The EEG signal reflects the summation of many (> 107) synchronously activated cortical neurons that have similar spatial orientation, and their combined activity may be conveniently conceptualized (or simplified) as a current dipole with respect to the macroscopic scale of the scalp-recorded EEG (Fig. 1). An electrical potential representing the dipole strength can be measured at scalp as the voltage difference between two different recording sites, with one site serving as the reference for the other (e.g., in Fig. 1A, site E2 is used as reference for site E1, resulting in a negative potential measurement for a current dipole with its negative pole directed towards site E1). The absolute value of this measured potential has no physical meaning (e.g., Nunez and Srinivasan, 2006), other than representing the potential difference between the two recording sites. Thus, if the same dipole is measured at the same two scalp locations but after reversing the assignment of ‘active’ and reference site, a positive potential measurement is obtained (Fig. 1B). Other than the reversal in sign, this positive potential has the same absolute value as
