Synesthesia, at first blush, seems to be something that would be very difficult to study, since it involves the subjective experience of the subject. If you claim to see certain numbers in color, I either have to take your word for it or declare you crazy. But actually there are some very clever tests that have been developed that provide some confirmation that synesthesia is a real effect. Number-color synesthetes can very rapidly identify a shape hidden in this image: http://mindbluff.com/syn3.gif while non-synesthetes must do a very slow manual search (the shape is hi-lighted here: http://mindbluff.com/syn4.gif).
The theory of cross-wiring as the neuronal basis of synesthesia very suggestive and there may be other instances of it in other domains. For example, on the human sensory homunculus, the representation for the feet and toes are located right beside the representation for the genitals. Some degree of cross-wiring in that region could explain foot fetishes. Likewise the sensory-cortical representation of the nipple is adjacent to the earlobe, and so on and so forth.
Interesting. I have number-color synesthesia but that test does nothing for me; I couldn't make out the image at all. For that test to work I would think that one would literally have to "see" the color of a number almost as though it were a hallucination.
I can only speak for myself obviously, but for me a number just invokes a certain color in my mind, almost as though the number has an aura. Two, for example, is blue-green, whereas five is just sort of black on white. But looking at a grey 2 on a background, I just see a grey 2 on a background... nothing more, unfortunately.
It was interesting nonetheless, just thought I'd share my own experience.
I suspect that there are varying degrees of synesthesia that range from the aura you describe to full-on hallucination. I have a friend that has an aural-color synesthesia who tells me that listening to music in the car (or for that matter having a conversation) is very difficult because the colors get in the way of her driving.
In any event, perhaps even in your case you could perform that test (or another like it) statistically significantly faster than persons without synesthesia.
For the bouba and kiki example, maybe the reason we associate shapes with sounds is because we already have words in our vocabulary that are associated with similar shapes?
For example, bouba is similar to bubble, which has no sharp edges. Kiki is similar to cut.
It'd be more interesting if you found similar associations in people who speak a completely different language with different origins.
If I were to graph myself speaking those two words I expect I would have two spikes on the k's of kiki, but bouba would be generally flatter.
We probably have abstractions, connotations, that these sounds share with the physical ideas of sharpness and soft-formed-ness (what word am I looking for). Then, we likely choose our words against these abstract connotations.
That's what I guess before I would look at your explanation as right.
Wow, I've never thought of this as synesthesia, or really given it much thought at all, but I think I exhibit the visual motion to sound symptom sometimes. It's rare, seemed to be more common during high school years (I'm 21 now). If it was really quiet and usually when I was relaxed and alone, it was almost as if the ambiance would get progressively louder. I'm not sure if it was associated with visual motion or repositioning my eyes, but I would also hear whooshing sounds. It seemed like I could also hear faint sounds louder, like typing or the creaking of a chair. At times it would actually get annoying, but it would go away when I went about and did something to take my mind off of it. Since the beginning of the year, I can only remember it happening once, for about half an hour .
I don't know if that's actually an example. While the perception of flavor is derived from both taste and smell, I don't think the two actually overlap. Although, you can certainly smell and taste something at the same time.
The theory of cross-wiring as the neuronal basis of synesthesia very suggestive and there may be other instances of it in other domains. For example, on the human sensory homunculus, the representation for the feet and toes are located right beside the representation for the genitals. Some degree of cross-wiring in that region could explain foot fetishes. Likewise the sensory-cortical representation of the nipple is adjacent to the earlobe, and so on and so forth.