The Brain from Inside Out

by György Buzsáki

The transmembrane potential of neurons can be measured by electrodes, which can report neuronal activity with submillisecond time resolution. Electric current flows from many neurons superimpose at a given location in the extracellular medium to generate a potential with respect to a reference site. We measure the difference between “active” and reference locations as voltage. When an electrode with a small tip is used to monitor the extracellular voltage contributed by hundreds to thousands of neurons, we refer to this signal as the local field potential (LFP). 10 Recordings known as the electrocorticogram (ECoG) are made with larger electrodes placed on the brain surface or its surrounding dura mater to sample many more neurons. Finally, when even larger footprint electrodes are placed on the scalp, we call the recorded signal an electroencephalogram (EEG).