On a mailing list for synaesthetes ages ago, I remember some discussion about autistic people sometimes having more perceptual synaesthesia than others at different times, and this having to do with overload. Also, some kinds of seizures can produce it, and some drugs can either reduce, create, or change it. None of those are considered classic synaesthesia, but then the guy who came up with the criteria for synaesthesia was studying people outside of those situations.
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"In my world it's a place of patterns and feel. In my world it's a haven for what is real. It's my world, nobody can steal it, but people like me, we live in the shadows." -Donna Williams