Embodied magnitude processing: On the relation between the SNARC effect and perceived reachability

Magnitude information, for instance, regarding weight, distance, or velocity, is crucial for planning goal-directed interactions. Accordingly, magnitude information, including numerical magnitude, can affect actions: Responses to small numbers are faster with the left hand than the right and vice versa (hand-based SNARC effect). Previous experiments found an influence of effector placements on the SNARC effect but also an influence of the mere distance between effectors and numbers. This indicates a sensorimotor grounding of space-number processing. In the current study, we investigated this grounding by probing the SNARC effect close to and far from the hands. We used a magnitude comparison task with a fixed standard of 5 (smaller numbers 1, 2, 3, 4; larger numbers 6, 7, 8, 9) and a sagittal response arrangement to measure hand-based and sagittal SNARC effects for digits presented at different sagittal distances to the hands, i.e., in peripersonal and extrapersonal space. A significant sagittal SNARC effect was found, with the largest effect size in extrapersonal space. Meanwhile, the hand-based SNARC effect appeared only descriptively, with the largest effect size between the hands, i.e., in peripersonal space. Additionally, a purely spatial congruency effect surfaced, prioritizing responses with the hand closer to the number. Together, these results emphasize that responses in simple decision-making tasks can be influenced interactively by a multitude of task-relevant axes and relative spatial locations, including effector placement and stimulus placement, as well as number magnitude.