It is known they have a significant effect when consumed together with a weak stimulant, like caffeine or nicotine. Tobacco smoke is another significant source in the human diet.
Interesting. Do you have a source on this? I can't seem to find any detailed data on this. Only some suggestions that since they're lipophilic, and are found in higher concentrations in brain tissue, they might accumulate there. I can't find any research analyzing whether they do accumulate in brain tissue and how long they stay there.
It is known they have a significant effect when consumed together with a weak stimulant, like caffeine or nicotine. Tobacco smoke is another significant source in the human diet.