Figure 1: Abacus addition procedure and design of the experimental tasks. (a) An addition example on the abacus (). The left abacus schematic represents the number 9 (one heaven bead equals to 5 and 4 earth beads equal to 4). The middle abacus schematic represents the addition procedure: subtract the complement of the addend to 10 (2 here) by pushing down the 2 yellow beads (near the blue arrow) with the index finger, then add 1 to the tens column by pushing up the yellow bead (near the red arrow) with the thumb. The right abacus schematic represents the result. (b) The task blocks that were used during data acquisition. The blocks were separated by a resting period of 16 s. Each block started with a cue of 4 s to remind the subject of the type of tasks and followed by 12 continuous addition problems. (c) The task design that is used during data acquisition. On each trial, an addition problem is presented for 400 ms and then followed by two alternative answers (also presented for 400 ms). Subjects are instructed to choose either the correct answer (EX task) or the most plausible answer (AP task) as soon as possible.