They aren't relying strictly on subjective reports, though. They are also doing eye tracking. It'd be more solid if it was backed up by MRI or EEG or some other

They aren't relying strictly on subjective reports, though. They are also doing eye tracking. It'd be more solid if it was backed up by MRI or EEG or some other kind of brain function measurement, but that's something, at least. I've experienced this effect numerous times (for my whole body, not just my hands), and I always figured it was due to proprioception cross-talking with the geometry-extracting part of visual processing. The result that synaesthetes report more vivid images seems to support something like that. Definitely worth doing neurological studies on to see exactly how it works, but I never realized it was the sort of thing that anybody would be surprised by.