Due to increasing life expectancy and a rising elderly population in Europe, the incidence of mild cognitive impairment which may predict diseases like Alzheimer's Disease or Vascular Dementia, is rising. Neurophysiological techniques are simple and inexpensive tools for early diagnosis and provide useful and objective correlates of cognitive activity both in normal subjects and patients suffering from the above conditions. Cognitive impairment due to different mental disease is characterized by decreased power and coherence in the alpha/beta band, which suggests functional disconnection among cortical areas, whereas both power and coherence in the delta and theta bands increase as a sign of cortical deafferentation from subcortical structures. Quantification of power and phase relationship by bispectral analysis suggests the Bispectral Index could be a useful but simple tool for early diagnosis of mental disease.