Common reference coherence data are confounded by power and phase effects

EEG synchrony during a perceptual-cognitive task: Widespread phase synchrony at all frequencies

“…3-10. As shown above, any synchrony that remains in average-referenced data cannot logically be due to the artifact postulated by Fein et al (1988). Episodes of synchrony in average-referenced data are genuine.…”

“…Any EEG measurement represents the voltage difference between a recording site, V 1 and a reference site, V ref . As pointed out by Fein et al (1988), any major reduction in power under electrode V 1 will cause the measurement V 1 À V ref to be dominated by activity under the reference electrode, V ref . This has the potential to cause artifactual synchrony, due to the comparison of two measurements both of which are dominated V ref .…”

“…EEG data are easy to collect, but methods of measuring phase synchrony in them are fraught with pitfalls. For example, Fein et al (1988) have rejected the idea that synchrony can validly be measured in data recorded using a common recording reference. They point out that it is theoretically possible for low power under both of two recording electrodes to exaggerate the common contribution of the reference electrode, so that any coherence or synchrony measures are essentially comparing the reference with itself.…”

Decrease in early right alpha band phase synchronization and late gamma band oscillations in processing syntax in music

“…The use of a common reference for coherence and phase synchronization analysis has been criticized in recent years based on the argument that the common reference introduces spurious correlations uniformly distributed over the scalp [Fein et al, 1988;Guevara et al, 2005;Nunez et al, 1997;Schiff, 2005]. Instead, the use of surface Laplacians has been encouraged because they are essentially reference-free and additionally reduce the volume conduction effect [Nunez, 1995;Pernier et al, 1988;Perrin et al, 1987Perrin et al, , 1989Schiff, 2005].…”

Classification and interactive segmentation of EEG synchrony patterns

“…Both of these problems may introduce spurious correlations between different EEG channels, even if they are relatively apart [15]. Various methods have been proposed to deal with these issues; particularly, in the case of cognitive tasks, by adequately choosing the reference site (e.g., linked earlobes), one may assume that any activity at the reference is not related to the task, and thus its effect may be greatly reduced by averaging over multiple trials.…”