I actually have this condition. My mother is a schizophrenic, (modern drug therapy has given her a normal life)and I was diagnosed with schizophrenia as a child, and then later as an adult, both in and out of the military. The condition is a poorly understood one, and often misdiagnosed. In any event, synaesthesia is a common manifestaiion of certain aspects of my particular mental makeup, and was, in fact, formally diagnosed after quite a few visits to a variety of so called 'experts'.
I can tell you that it is not all that similar to the sense crossing that occurs on most psychedelics, with perhaps the exception of the audio-visual elements of a high dose LSD trip. However, there is an element of the condition that is VERY difficult to describe, that being the contextual crossing of sensory data input. Sensory events can actually be transcribed internally as IDEAS, or EMOTIONS, or MEMORIES, which are totally unrelated to the actual sensory event. It can be something as inocuous as leaning on a counter in a certain way, and for that instant feeling as though you are a writer living in Pennsylvania when in fact you are an actor living in LA.
Also, the crossing seems to be less obvious than people might imagine. I have seen traffic lights in colors that are not actually colors, but feelings or memories. I am not talking about the light REMINDING me of something, I am talking about the light actually being the memory itself.
Its very very difficult to describe.
I am ambidextrous, btw, and I have an excellent sense of direction. So I guess I am not the typical, synaesesthiac.
I can tell you that my drug experience has helped me greatly. I am able to ignore outright hallucinations, and I am able to recognise them more easily, as well as recognise any sensory crossing that occurs, with the exception of some occaisional contextual issues.
I have studied my internal syntax extensively, and I believe that all information is language, and vice versa, and that the more incomplete the language is, the more occluded, necessarily , we are. This has prompted me to search for alternate syntax, and the quickest way to discover that is with drugs.
An interesting concept, which was chronicled by P.K. Dick, but which I independently considered as well, is the concept of 'living information', which is immediately and spontaneously illuminating.
In short, information can be thought of as a type of organism, which requires a 'host', or brain, in which it resides in its dormant form, growing, eventually replicating itself in either the spoken word, or a drawing, and reaching its final form in becoming an artifact, or physical object, which is the ultimate form of language, or information. Of course, the relationships of each piece of matter, be it a car or an atom, can also be thought of as syntax, and in fact is necessarily just that.
The whole line of thought has led me to search for a complete language, which, by definition, cannot exist inside one's head, but there might, perhaps, exist a more accurate translation of life.
In my opinion, chemistry is the finest order of language currently available for practical interface.
Anyway, I am rambling again, another trait. Sorry.