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

The neural mechanisms underlying real-time social interaction remain poorly understood. While hyperscanning has emerged as a popular method to better understand inter-brain mechanisms, inter-brain methods remain underdeveloped, and primarily focused on inter-brain synchronization (IBS).

We developed a novel approach employing two-brain EEG microstates, to investigate neural mechanisms during symmetric and asymmetric interactive tasks. Microstates are quasi-stable configurations of brain activity that have been proposed to represent basic building blocks for mental processing. Expanding the microstate methodology to dyads of interacting participants enables us to investigate quasi-stable moments of inter-brain synchronous and asymmetric activity.

Conventional microstates fitted to individuals were not related to the different interactive conditions. However, two-brain microstates were modulated in the observer-actor condition, compared to all other conditions where participants had more symmetric task demands, and the same trend was observed for the follower-leader condition. This indicates differences in resting state default-mode network activity during interactions with asymmetric tasks.

Hyperscanning studies have primarily estimated IBS based on functional connectivity measures. However, localized connections are often hard to interpret on a larger scale when multiple connections across brains are found to be important. Two-brain microstates offer an alternative approach to evaluate neural activity from a large-scale global network perspective, by quantifying task-driven asymmetric neural states between interacting individuals.

We present a novel method using two-brain microstates, including open-source code, which expands the current hyperscanning-EEG methodology to measure and potentially identify both synchronous and asymmetric inter-brain states during real-time social interaction.