it is equally as plausible as it being the souls of the dead who left something behind on earth. it requires COSMICALLY lucky strings of coincidences and data and yes it requires you to accept that most people slip in and out of relative insanity on a regular basis in a way that is far more than color or shape hallucinations.
it's nice to set up a thought experiments where every factor coincides with what you already assume to be true of the brain but, as the example i already gave, there are instances that just shatter those thought experiments. "well you slip in psychosis" isn't really a good enough hypothesis.