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sensors can also be used to eliminate noise and to synthesize higher order virtual
gradiometers [97]. Also, cost-efficient active shielding technology has been devel-
oped to further reduce the effects of sources outside the MSR and to reduce the
need for heavily shielded rooms. Recent significant progress on active shielding has
allowed the installation of MEG systems in a greater variety environments, with
reduced MSR bulk and weight.
8.2.2 Electroencephalography (EEG)
In EEG, an array of electrodes is placed on the scalp surface to noninvasively sample
the scalar field of electric potentials relative to a reference electrode [58, 61]. EEG
recording technology has progressed much since the first human recordings by Hans
Berger in 1924 [8] and the later work by Edgar Adrian [1]. Due to its relative low-
cost and portability, EEG has become a standard technique for clinical monitoring.
Modern state-of-the-art research systems use electrode caps with as many as 256
sensors. It is sometimes considered as a bridge-technique between brain imaging
modalities. Indeed, some EEG systems are used for simultaneous EEG/MEG or
EEG/fMRI recordings. Research is being conducted on wireless acquisition and on
dry electrode technologies that do not use conductive gel, thereby reducing prepa-
ration time.
8.2.3 Electrocorticography (ECoG)
In patients undergoing ECoG, grid or strip electrode arrays are neurosurgically
placed to record the electric potential more closely to the neural sources and undis-
torted by the skull [40,11]. Grid arrays have typical interelectrode distances of about
1 cm or lower. Invasive measurements of the local field potential (LFP) can be
recorded by depth electrodes, electrode grids, and laminar electrodes [67, 91, 77].
Although the variations of electrical potentials captured by invasive recordings are
usually considered as being locally generated (that is, within the immediate vicinity
of the electrode), intracranial electrodes can pick up contributions from remotely
located sources when they are strongly or coherently activated.
8.3 Data Preprocessing
Data is usually preprocessed with a variety of methods prior to localization, the pur-
pose being correction and/or rejection of cardiac, eye, muscle, respiratory, and envi-
ronmental artifacts, and the extraction of features of interest. For that purpose, data
channels may be processed either sequentially or all at once. In the former, so-called
