Simon Effect and Conflict Adaptation in Unimanual Finger Press and Bimanual Tool-Mediated Responses.

Two-handed tool use has long supported problem-solving in everyday life, yet its role in cognitive control remains poorly understood. This study investigated how response mode-unimanual finger presses versus coordinated two-handed stick use-affects cognitive performance in the Simon task, a classic measure of inhibitory control. Thirty-two participants completed the task by responding to the color of a stimulus while ignoring its spatial location, using both response modes in separate blocks. Analyses of reaction time and accuracy as a function of stimulus location (left, right) and response side (left, right) identified four distinct Simon effects and their corresponding conflict adaptation patterns across response modes. While the magnitude of the Simon effect was larger during stick use, sequential conflict adaptation-reflected in reduced interference following incongruent trials-was more pronounced for finger responses. No reliable practice-related changes in the Simon effect were observed across blocks for either effector. Delta plot analyses further revealed effector-specific differences in the temporal dynamics of spatial conflict across the RT distribution. By comparing unimanual and bimanual response modes across multiple interference types, this study highlights distinct contributions of motor implementation to cognitive control and supports the value of more ecologically valid action-based paradigms.