Quantifying the dynamics of amplitude- and phase-coupling in the human brain

Local brain activity and interactions between areas are modulated on various timescales from the order of milliseconds to several minutes. Two distinct modes of functional interactions can be defined based on the phase- and amplitude-relations of large-scale brain activity, respectively. However, little is known about the dynamics of both coupling modes over the various timescales. Moreover, it is traditionally assumed that phase-coupling displays faster dynamics than amplitude-coupling without clear empirical evidence. Here, we leveraged the high temporal precision of magnetoencephalography (MEG) to quantify both functional coupling modes. We applied novel analysis approaches to time-resolve coupling on the millisecond scale and directly compared amplitude- and phase-coupling dynamics. We found the strongest functional coupling modulations in a frequency range from 0.01 Hz to about 1/10 of each carrier frequency. Importantly, phase-coupling modulations appeared to be relatively slower than for amplitude-coupling. Within each carrier frequency and coupling mode slower coupling dynamics correlated with stronger time-averaged functional coupling. Overall, we found both amplitude- and phase-coupling to be modulated over a variety of temporal scales. Our findings hence outline how to assess fast changes in functional coupling during cognitive and task-related processing. Moreover, and contrary to intuition, phase-coupling was more stable over time than amplitude-coupling. Our results, thus, further underline the dissociation between amplitude- and phase-coupling and their distinct characteristics as neuronal coupling modes.