Propagating neocortical gamma bursts are coordinated by traveling alpha waves

Abstract

Neocortical neuronal activity is characterized by complex spatiotemporal dynamics. Although slow oscillations have been shown to travel over space in terms of consistent phase advances, it is unknown how this phenomenon relates to neuronal activity in other frequency bands. We here present electrocorticographic data from three male and one female human subject and demonstrate that gamma power is phase locked to traveling alpha waves. Given that alpha activity has been proposed to coordinate neuronal processing reflected in the gamma band, we suggest that alpha waves are involved in coordinating neuronal processing in both space and time.

Figure 1.

Power spectra averaged over all trials and electrodes for each participant. The analysis reveals dominant peaks in the alpha band (8–10 Hz).

Figure 2.

Phase-amplitude coupling over the most posterior ventral electrodes. The color scale reflects the degree of nonuniformity of the distribution of high-frequency amplitude (y-axis) with respect to the phase of the low-frequency oscillations (x-axis). Note that the coupling was constrained to the alpha band with respect to phase and high gamma band with respect to amplitude.

Figure 3.

Left: Number of times that each group of electrodes (a motif) showed the traveling wave pattern for all possible reference locations. Right: Average alpha phase difference between all electrodes and the reference location (white disc). The color code shows the phase difference (−180 to 180). The size of each square shows the magnitude of the PLV between a given electrode and the reference. The red arrow indicates a linear arrangement of electrodes where a systematic phase difference was observed (i.e., a traveling wave). The p-values refer to the statistical significance of systematic phase advances (or decays).

Figure 4.

Top: Phase aligned to the alpha activity in the reference location and either five or six consecutive electrodes (Fig. 3). Note that the averaged traces are not band-pass filtered, but they are normalized to have zero mean and SD one. Bottom: Normalized temporal evolution for the high gamma band. The rose plots show the distribution of alpha phase (in blue) corresponding to zero phase of the reference location, and the red line shows the mean. The green regions show the gamma power distribution corresponding to the zero phase of the reference location.