagnosis of previous or current major psychiatric disorder (schizophrenia, bipolar disorder, psychosis) within last 2 years Excessive alcohol intake or substance abuse within the last 2 years Other known brain disorder Ophthalmological disorders that may affect pupillometry