Task demands determine hand posture bias on conflict processing in a Simon task.

A huge body of research in humans and monkeys has provided evidence for altered processing of items that are presented close to the hands. At the same time, the underlying mechanisms that explain why objects close to the hands are processed differently from objects far from the hands are still debated. Empirical demonstrations have provided evidence for the involvement of bottom-up influences, but also for top-down influences of task relevance. Objects close to the hands change spatial attentional processing or are subject to increased cognitive control. The present study demonstrated that variations in the task-processing demands predicted the hand posture influence on conflict resolution in a Simon task. Participants responded with their hands either at the monitor (close to the stimuli) or on their knees (far from the stimuli). The Simon effect was significantly reduced for the hands-close as compared to the hands-far condition when participants performed a numerical size judgment (Exps. 1 and 2). In contrast, the Simon effect was significantly increased for the hands-close condition when the Simon task consisted of a low-level perceptual feature discrimination (i.e., color task, Exp. 2). The obtained task-processing specificity provides further evidence that a highly flexible system underlies hand posture effects on stimulus processing.