Electroencephalography and magnetoencephalography

Introduction The brain is an electrochemical organ. Most of the brain's energy metabolism is devoted to maintaining electrical gradients, which are the basis of resting state potentials and neural transmission. Three neuroimaging techniques create images using these potentials and may be used to examine brain function: electroencephalography (EEG), evoked potentials (EPs), or event-related potentials (ERPs), and magnetoencephalography (MEG). EEG was introduced to clinical practice in the early 20th century by a German psychiatrist, Hans Berger, who demonstrated changes in the EEG in response to arousal, emotions and activities. Interest in clinical EEG was fueled by Berger's study of the EEGs of institutionalized patients (Berger, 1937), many of whom showed distinct abnormalities, particularly diffuse slowing. Berger also made clear that the EEG has a special role in the assessment of older adults, by reporting that the EEG changed with aging and was abnormal in a high proportion of individuals with dementia. Much of Berger's work is still relevant today; the EEG remains the single most costeffective physiologic test to indicate the presence of delirium or dementia in an older adult. The usefulness of conventional EEG has been limited, however, by limited reliability of interpretations and unclear physiologic meaning of results. Over the past several years, the usefulness of EEG has been enhanced through the application of computerbased methods for signal analysis. These methods, classed under the broad rubric of quantitative electroencephalography (QEEG), have rendered EEG measurements more reliable and reproducible and have aided physiologic interpretation of abnormalities.