It is mostly told in flashback. The narrator in the asylum only has a mushroom look to prove it. No other proof--none we can see. Was he chemically changed into something similar to a mushroom --something could the scientists see in a blood test??

Another point. What is the meaning of some of this film? When one guy eats mushrooms he daydreams of beautiful showgirls back in Tokyo. Is this film for or against western ideals? How did the narrator know the other guy daydreamed of showgirls?