>>So that is why I ask myself is it an illusion produced by chemical and neurological changes within me - or is it an external phenomenon that I can only perceive when I am in an altered state of consciousness?<<
I don't think anyone knows the true answer to that, Peter. For myself, all I can say is it certainly doesn't feel to be an illusion, but simultaneously, it's more real than "normality".
There are many esoteric writings which direct us to look within ourselves for all the answers and many religious doctrines pointing us to an external source. Who knows, do we need to look at this another way and stop thinking if one way is "right" the other way must be "wrong". Perhaps they are both right, maybe what we perceive at the ancient places is a change in ourselves <i>and</i> a change in something external which combines in that instant to create our experiences.