By definition, brain activity that is volume-conducted to the scalp must be open field activity. However, even within a laminated cortical structure populated by asymmetric projection cells, considerable activity is locally cancelled (cf. CSD profile in Fig. 2B), making the notion of a closed field a relative concept. Ahlfors et al. (2010) also recognized the need to analyze the cancellation effects for distributed EEG and MEG generators. Caution must therefore be exercised when attempting to attribute ERP generators to regions identified by imaging methods that are not constrained by the properties of volume conduction. A non-electrical measure may identify activity corresponding to a predominantly closed field, phasic inhibitory activity, or patterned activity that is not represented by the scalp potential. Moreover, even when local intracranial activity appears to correspond to that recorded at the scalp, the different properties of intracranial and scalp electrodes must be carefully evaluated (e.g., contact size, impedance, and the tissue interface).
