Finally, the role of volume conduction is important to evaluate. Volume conduction is the propagation of an electomagnetic field at the speed of light or 3 × 1010 cm/s (Feynman et al., 1963; Malmivuo and Plonsey, 1995). For the distance of the order of centimeters the delay is approximately 3.3 × 10−9 s which is a delay that is extremely small and therefore approximates zero phase difference at all points in a volume. Phase difference is measured by the “imaginary number” component of the cross-spectrum and PR is the 1st derivative of the “imaginary number” that by definition is not volume conduction if a phase difference is greater than zero. A test of the possible influence of volume conduction involved measuring the absolute phase difference between Brodmann areas and then determining if the phase difference was greater than zero and how phase difference changes with distance. If greater than zero then the phase lock measures cannot be explained by volume conduction. Further, because connection density decreases as a function of distance and conduction delays linearly accumulate with distance then if
