Dangerous phase.

"Use a quiet reference." How many times have we heard this mantra during training or practice, interpreting electroencephalogram (EEG) tracings, or implanting intracranial electrodes? How many of us have used common reference EEG for synchrony studies in recent years? Far too many.Perhaps one source of this problem is the number 104. This is the relatively small number of citations to the reference Fein et al. (1988), which should have put to rest any further use of referential EEG for coherence measurements. And in retrospect, a more careful reading by us of Nunez's (1981) text would have instructed us not to do this. How such warnings have managed to escape integration into common knowledge and practice is troublesome. Electrical potentials are all measured with respect to other potentials. Technically, a potential difference is calculated by integrating the electrical field over a given path from one place to another in EEG terms, we mea sure a potential with respect to another potential, measured at one or more electrodes. All EEG potential measurements reflect the paths used to measure those potentials, and do not directly reflect localized regions of the brain beneath one electrode. Worse, in scalp EEG, the layers of cerebrospinal fluid, dura, skull, and scalp serve to smooth, filter, spread out, and redirect currents generated within the brain so that the measured scalp potentials bear a rather tenuous relationship to the underlying (presumably dipole) current sources. In calculating coherence, it is easy to show that if the potential differences are all made with respect to a common reference, then the amplitude of the reference can dominate the coherence estimate (Fein et al., 1988). In recent years, phase synchronization has been increasingly applied to analyze the dynamics of nonlinear systems (Pikovsky et al., 2000). In Guevara et al. (in this issue), we see the extension of Fein's results for phase coherency. The geometry of Fig. 1 in Guevara et al. should be imprinted on all of us the amplitude of a common reference can dominate the calculated phase syn chronization. There is far too much literature within the past decade that calculated phase synchronization from common referenced EEG. The good news is that the fix to remove common reference artifacts is simple. The bad news is that the interpretation of reference- free synchronization results from brain signals requires considerable caution.