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Hallucinogens would have been used. No question. But probably only by an elite group, who held knowledge of the deep unknown. That aid, I strongly suspect that shamanic trances would have been far heavily than we can perceive of today. They would have been literally death states, where the practioner would not have known whether they were going to come back from the experience, for in their minds they would have been "dead to the world". Indeed, there is every reason to believe that they would have believed they were "dead", the reason why they would have been able to access the afterlife in a similar manner to the soul of a deceased person.
Yet th shaman was able to come back from the "dead", enabling them to share their experiences with those belonging to the land of the living.