One of the books I am reading for a class claims there is evidence that, as the title says, psychedelics (specifically mushrooms, more specifically amanita muscaria) were used by Europeans until 1100-1300. The author cites what he claims is plentiful mushroom iconography throughout chapels and other art during this period.

Is there any truth to this, or at least to the idea that Europeans of this period had some knowledge/cultural memory of such a practice?

EDIT: The particular book in question is The Psychedelic Gospels by Jerry Brown.