In this survey of studies about interbrain synchronization, the authors talk about subjective reports of 'self-other merging' (non-singular experiences of consciousness).
The supposition is supported by hyperscanning studies demonstrating the correlation between objective measurements of brain-to-brain synchrony such as Phase Locking Value or Wavelet Coherence, with situations of shared social intention such as cooperative games.
Notably, in order to establish true causality for the social/intentional basis of interbrain synchrony (as opposed to shared external stimuli), many of these studies perform controls which rule out different forms of stimulus.
- For example, this singing/humming study placed an opaque partition between participants to eliminate visual communication:
- In this study where pairs of participants interacted indirectly by playing the Prisoner's Dilemma game on computers, interbrain synchrony was observed when participants were told they were playing with a human partner, but was not when they were (falsely) told they were playing with a machine
- An alternative summary
My question is: if interbrain synchrony is real, and is triggered by social intention instead of shared sensory stimulus, what is the physical explanation for the observed phase locking between multiple brains? How can physically distant brains synchronize their electrical oscillations? Can human brains really communicate directly with each other?
In non-human animals: In vertebrates, electroreception is only known to occur in species with specifically evolved sensory organs, and generally requires water or humidity for efficiency. Furthermore, even if electrocommunication could account for electric signals travelling the distance between brains, it cannot explain the physiological aspect of interbrain synchrony (which is measured in terms of synchronized readings from similar brain regions).
Conversely: Mirror neurons may explain the physiological correlations of interbrain synchrony, but seem to be dependent on external sensory stimulus. This appears to be at odds with experimental setups which purposely introduce indirections between participants. Additionaly, mirror neurons are compatible with an internalist explanation of interbrain synchrony which denies the first authors' suggestions regarding a non-private consciousness.
Both mechanisms are compatible with a separate experimental observation that interbrain synchrony is positively correlated with oxytocin, which I understand to be a neuropeptide that encourages neural (electrical) activity.
What I would like to know is the scientific consensus on how interbrain synchronization is possible. There seems to be enough evidence to establish that it is a phenomenon which really occurs. Researchers also seem clear/insistent on describing brain-to-brain synchrony as distinct from correlated reactions to shared sensory stimuli. In that case, it seems that there must be some alternative hypothesis for how synchronization is mechanically possible.
Edit: Here are some additional references to help elucidate the primary question, which is whether or not it is possible for interbrain synchrony to be mediated purely by shared stimuli.
Among the indirect channels of sensory stimulus, various forms of touch, visual, and auditory stimulus have each been discovered to have strong correlations to b2b synchrony. However, the fact that they are not exclusive channels raises the question of how/whether they have the bandwith to carry the necessary amount of information to produce the observed synchronization patterns:
- link Synchrony is enhanced by touch (though does not require it).
- link Synchrony is associated with nonverbal positive affect (gaze, smiling) even in no-speech conditions.
- link Egyption fruit bats exhibit synchrony in a dark room. (Synchrony was not strongly correlated with sound, but hearing was the only sense generally present/available.) This study has a novel methodology and several notable results, including the fact that rises and falls in neural synchrony were predictive of changes in social activity -- in other words, neural changes occured before behavioural ones.
Conversely, hypotheses not mediated by sensory stimuli rely on electro- or magneto-reception. Humans are not known to possess specific electroreceptors, but they do appear to possess the best known magnetic candidates (cryptochromes and magnetite). However, explanations based on electro- or magneto-reception do not seem fully satisfactory on account of the weakness of human-generated electromagnetic fields and their rapid attenuation with distance.
Some authors do go so far as to say that the observed patterns of brain-to-brain synchrony are too improbable to be explained by intermediary sensory stimuli; to quote, interaction between "neural oscillators from two autonomous human brains".
Unfortunately, I lack the background in mathematics, physics and biology to determine which of the hypotheses is more improbable. Is the increased intensity of interbrain synchronization correlating to various control conditions (e.g. touch vs no touch, nonverbal affect with no speech, belief in playing with humans vs computers) really too mathematically improbable to be explained by biological factors like oxytocin (associated with affective touch) or increased cognitive attention? Or are the electromagnetic fields generated by human neural activity really too weak to be detected by biological electro-/magneto-receptors, at distances of a few meters?