Two-brain microstates: A novel hyperscanning-EEG method for quantifying task-driven inter-brain asymmetry

Joint action and interpersonal coordination between individuals are integral parts of daily life, and various behavioral tasks have been designed to study their emergence and maintenance. One example is the mirror-game paradigm, which examines the dynamics of two people improvising motion together. However, the underlying neural mechanisms remain poorly understood, and inter-brain methods underdeveloped. Previously, we reported unique individual behavioral and neural signatures of performing actions when observed by others using a mirror-game paradigm. Here, we explored inter-brain synchronization during the mirror-game paradigm using a novel approach employing two-brain EEG microstates. Microstates are quasi-stable configurations of brain activity that have been reliably replicated across studies, and proposed to be basic buildings blocks for mental processing. Expanding the microstate methodology to dyads of interacting participants (two-brain microstates) enables us to investigate quasi-stable moments of inter-brain synchronous and asymmetric activity. Interestingly, we found that conventional microstates fitted to individuals were not related to the different task conditions; however, the dynamics of the two-brain microstates were changed for the observed actor-observer condition, compared to all other conditions where participants had more symmetric task demands (rest, individual, joint). These results suggest that two-brain microstates might serve as a method for identifying inter-brain states during asymmetric real-time social interaction.