fennel contains methyl chavicol (and things to help activate it) witch is the primary psychedelic compound in sweet basil oil.
i find especially when taken after certain inhibitor vitamins and oils both fennel and basil oil dramatically improve the overall quality of my vision in a long term manner not just during the experience.
during the vision becomes astoundingly clear and strong, afterword my vision is permanently better but not as strong as during, but it is cumulative so if you keep using it and letting it wear off you eventually get the astounding vision permanently after it wears off.
fennel, cardamom, anise, and aniseed are supposed to balance chakras 2 and 6.
6 is heavily tied to vision this makes sense.
the way it improves vision long term makes it feel like before trying it you had grit in your eye that you could not see but the irritation was hindering your vision, it takes less effort to exert visual concentration.