When adding is right: Temporal order judgements reveal spatial attention shifts during two-digit mental arithmetic

Recent research suggests that addition and subtraction induce horizontal shifts of attention. Previous studies used single-digit (1d) problems or verification paradigms that lend themselves to alternative solution strategies beyond mental arithmetic. To measure spatial attention during the active production of solutions to complex two-digit arithmetic problems (2d) without manual motor involvement, we used a temporal order judgement (TOJ) paradigm in which two lateralised targets were sequentially presented on screen with a varying stimulus onset asynchrony (SOA). Participants verbally indicated which target appeared first. By varying the delay between the arithmetic problem presentation and the TOJ task, we investigated how arithmetically induced attention shifts develop over time (Experiment 1, n = 31 and Experiment 2, n = 58). In Experiment 2, we additionally varied the carry property of the arithmetic task to examine how task difficulty modulates the effects. In the arithmetic task, participants were first presented with the arithmetic problem via headphones and performed the TOJ task after the delay before responding to the arithmetic task. To account for spontaneous attentional biases, a baseline TOJ was run without arithmetic processing. Both experiments revealed that addition induces shifts of spatial attention to the right suggesting that visuospatial attention mechanisms are recruited during complex arithmetic. We observed no difference in spatial attention between the carry and noncarry condition (Experiment 2). No shifts were observed for subtraction problems. No common and conclusive influence of delay was observed across experiments. Qualitative differences between addition and subtraction and the role of task difficulty are discussed.