Number, size, and space associated in a common system by distinct mechanisms.

The spatial numerical association of response codes (SNARC) effect demonstrates that people respond faster to small numbers with their left hand and faster to large numbers with their right hand. The size congruity effect (SCE) refers to the fact that congruent trials between numerical values and physical sizes are faster than incongruent trials. Previous studies have found that the SNARC effect and SCE are independent when magnitudes or sizes are processed explicitly. This study aimed to explore whether number, size, and space are common and distinct mechanisms using an implicit parity judgment task. The results showed that the SNARC effect, SCE, and SNARC-like effect all co-existed. Furthermore, there was a significant interaction between the SNARC effect and SCE, in which the SNARC effect in the SCE-congruent condition was larger than in the SCE-incongruent condition, whereas SCE merely emerged in the SNARC-compatible trials. However, participants responded to small numbers in large size faster than to large numbers in small size with the left hand in SCE-incongruent trials, which reflected that number-space mapping (SNARC effect) was stronger than size-space mapping (SNARC-like effect). These findings provide new evidence for A Theory of Magnitude (ATOM), which suggests that number, size, and space are associated with a common generalized magnitude system through distinct mechanisms.