Inter-brain synchrony (neural coupling)

Inter-brain synchrony is the observation, drawn from hyperscanning, that two interacting brains show correlated or phase-aligned activity — for example a listener's cortical activity tracking and even anticipating a speaker's (Stephens, Silbert and Hasson 2010), or coupled oscillations during cooperation and joint attention. It is measured with correlation, coherence, phase-locking, or Granger-style directed influence between the two recordings.

Interpretation is contested. Synchrony can reflect genuine mutual adaptation, but it can equally arise because both brains process the same stimuli, make similar movements, or share the same environmental and physiological rhythms — so a correlation between brains is not evidence of a direct inter-brain influence. The construct is scientifically useful as a marker of shared processing, but it is not a communication channel and does not imply information flowing brain-to-brain in the BBI sense.