"It was once assumed that synesthetic experiences were entirely different from
synesthete to synesthete, but recent research has shown that there are
underlying similarities that can be observed when large numbers of synesthetes
are examined together. For example, sound-color synesthetes, as a group, tend to
see lighter colors for higher sounds and grapheme-color synesthetes, as a
group, share significant preferences for the color of each letter (e.g., A tends
to be red; O tends to be white or black; S tends to be yellow etc."
The phenomenon of synesthesia isn't that odd if you know that our experienced reality is really the brain's interpretation of a field of pure information made of vibration. Our senses don't perceive different things, but the same "thing" differently interpreted.
Ceremonial magick techniques make use of this by employing artificial synesthetic stimulation. Equivalent images, colours, shapes, numbers, scents and sounds are used to focus consciousness on a specific vibration. Advertising and propaganda use these also. So does Fung Shiu.
As far as I can tell, synesthesia research has only examined the relationship between "physical" senses. I think it also includes much of what we call extrasensory perception. Abilities like clairaudience, psychometry and clairvoyance all manifest through physical senses. Many people are able to see auras and yet the aura is a non-physical energy field. I started wondering about this because when I look at photos or video of certain people (people like Blair, Cheney, Rumsfeld, Rice, Kissenger, and most other serial killers) they appear so obviously wrong. Their faces are cruel and insanity glitters behind their eyes. I couldn't stop asking myself how anyone could trust them. The more I thought about it, the more convinced I became that I couldn't be seeing the same thing they're seeing. How common is this? I think it is very common, but most of us take our subtle perceptions for granted if they've always been there.
Some other common examples of subtle/physical synethesia:
Auditory- When listening to a piece of music played with emotion, it isn't unusual to "hear" what the artist was feeling. How can one hear an emotion?
Olfactory- It is a well known fact that certain scents produce emotional responses. It's partly associative but not entirely, since some smells have consistent effects. When I am asked how I knew that my partner was the "right one", smell was the deciding factor. That might sound riddiculous but it's true. It isn't just that he smells glorious. I've smelled glorious before. He smells right, and I can't explain it better than that. I can't even describe the smell itself, since it's not like anything else. I left my previous lover because his smell changed. About a year into the relationship, he started smelling like creamed corn. I'm not a picky eater and I could count the things I won't eat on the fingers of one hand. Creamed corn is one of them. I felt bad about having to do that because he is a lovely person and he didn't understand at all. Honestly, I don't know if this is a subtle/physical cross-over or simple hypersensitivity or both combined.
Tactile- This one is often experienced as "the sense of being stared at". The etheric body responds to touch even when there is no physical contact. Healing touch and some martial arts utilise non-physical touch.
All this would suggest that the distinction between physical senses lies in the mode of perceiving rather that the stimulus itself. Also the line between sensory and extrasensory is more conventional than real. It certainly makes me wonder how different the "worlds" we individually inhabit really are.