The ups and downs (and lefts and rights) of synaesthetic number forms: Validation from spatial cueing and SNARC-type tasks

Typically, numbers are spatially represented using a mental ‘number line’ running from left to right. Individuals with number-form synaesthesia experience numbers as occupying specific spatial coordinates that are much more complex than a typical number line. Two synaesthetes (L and B) describe experiencing the numbers 1 through 10 running vertically from bottom to top, 10–20 horizontally from left to right, 21–40 from right to left, etc. We investigated whether their number forms could bias their spatial attention using a cueing paradigm and a SNARC-type task. In both experiments, the synaesthetesʹ responses confirmed their synaesthetic number forms. When making odd–even judgments for the numbers 1, 2, 8, and 9, they showed SNARC-compatibility effects for up–down movements (aligned with their number form), but not left–right (misaligned) movements. We conceptually replicated these biases using a spatial cueing paradigm. Both synaesthetes showed significantly faster response times to detect targets on the bottom of the display if preceded by a low number (1, 2), and the top of the display if preceded by a high number (8, 9), whereas they showed no cueing effects when targets appeared on the left or right (misaligned with their number forms). They were however reliably faster to detect left targets following the presentation of numbers 10 and 11, and right targets following numbers 19 and 20 (since 10–20 runs from left to right). In sum, cueing and SNARC tasks can be used to empirically verify synaesthetic number forms, and show that numbers can direct spatial attention to these idiosyncratic locations.