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>> Have you ever hooked up any sort of bio-feedback mechanism while meditating?
I haven’t, personally. There’s lots of peer-reviewed research on EEG changes during normal waking state and meditative states between meditators and non-meditators, as far as I know. And there’s a cottage industry of brainwave feedback devices, too, I think, not to mention EEG video games.
The youtube video below provokes lots and lots of vitriol, but it’s fun to watch, and I generally vouch for it’s authenticity as well as the spoken content (though I would use different language, probably). (This does not imply vouching for the rest of Ken Wilber’s oeuvre.)
I can’t do anything like what’s in the video.
>> Can these states be trained with bio-feedback I wonder?
Certainly. Also, you don’t actually *need* biofeedback as long as you have a clear idea of what you’re looking for phenomenologically. When you catch a hint of it, you use that as your feedback. Lots of meditative traditions put strong emphasis on endogenous state generation and maintenance and some emphasize bringing those states into daily life.
Strategically, philosophically, and aesthetically, I prefer the traditions that emphasize carrying qualities and intentions across any state you find yourself in, because it’s hard and often counterproductive to try and police what’s going on inside you. This is all semantically fuzzy though, and there’s lots of overlap to all meditative traditions.