Fehmi Alpha Synchrony neurofeedback
In the past I've done a bit of work with Les Fehmi's Open Focus™ (book) synchrony training technology (synchrony paper). This is doing the synchrony in conjunction with the OpenBCI hardware. This is a five channel alpha synchrony protocol that is used with Fehmi's nondual oriented guided meditations. Les first developed Open Focus meditations and neurofeedback in the late 1970's. And I believe his approach may be under-appreciated relative to later nondual 'schools' and attention training approaches.
The idea in this iteration of the BrainBay protocol was for the neurofeedback practitioner to very gently (but not obsessively) monitor the realtime spectrograms of all 5 channels (Fpz Cz Oz T3 T4), and adjust the weighting factors so that synchrony comes up evenly in all quadrants. That is, to use the reward thresholds to ask the weak quadrants to proportionately raise their synchronous alpha amplitude, so they "join the conversation".
If you're curious how this relates to the neuroscience of holistic or integrative perspectives, see this paper from 2012: Alpha-Band Phase Synchrony Is Related to Activity in the Fronto-Parietal Adaptive Control Network. Synchronized alpha appears to increase communications pathways between frontals and attention / sensory networks in the parietals. In other words, new forms of "attention" and perspective. Exactly what Fehmi has been getting at for 30+ years.