When I tell people that I have synesthesia, they either give me blank looks or else they get a little excited, imagining I live in a heightened psychedelic state; the reality is more mundane (though I occasionally have "heightened experiences" that while dramatic, I would actually prefer to avoid). It's quite hard for me to describe it, but I'll try here to give some idea of my experience and what I know about it.
Wikipedia offers an obtuse definition of synesthesia: "a neurological phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway." ...Huh? I doubt anyone reading that would know that synesthesia is more commonly defined as "seeing sounds" or "hearing colors." But then again, I don't exactly "see scents." It's not like I see a kaleidoscope pulsing in the air when I smell a rose. It's simply that scent "has color."
The closest description of synesthesia I've found to my own experience is from Lucy Rauberta, who described it as: "a form of perception that crosses over taste, sound, smell, touch, sight and very likely memories and words, receiving different senses simultaneously." In day to day experience, this isn't so fantastical as it sounds and manifests in a rather ordinary way. Ordinary sensory experience, however, is actually rather fantastical but we're so busy intellectualizing and categorizing it that we miss how slippery, subtle and beautiful it really is. When we take time to pause and give all our attention to senses for a sustained time (say, in meditation) we learn that our senses are a continuum of vibrations. We call one part of that continuum seeing, another part hearing—at some point the vibration becomes coarser and so we call it touching, etc.
I used to think that everyone experienced the color that to me is inherent in scent. In fact, I used to say that I didn't have synesthesia, because like many people, I imagined a big, continuous psychedelic experience. So I would continue to tell people how clary sage was chartreuse with little violet textures—thinking it was sort of obvious. And they would say, "do you have synesthesia?" and I'd say, "no." But then after one particularly "dramatic experience" that was followed by a visit to a neurologist I learned that synesthesia describes a spectrum of experiences which can be quite different from individual to individual. What they have in common is this sensory "crossing-over" thing.
Apparently, the most common type of synesthesia is "grapheme-color synesthesia" where numbers and letters are associated with colors—so the letter S might be light yellow. The actual colors are idiosyncratic: people with grapheme synesthesia don't all see S as light yellow. In the same way, people with sound-color synesthesia don't all see the same notes in the same colors.
I don't have grapheme-synesthesia at all—in fact, the notion seems a little odd to me. But with some effort, I think I can imagine it: the letter S doesn't radiate pink rays of light, pink is simply a quality inherent in the experience of the letter S.
So what does that actually feel like? Sensory experience is very difficult to describe if people don't already know what you're talking about (think for a moment how you might "explain" the color red to a friend who is blind). Our words are verbal pointers that point to experiences we already agree on. For that reason, it's not really possible to describe or explain the experience of synesthesia if you don't already know it. Still, since people keep asking me to, I'm going to try with an analogy about lemons.
Consider how yellow is a quality of the experience of lemons. It's very natural to know that an experience of lemons includes yellow—but not exclusively, since your actual experience of a lemon is multi-sensory. When you experience a lemon, you don't isolate a single aspect. The tactile quality of the skin, the scent, the color, the sharp, sour taste, all of these are mixed together. The yellow is not wiping out the rest of your experience with its yellow-ness. And so you know the lemon to be yellow in a very ordinary way. In your actual experience, there is one, undivided, ever shifting, smelling-vibrating-seeing-touching-thinking-tasting experience that is the experience of a lemon—and that experience includes yellow-ness.
I'm not sure that describes the experience of synesthesia very well, but maybe it demystifies it a little. For me, scent "has color" in the same way. Complex scents like rose oil or jasmine absolute, have several colors and those colors change and shift (the images above are from my own studies of rose oil). One of my favorite composers, Olivier Messaien had sound-color synesthesia and often made color notations in his musical scores. He described the shifting of his experience like this:
"I too see colors—if only in my mind—colors corresponding to sound. I try to incorporate this in my work, to pass on to the listener. It's all very mobile. You've got to feel sound moving. Sounds are high, low, fast, slow etc. My colors do the same thing, they move in the same way. Like rainbows shifting from one hue to the next. It's very fleeting and impossible to fix in any absolute way.... It's true I see colors, it's true they're there. They're musician’s colors, not to be confused with painter's colors. They're colors that go with music. If you tried to reproduce these colors on canvas it may produce something horrible. They're not made for that, they're musicians colors. What I'm saying is strange but it's true."
In my own work, I'm not seeking to recreate the experience of synesthesia for someone else—which is basically impossible. Instead, I'm using synesthesia as a point of departure to expand the space of painting and of perfume—to show that in fact, these two arts aren't quite so different from each other as they seem.