Figure 1. Examples of the stimuli used in the arithmetic task and Match task.

Five yellow digits, together with a green character as a cue, were presented for 5 s, followed by the presentation of one white digit (matching stimulus) for 3 s. For each example, the task-relevant digits and a brief outline of the calculation processes are shown in the right panels, where the digits circled in red are the digits stored in memory for digit-matching. The participants judged whether one of those stored digits appeared as a white digit; all white digits shown here are correct examples. (A) Under the Simple condition, indicated by the presentation of “±” as a cue, the addition and subtraction of the upper two digits (i.e., two of the five yellow digits) were mentally performed by the participants. We instructed the participants to always subtract a smaller value from a larger value. (B) Under the Linear condition, indicated by the presentation of “a_n” as a cue, the third term of a linear sequence initiated by the upper two digits was calculated by the participants. The first row at the top of the right panel represents the linear sequence, and the second row with branches represents the constant differences between each pair of adjacent terms in the linear sequence, as often used in math textbooks. (C) Under the Quad condition, indicated by the presentation of “b_n” as a cue, the following arithmetic calculations were performed by using the lower three digits (i.e., three of the five yellow digits). The first row in the right panel represents the given quadratic sequence. The second row represents the differences between each pair of adjacent terms in the given sequence. The participants were asked to regard the resultant second row as a linear sequence, whose third term was then calculated in the same manner as the Linear condition. (D) In the Match task, indicated by the presentation of “m” as a cue, the participants stored all of the five digits in memory.