Sentence:  For one of the enigmatic features of <WSD>synaesthesia</WSD> is that, within a given cultural group, the kinds of associations made by different subjects occur according to statistically verifiable recurring patterns. As Jakobson explains, ‘when we ask whether /i/ or /u/ is darker, testing such phonic oppositions as grave vs. acute, some of the subjects may respond that this question makes no sense to them, but hardly one will respond that /i/ is the darker of the two'nb.... 
Meaning of the word marked with <WSD> tags:  The association of one sensory perception with, or description of it in terms of, a different perception that is not experienced at the same time.