"synaesthetes who experience colour show enhanced perceptual processing of colour compared to non-synaesthetes. This study aimed to investigate whether enhanced perceptual processing was a core property of synaesthesia by contrasting tactile and colour sensitivity in synaesthetes who experience either colour, touch, or both touch and colour as evoked sensations. "

https://www.ncbi.nlm.nih.gov/pubmed/19533108