Journal Article

The origin of extracellular fields and currents — EEG, ECoG, LFP and spikes

Anastassiou, Costas A., Buzsáki, György, Koch, Christof

2012

Overview

Key Points All currents in the brain superimpose to yield an 'electric field' at any given point in space. The current sources and sinks form dipoles or higher-order n-poles. Extracellular currents arise from many sources, including synaptic currents, fast action potentials and their afterpotentials, calcium spikes and voltage-dependent intrinsic currents. The magnitude of extracellular currents depends critically on two factors: the cytoarchitectural organization of a network and the temporal synchrony of the various current sinks and sources. Depending on the recording method, neuroscientists distinguish between electroencephalogram (EEG), electrocorticogram (ECoG) and local field potential (LFP; also known as micro-, depth or intracranial EEG), although all of these measures refer to the same biophysical process. The electric field is the force 'felt' by an electric charge, and can be transmitted through brain volume. The extent of volume conduction depends on the relationships between the current dipole and the features of the conductive medium. High-density sampling of the extracellular field with contemporary methods enables the calculation of current source density, and therefore the localization of current sinks and sources. The voltage gradients generated by highly synchronous activity of neuronal groups can affect the transmembrane potential of the member neurons and alter their excitability through ephaptic coupling. Synchronous spiking of nearby neurons is the main source of the high-frequency components of the local field. There is a discernable relationship between the temporal evolution of cell assemblies and the time-dependent changes of the spatially distributed currents. High-density, wide-band recordings of the local field can therefore provide access to both afferent inputs and the spiking output of neurons. Neuronal activity in the brain gives rise to transmembrane and extracellular electromagnetic fields that can be measured in the extracellular medium using several approaches. In this Review, Buzsáki and colleagues provide an overview of the mechanisms that underlie the generation of extracellular currents and fields. Neuronal activity in the brain gives rise to transmembrane currents that can be measured in the extracellular medium. Although the major contributor of the extracellular signal is the synaptic transmembrane current, other sources — including Na + and Ca 2+ spikes, ionic fluxes through voltage- and ligand-gated channels, and intrinsic membrane oscillations — can substantially shape the extracellular field. High-density recordings of field activity in animals and subdural grid recordings in humans, combined with recently developed data processing tools and computational modelling, can provide insight into the cooperative behaviour of neurons, their average synaptic input and their spiking output, and can increase our understanding of how these processes contribute to the extracellular signal.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 631/378/548 / 631/80/86/820 / 87 / Animal Genetics and Genomics / Animals / Behavioral Sciences / Biological and medical sciences / Biological Techniques / Biomedical and Life Sciences / Biomedicine / Calcium Signaling - physiology / Cell membranes / Cellular signal transduction / Central nervous system / Electrical Synapses - physiology / Electrodiagnosis. Electric activity recording / Electroencephalography / Electrophysiology / Evoked Potentials - physiology / Extracellular Space - physiology / Fundamental and applied biological sciences. Psychology / Humans / Investigative techniques, diagnostic techniques (general aspects) / Ligand-Gated Ion Channels / Magnetoencephalography / Medical sciences / Nervous system / Neural Conduction - physiology / Neurobiology / Neuroglia - physiology / Neurons / Neurons - physiology / Neurons - ultrastructure / Neurosciences / Physiological aspects / review-article / Synapses - physiology / Synapses - ultrastructure / Vertebrates: nervous system and sense organs / Voltage-Sensitive Dye Imaging