I wonder how research take into consideration all the human senses outside the usual five suspects. We have way more senses than the ones teached in school, intuitively this should lead to a much more widespread synaesthetic condition in our species, than what is already observed.

People don't give proprioception enough credit until they end up losing it. Proprioception is the sense of where in the world our bodies are.

I hold the belief that proprioception is more than a sense, but rather, part of what makes us conscious as it requires a sense of 'I'. Toddlers are able to identify themselves in mirrors, and it is usually preceded by movements, and identifying that "the mirrored human performs the same movements as I do", which requires some form of proprioception.

Oliver Sacks talks a lot about other senses beyond those covered in school. I can recommend "The man who mistook his wife for a hat", "Hallucinations", and "Awakenings".

TL;DR: Synesthesia