Intracranial EEG (iEEG) is an invasive technique based on recording electroencephalography (EEG) signals directly from the human cortex, as opposed to surface recordings in scalp-EEG. This is achieved either by using subdural grids or strips of electrodes placed directly on the surface of the cortex (also known as Electrocorticography), or using multi-lead depth electrodes.[1] In some cases, such as epileptic studies, deeper brain activity may not be recorded accurately by EEG. This is when a deeper EEG is required, due to the shielding effect of the scalp and skull. The EEG electrodes are placed under the surface of the scalp and skull directly on the brain surface. This is also called "electrocorticography" (ECoG). Electrodes can even be placed into specific brain areas, such as the hippocampus. The EEG signal then processes in the same manner as with a surface EEG, but with higher recording rates because of higher frequencies and little to no interference from outside sources — as with surface EEG.
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Intracerebral recordings in humans can be performed using a range of techniques that differ according to the neurological disorder and therapeutic strategy under consideration. Depth recordings are for instance used during positioning of a deep brain stimulation (DBS) electrode in Parkinson's disease [Benazzouz et al., [2002]; Engel et al., 2005. Furthermore, invasive recordings are used in patients with drug-resistant intractable epilepsy in order to chronically monitor neural activity in multiple brain structures during pre-surgical evaluation.
In patients with pharmacologically resistant epilepsy, iEEG is used to identify cortical regions critical for seizure onset and identify others that need to be spared at the time of surgery [e.g., Kahane et al., [2004]]. Intracerebrally implanted electrodes sometimes stay in place for more than two weeks in order to localize the origin of fast electrophysiological rhythms that precede seizure onset and that are at the core of the epileptogenic network. Intracranial EEG is used to test one or several hypothesis regarding the anatomical organization of the epileptogenic network. This sometimes implies that intra-cerebral electrodes are positioned in widely distributed brain regions that include pathological but also healthy tissue. as a result, such a clinical context can also provide a unique opportunity to study fundamental questions about neural coding and cognition (Jerbi et al. 2009).[2]
Although microelectrodes have been used in humans to acquire single-neuron spiking data [Fried et al., 1997; Heit et al., 1988; Ojemann et al., 2002; Ward and Thomas, 1955], clinical recordings in epilepsy patients are generally performed using macroelectrodes that measure coherent activity of local neuronal populations in the vicinity of the recording site . The most common choice in the clinical routine is to use either stereotactic electroencephalography (SEEG) or electrocorticography (ECoG) which acquire intracranial data using multilead depth electrodes or subdural grid electrodes respectively. Subdural grids consist of 2D arrays (or sometimes one-dimensional strips) of electrodes positioned directly on the lateral surface of the brain, with a typical inter-electrode distance of 1 cm [Engel et al., 2005]. In contrast, depth electrodes are semi-flexible one-dimensional linear arrays, shaped as narrow needles that penetrate deep into the brain. Such depth electrode implantations are often referred to as Stereotactic EEG because a stereotactic technique developed by Talairach and Bancaud is used to localize the electrodes [Kahane et al., 2006]. While subdural grids provide widespread cortical coverage and cortical maps of gyral activity, the multilead depth electrodes record from both sulci and gyri and go beneath the cortical surface to probe deep cortical structures, such as the cingulate gyrus, and occasionally subcortical structures, such as the lateral geniculate nucleus [Krolak-Salmon et al., 2003] or the nucleus acumbens [Münte et al., 2008]. A further difference between SEEG and ECoG is that while depth electrodes require small burr holes for implantation the implantation of two-dimensional subdural grids involves a larger craniotomy. Converging evidence suggests that both grid [Menon et al., 1996] and depth-electrodes recordings [Lachaux et al., [2003]] provide sufficient spatial resolution to localize neural activity at the gyral level, a precision that is as good as, if not better, than what is achieved with fMRI. In addition, the spatial precision of the analysis also depends on the accuracy of electrode localization [Dalal et al., 2008] and choice of reference electrode. In SEEG data, the local precision is highest when each recording site is referenced to its nearest neighbor (bipolar montage) than when one remote site is used as reference for all channels (common reference) [Lachaux et al., 2003].
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