The influence of distractors and numerical direction on mental number line bisection

Spatial relationships are used to mentally represent numerical information. We visualize small numbers on the left and large numbers on the right, forming a mental number line. When determining the midpoint between two numbers, healthy participants show a bias to the left, similar to the bias on line bisection. Upper space distractors increase leftward line bisection biases and the current study examined the influence of distractors on mental number line bisection. Participants viewed sequential number triplets, presented concurrently with distractors. In Experiment 1, participants performed more accurately when numbers were presented in ascending order and showed stronger left biases for descending trials. Further, a responding bias was observed, where participants pressed the response key on the same side as the distractors. In Experiment 2, an orthogonal response mapping was used, which confirmed the occurrence of a responding bias. Interestingly, distractors weakened pseudoneglect, demonstrating a dissociation between line bisection and mental number line bisection in terms of distractibility. Numerical direction influenced asymmetries, such that leftward biases were stronger on descending trials. It is suggested that a right-to-left orientation of attention on descending trials might lead participants to overestimate how far along the number line they have moved, which increases leftward biases.