These posters are straight from the mind, the movement of thought, emotions, music. Which is why they are such amazing ads for music, they're visual music.
Unfortunately, I don't know much about the individual artists who made them. I've looked it up and there's no specific one whom I admired most, it's the general style I find inspiring. Jefferson Airplane seems to have the best/most posters in this style.
Also, I separate these posters from the 60's/70's blacklight style which will probably be another post, and again, Jefferson Airplane blacklight posters really stepped up and pushed boundries. But more about that later! :)