I have no idea if someone made a research about how many members of the contemporary hunter-gatherer societies hear "voices" and see "spirits". But it seems to be a standard trope.
The part about koans is just my idea; it's not from the book. Actually, I later realized it could easily be the other way round. High stress induces bicameral thinking, and giving someone an unsolvable puzzle and saying his future incarnations depend on it could be quite stressful. The non-dominant hemisphere is supposed to be the one that matches patterns, so it could just as well be an exercise to activate it. And the "enlightement" could mean activating the inner voice. (In other words, it could be a culturally different way to achieve what charismatic Christians are achieving by "speaking in tongues".) Well, if you can easily argue either way...
hear "voices" and see "spirits"
The thing is, it's not limited to primitive tribes. Try a Pentecostal church in a XXI century first-world country :-)
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.