@joshmillard My guess is it's (for me) something to do with memory function. I used to get very strong déjà vu and a sort of jamais vu like dissociative experience where I couldn't experience the connections between moments in time (while still understanding them conceptually). Synaesthetic impressions have the same quality to them: a sense of "snagging" the same part of my mind. I wonder if dyspraxia (with poor working but excellent long-term memory) makes me more prone to it.