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Abstract
Globally coherent patterns of phase can be obscured by analysis techniques that aggregate brain activity measures across-trials, whether prior to source localization or for estimating inter-areal coherence. We analyzed, at single-trial level, whole head MEG recorded during an observer-triggered apparent motion task. Episodes of globally coherent activity occurred in the delta, theta, alpha and beta bands of the signal in the form of large-scale 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 task-relevant intervals; direction of motion was more predictable during the latter. A large proportion of the cortical signal, measurable at the scalp, exists as large-scale 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.
Highlights
Local modules and large-scale traveling wavesCurrent neuroscience tends to regard the cortex as a network of semi-autonomous regions [1]
Considerable progress has been made in determining the functions of individual regions at various scales, from cortical columns to whole Brodmann areas. Since these functions are hardly ever performed in isolation, fMRI, EEG, and MEG studies typically observe activity distributed among multiple centers
A growing literature considers the functional significance of traveling waves (TWs) at multiple spatial scales. These include the columnar scale [8,9] Brodmann area [10,11,12,13] and in large scale cortex [14,15,16,17]. How these different scales interact is an interesting question in itself, but in the present research we focus on the largest scale of TW dynamics
Summary
Local modules and large-scale traveling wavesCurrent neuroscience tends to regard the cortex as a network of semi-autonomous regions [1]. Considerable progress has been made in determining the functions of individual regions at various scales, from cortical columns to whole Brodmann areas. Since these functions are hardly ever performed in isolation, fMRI (functional magnetic resonance imaging), EEG (electroencephalogram), and MEG (magneto-encephalogram) studies typically observe activity distributed among multiple centers. Such observations have raised interest in how the centers are coordinated [2,3] and, in particular, the role of synchronous neural oscillations over multiple frequency bands therein [4,5,6]. The shift in focus from single to multiple interacting centers of activation has not changed the predominance, of the view that these activity centers are static.
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