Implicit and explicit spatial-numerical representations diverge in number-form synesthetes

•We tested spatial-numerical associations (SNAs) in number form (NF) synesthetes.•NF characteristics were evaluated using both qualitative and quantitative methods.•Surprising incongruencies emerged between explicit NFs and implicit tasks.•This suggests separate underlying mechanisms for NF synesthesia and SNAs.•Multiple factors are likely responsible for individual SNAs, even in synesthetes. In number-form (NF) synesthesia—a condition in which people report vivid, automatic and consistent mental layouts for numerical sequences—numbers and space are closely linked. These explicit associations are similar to the implicit associations demonstrated by the Spatial-Numerical Association of Response Codes (SNARC) effect. Thus, NF synesthesia offers a unique opportunity to investigate spatial-numerical associations. We tested implicit and explicit representations in NF synesthetes using a multiple case-study design. Over two sessions, synesthetes participated in a semi-structured interview focusing on the nature of their associations, as well as SNARC and number line estimation tasks. Contrary to our hypotheses, only one synesthete demonstrated SNARC effects congruent with her reported form, whereas two others exhibited SNARC effects that were the opposite of their explicit NFs. While this inconsistency between implicit and explicit representations may indicate separate underlying cognitive mechanisms, factors such as task-specific constraints and strategic variability must also be considered.