“Mind the thumb”: Judging hand laterality is anchored on the thumb position

People can decide whether the image of a hand represents a left or a right one. The laterality judgment mainly implies mentally imaging own hand movement (motor simulation) if the stimulus represents a palm, or analysing visual cues, as hand asymmetry, if the stimulus reproduces a dorsum. Here, by capitalizing on evidence underscoring the key role of thumb-palm complex in motor dexterity of human hand, we hypothesise that activation of motor or visual processes when judging hand laterality is due to the different relevance of palm-thumb and dorsum-thumb combinations to hand action. To test this thumb-anchored strategy, in a laterality judgment experiment, we concurrently manipulated the thumb position (flexed or extended) with respect to palm and dorsum, and the human likeness of the hand shape (influencing the salience of the thumb with respect to the hand shape). The main results demonstrated that viewing the flexed thumb from palm or dorsum elicited motor simulation, while viewing the extended thumb activated motor simulation when combined with palm but not dorsum. The present data highlight the pivotal role of the thumb in hand laterality judgments, consistent with its key role in human in-hand manipulation.