Thanks Rhody, if you meant to point to this definition from your link in post 74. I'll just address that quickly. It is different to any definition I’ve seen before, but still don't think it helps to clarify things as suggested in your points.
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Definition of "synesthesia"
Synesthesia is the general name for a related set (a "complex") of various cognitive states. Synesthesia may be divided into two general, somewhat overlapping types. The first, which I sometimes call "synesthesia proper", is as described above, in which stimuli to a sensory input will also trigger sensations in one or more other sensory modes. The second form of synesthesia, called "cognitive" or "category synesthesia", involves synesthetic additions to culture-bound cognitive categorizational systems. In simpler words, with this kind of synesthesia, certain sets of things which our individual cultures teach us to put together and categorize in some specific way – like letters, numbers, or people's names – also get some kind of sensory addition, such as a smell, color or flavor. The most common forms of cognitive synesthesia involve such things as colored written letter characters (graphemes), numbers, time units, and musical notes or keys. For example, the synesthete might see, about a foot or two before her, different colors for different spoken vowel and consonant sounds, or perceive numbers and letters, whether conceptualized or before her in print, as colored. A friend of mine, Deborah, always perceives the letter "a" as pink, "b" as blue, and "c" as green, no matter what color of ink they are printed with.”
The first type, according to this, of synaesthesia involves two or more senses. Does this include music, which may be culturally defined, or chords, too, for example, which could be cultural
http://www.newscientist.com/article...sal-music.html?DCMP=OTC-rss&nsref=online-news
(there is quite a bit about this, but this will do for now)?
http://www.wjh.harvard.edu/~mnkylab/publications/recent/McDermottHauserMusicOriginsMP.pdf
It excludes number graphemes, for example, because they are learned and cultural, but it may not be about the grapheme as what it represents in a possibly heritable ‘number line’, (Deheane).
http://books.google.co.uk/books?hl=...23YdQATlvAx2HMjb_J8SRGrS0#v=onepage&q&f=false
“Synesthesia is additive; that is, it adds to the initial (primary) sensory perception, rather than replacing one perceptual mode for another. With my colored musical timbres, I both hear and "see" the sounds; the visual images don't replace the audial sensations. Both sensory perceptions may thus become affected and altered in the ways they function and integrate with other senses. Synesthesia is generally "one-way"; that is, for example, for a given synesthete, tastes may produce synesthetic sounds, but sounds will not produce synesthetic tastes. However, there have been a few rare cases of synesthetes who have had "bi-directional" synesthesia, in which, for example, music induces (synesthetic) colors and seeing colors induces (synesthetic) sounds – the correspondences, however, may not be the same in both directions!”
Fechner’s illusion, for example,
http://dogfeathers.com/java/fechner.html
is additive.
Synaesthesia may be ‘bi-directional”, for example,
http://eprints.ucl.ac.uk/2934/1/2934.pdf
(edit: I had accidentally said "may not be "bi-directional")
“Regarding synesthesia “proper”, stimuli to one sense, such as smell, are involuntarily simultaneously perceived as if by one or more other senses, such as sight or/and hearing. For example, I myself have three types of synesthesiae: The sounds of musical instruments will sometimes make me see certain colors, about a yard in front of me, each color specific and consistent with the particular instrument playing; a piano, for example, produces a sky-blue cloud in front of me, and a tenor saxophone produces an image of electric purple neon lights. I also have had colored taste and smell sensations; for example, the taste of espresso coffee can make me see a pool of dark green oily fluid about two feet away from me.
The word "synesthesia" comes directly from the Greek (syn-) "union", and (aísthesis) "sensation", thus meaning something akin to "a union of the senses". "Synaesthesia" is the British English spelling of the word; in American English, it is often spelled "synesthesia", without the "a". The concept appears in other European languages, too: In Danish it is synæstesi . The Dutch word is synesthesie . In Finnish, synestesia. In French, it's synesthésie , one type of which is audition colorée , "colored hearing". In German, it's Synästhesie , and colored hearing is Farbenhören. In Italian, sinestetici ; in Polish, synestezja; in Russian, ( sinestezia ); in Swedish, it's synestesi.”
The name suggests the combination of two senses, yet the first paragraph includes both sensory and cultural combinations, and as it states this only applicable to one type.
“Synesthesia has definite neurological components and is apparently partially heritable, one component perhaps passed down genetically on X-chromosomes. The percentage of the general human population which has synesthesia varies with the type involved; estimates run from 4 in 100 for basic types of cognitive synesthesia (colored letters or musical pitches), to 1 in 3,000 for more common forms of synesthesia proper (colored musical sounds or colored taste sensations), to 1 in 15,000 or more for people with rare (such as one synesthete I know of who synesthetically tastes things she touches) or multiple forms of synesthesia proper. Perhaps more than half of all humans have a basic form of synesthesia in which they consider "higher" sounds to be "brighter" and "lower" sounds to be "darker".”
The “definite neurological components” may not include abnormal neuronal connections. (http://docs.google.com/viewer?a=v&q=...EZbn9oSbT62Dyw )
I think the mention of x-chromosomes may be based on the idea from old data that this occurs more often in women, which has since been discounted. Newer information involves a survey held at the Science Museum in London, which I’ll try to find. (Haven't yet, but is backed by Simner 2006.)
The last sentence refers to understanding metaphors, which I believe is a human capacity.
