http://synesthete.org/

Personally, I would recommend synesthetes to contact synesthesia research centers at near universities. There are many forms of synesthesia out there (combinations of color, emotions, sequences, spatial locations, graphemes, sensations, sounds, etc.), and usually the researchers need new participants for their experiments. It can be an interesting experience and you’ll be contributing to science.

http://michaelgr.com/2007/09/14/my-experience-with-synesthesia/

I even used pictures of the Rama test.

Likewise, in the past I was always accused of lying when I said my 3’s were red. I didn’t know that others didn’t see the world this way, or that there was a name for the “condition,” until about November of 1999 when Discover Magazine published a popular article on the subject.

I’ve heard that synaesthesia is not all that uncommon, but I’ve never met another synaesthete to my knowledge. Admittedly it’s not something that comes up in conversation due to the oddity and the potential embarrassment factor.

I can address a few points from my point of view:

1. Yes, I see mathematical expositions in a flurry of colors.

2. For color-grapheme associations, context does matter, to a certain extent. For example, the color of a character will change depending on whether I think it’s a number 5 or a letter S. I can force a character to “change” colors.

2a. For some reason this doesn’t apply to Roman numerals, which for me always retain the colors associated with their “alphabetness.”

3. Despite point 2, I have noticed that I can sometimes read highway signs from a distance based on the colors I see even if the characters are a bit too blurry for me to consciously make them out. But perception of perception is fuzzy — my brain must be recognizing the graphemes somehow even if I don’t.

4. I occasionally read things written in non-Roman alphabets. The colors of the characters in these non-Roman alphabets are frequently colored by their corresponding sound, and thus letter, in the Roman alphabet. Amusingly, these non-Roman characters may appear “two-toned” if they carry one sound/color in the corresponding Roman character but also physically resemble another of the non-Roman characters that carries a different sound/color.

5. The triangle of 2’s doesn’t pop out immediately for me, although certainly more quickly than the 20-second average alleged for the non-synaesthetes. I have to recognize the 2’s first.

I would like to know whether color-grapheme synaesthesia in particular is more common among people who learned to read comparatively early. (This would probably have to be disentangled somehow from the tendency of people with higher-than-average IQs to learn to read early; I recall hearing that higher-than-average IQ is associated with synaesthesia, as is autism.) My understanding is that infants don’t prune away these neurological cross-modal associations until age 2 or 3.

I laid out my colours a few months ago:
http://www.booberfish.com/blog/2007/01/everything-has-a-colour/

http://www.sitemaker.umich.edu/satran/files/atran_et_al_science_mag_240807.pdf

More relevant, perhaps, is the PNAS article:

http://www.sitemaker.umich.edu/satran/files/pnas_sacred_bounds.pdf

http://en.wikipedia.org/wiki/Daniel_Tammet

It’s thought that his savant abilities are facilitated by the associations synaesthesia provides. When I first heard about Tammet, I immediately thought of the Feynman quote in #8, and wondered if he also benefitted from this quirk of neurology.