I know this is slightly off topic but might be interesting to some. What guys describe reminds me of some of the things people experience while doing kasina meditation.

Creating hallucinations without drugs is surprisingly easy

Another way is dreaming. If you learn to blur the threshold between wakefulness and sleep by getting acquainted with the falling-asleep process you can evoke hypnagogic imagery rather consistently while "awake" and without relying on gimmicks.

And then there is this:

http://youtu.be/FYTeFJvY-oQ