are measured from two scalp sites placed at a right angle to the orientation of the green dipole (Fig. 14B), a greater potential will be measured for the red dipole. Hence, this reference scheme will lead to the opposite conclusion (i.e., the second process yielded a larger EEG signal than the first process). Even if the two processes are represented by the same dipole (same orientation), although to a different degree, the choice of EEG reference will influence the measured SP amplitudes. Whereas the in-line reference arrangement will render a green-larger-than-red result (Fig. 14C), the orthogonal reference arrangement will reveal no differences (zero amplitudes) between the two processes (Fig. 14D). These unfortunate ambiguities apply to all surface potential measures in time, frequency and time-frequency domains, however, becoming increasingly more convoluted with multiple neuronal generators overlapping in space and time and greater complexity of the EEG measure. Without doubt, many misconceptions and controversies in EEG and ERP research have their root in the use of difference reference schemes (e.g., Joyce and Rossion, 2005).
