In streams of light I clearly saw The dust you seldom see, Out of which the Nameless makes A Name for one like me. . . All busy in the sunlight The flecks did float and dance, And I was tumbled up with them In formless circumstance.
With any hallucinations, if you can do functional brain imagery while theyre going on, you will find that the parts of the brain usually involved in seeing or hearing - in perception - have become super active by themselves. And this is an autonomous activity; this does not happen with imagination.