The human brain consists of neural networks, which are usually in different states of activity. The activity of the brain can be measured by several indicators, e.g. by the changes of the electric potentials, which are recorded by an EEG (electroencephalogram). Special wave forms in the EEG are the event-related potentials. In contrast to a spontaneously measured EEG (which appears independently of an "event"), these wave forms are caused either by sensory perception (e.g. visual, acoustic, motoric or somatosensory stimuli; evoked) or they are correlated to cognitive processes (e.g. vigilance, memory; event-related). Thus, evoked potentials represent the processing of physical stimuli, while event-related potentials are caused by "higher tier processes". One of the difficulties in interpreting EEGs of different individuals is based on the high degree of variability between individuals. This variability is less strongly pronounced in event-related potentials and evoked potentials.
There are several studies on the effects of magnetic fields on the activity of the human and animal brains, in which the power spectral density of the main frequency bands, evoked or event-related potentials, were examined in the EEG (a detailed list can be found in "Environmental Health Criteria 238" in WHO, p. 125 ff. and p. 151 or for magnetic fields within the EMF-Portal or for electric fields within the EMF-Portal). These studies are difficult to compare due to the differences in the experimental design and exposure conditions. Moreover, there are inconsistencies and contradictory results. Additionally, the European Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR 2015) summarizes in its Opinion on Potential health effects of exposure to electromagnetic fields (p. 169), that due to the heterogeneity of the present studies with regard to applied fields, duration of exposure, and statistical methods, these studies are not useful for drawing meaningful conclusions. According to WHO, there is still need for further research on this topic, especially regarding occupational exposure and children.