Although electrical activity recorded from the exposed cerebral cortex of a monkey was reported in 1875 , it was not until 1929 that Hans Berger, a psychiatrist in Jena, Germany, first recorded noninvasively rhythmic electrical activity from the human scalp , which has subsequently known as electroencephalography (EEG). Since then, EEG has become an important tool for probing brain electrical activity and aiding in clinical diagnosis of neurological disorders, due to its excellent temporal resolution in the order of millisecond. The first recording of magnetic fields from the human brain was reported in 1972 by David Cohen at the Massachusetts Institute of Technology , which led to the development of magnetoencephalography (MEG). Like EEG, MEG also enjoys high temporal resolution in detecting brain electrical activity. EEG and MEG have become two prominent methods for noninvasive assessment of brain electrical activity, providing unsurpassed temporal resolution, in neuroscience research and clinical applications such as epilepsy or sleeping disorders.