Global neuromagnetic cortical fields have non-zero velocity

Abstract

Globally coherent patterns of phase can be obscured by analysis techniques that aggregate brain activity measures acrosstrials, whether prior to source localization or for estimating interareal coherence. We analyzed, at singletrial level, whole head MEG recorded during an observertriggered apparent motion task. Episodes of globally coherent activity occurred in the delta, theta, alpha and beta bands of the signal in the form of largescale waves, which propagated with a variety of velocities. Their mean speed at each frequency band was proportional to temporal frequency, giving a range of 0.06 to 4.0 m/s, from delta to beta. The wave peaks moved over the entire measurement array, during both ongoing activity and taskrelevant intervals; direction of motion was more predictable during the latter. A large proportion of the cortical signal, measurable at the scalp, exists as largescale coherent motion. We argue that the distribution of observable phase velocities in MEG is dominated by spatial filtering considerations in combination with group velocity of cortical activity. Traveling waves may index processes involved in global coordination of cortical activity.