The iEEG and scalp EEG were acquired using a stainless steel suture placed in the vertex region of the scalp, midline between the Cz and Fz electrode positions (international 10–20) as a common reference. The scalp suture electrode is relatively isolated from the intracranial electrodes by the intervening layers of cerebrospinal fluid, bone, muscle, and scalp. These layers serve to distribute and attenuate the signal in such a way that approximately 7 cm2 of coordinated cortical activity is required to produce a clear detectable deflection on the scalp [41]. In practice, the reference electrode serves the purpose primarily of rejecting common-mode potentials generated by muscular contraction and body movement, which are conducted to the intracranial vault. Unfortunately, it also introduces artifacts unique to the scalp site.
