Electroencephalogram (EEG) Topography

In human brain electrophysiology, two broad classes of activation can be distinguished: (1) spontaneous neural activity reflected by the electroencephalogram (EEG) which constitutes the continuous brain activation; and (2) potential fields elicited by physical, sensory stimuli, or by internal processing demands. Such activity is named evoked potential (EP) when elicited by external sensory stimuli, and event-related potential (ERP) when internal, psychological events are studied. Conventionally, EEG measures are used to quantifying the global state of the brain system, and to study ongoing brain activity during long time epochs (from 30 minutes to several hours) such as during sleep or long-term epilepsy monitoring. The study of evoked activity aims at elucidating different brain mechanisms, and it allows assessing sensory or cognitive processing while the subject is involved in perceptual or cognitive tasks. As evident from the data and results presented in this chapter, mapping brain electrical activity constitutes a means for (1) visualization of brain electric field distributions on the scalp, (2) the adequate quantitative and statistical analysis of multichannel electrophysiological data, and (3) the computational determination of possible underlying neuronal populations that are spontaneously active or are activated by sensory stimulation or psychological events. Thus, electrical brain activity can be characterized in terms of latency (i.e., processing times), synchronous involvement and extent of neuronal populations (i.e., field strength), and topographical distribution of frequency content or potential components. Practical applications are twofold: studies of functional states of the human brain, information processing, and motor planning in healthy subjects, and clinical questions on the intactness and functionality of the central nervous system of patients suspected of nervous or psychiatric disease. For clinical purposes, the question of deviation from normal is relevant. Such deviations may often not be obvious in visual inspection of EEG or evoked potential traces but are detectable in quantitative, numerical evaluation. The mapping of the electric scalp distribution patterns of healthy volunteers allows establishing normative data. It has been shown that neurological impairment or psychiatric disorders are characterized by distinct profiles of abnormal brain electric fields. Such significant deviations can statistically validated and used for clinical diagnosis and evaluating treatment efficacy. Brain topography also allows drawing conclusions about the possible localization of pathological activity such as spikes or “spike and wave” patterns in epilepsy or the location of abnormal activity caused by structural damage such as lesions or tumors. Future applications of topographic mapping of electrophysiological activity will include coregistration of high time resolution EEG recordings with brain imaging methods like functional MRI. It is to be expected that the collaboration of the fields will lead to functional imaging of brain activity with high temporal and high spatial resolution. Keywords: EEG topography; tomographic imaging; electrocepholographic mapping; scalp distribution; evoked potential; intracranial brain fields