Stimulus-hand proximity shapes conflict processing by task-dependent modulation of distractor activation

Placing the hands near stimuli modulates interference effects in conflict tasks. Liepelt and Fischer (2016) found that the direction of this near-hand effect depends on task demands: In a complex number-categorization Simon task, conflict was reduced when visual stimuli appeared near the hand, whereas in a simpler color-discrimination Simon task, conflict increased. They attributed this result pattern to a flexible attentional weighting of bottom-up vs. top-down processing. The present study specified and tested this hypothesis by applying the Diffusion Model for Conflict (DMC) to their data. The DMC assumes two parallel evidence accumulation processes: a transient distractor-based and a constant target-based diffusion process, which combine additively to drive response selection. Across two experiments, model fits revealed that stimulus-hand proximity selectively modulated the strength of distractor-based activation, while other parameters remained relatively stable across hand conditions. Crucially, this modulation mirrored the behavioral pattern: greater distractor activation near the hands in the color task producing increased conflict, but reduced activation near the hands in the number task leading to reduced conflict. These findings suggest that conflict is shaped by stimulus-hand proximity through selective adjustment of distractor-based processing, not target-based processing: location signals are typically amplified near the hands but down-regulated under higher cognitive demands. The results further illustrate how embodied manipulations can selectively impact specific processing components as mapped within a formal decision-making model.