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Synaesthesia is often triggered by numbers, although it is conceivable that different aspects of numerical representation are responsible for different variants of synaesthesia. For individuals with “higher synaesthesia” it is assumed that number meaning (or numerosity) is responsible for the elicitation of synaesthetic experiences. This study documents a case study of a synaesthete, TD, who broadly fits this profile. TD reports that the same colours are elicited from physically different representations of number (digits, dice patterns and finger counting) provided that they share the same numerosity. The authenticity of his synaesthesia is established using Stroop-like priming and interference paradigms. Not only does synaesthetic colour interfere with veridical colour judgements, but also veridical colours can interfere with numerosity judgments. This suggests a close bi-directional coupling between numerosity and …