I have grapheme-color synesthesia.
Something I would like to clear up is that many of you might expect natural synesthesia to be similar to drug-induced synesthesia. It is not really like the sort of synesthesia you would experience on a psychedelic much at all.
With psychedelic synesthesia, you get an actual illusory sense that somehow connects to another sense. Natural synesthesia is not at all a hallucination/visual in the way you would experience with a psychedelic. With natural synesthesia, there is a very clear distinction between your actual senses and your synesthetic sense. Because of this, it is never distracting or annoying at all, at least for me.
What I mean by this is, despite my synesthesia, when I look at this post I am writing, I literally see every letter as black, exactly as anyone else would. Synesthesia in no way alters or changes my normal sensory perception. However, kind of in parallel with my normal sight, there is an additional, perceived color for each letter/digit.
This is not the same as an association. It's not how a non-synesthete would associate "orange" with the color orange. In that case, there is some thought involved, some sort of mental connection to be made. For me, my synesthetic colors are perceived instantly, immediately, just as readily as any other normal sense perceives. I do not have to call up anything from memory, or think about it. It's just always, involuntarily there.
Serotonergic psychedelics sort of increase my synesthesia. They don't just `turn up the volume' though, they actually change the subjective quality of the synesthesia. All the synesthetic colors remain the same, but the synesthetic sense begins to blend a bit with my actual, real vision. This was most obvious when on 2C-E, when looking at some text I would literally see the synesthetic colors seep into my 'normal' sensory channels. It changed my synesthesia into a true visual effect.
If anyone else has questions about synesthesia, I would love to answer them!