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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.

Behavioural evidence suggest that males outperform females in mentally transforming objects, whereas whether sex differences exist in mentally transforming body part images (implicit motor imagery) is an open issue. The aim of the present study was to fill this gap testing performance of 360 healthy participants on a classical behavioural measure of implicit motor imagery: the hand laterality task. Participants had to judge handedness of hand images portrayed from back and palm and presented in different spatial orientations. Two main findings emerged. First, males were significantly faster than females in judging hands portrayed from palm, in particular left palms at 0°, 90° and 180° orientation, whereas females were faster than males in judging backs, in particular left and right backs at 0° and the left back at 90°. Second, both males and females showed a significant biomechanical effect (faster responses for hands portrayed in medial vs. lateral positions) while judging palms, albeit the effect was stronger in males, whereas only females showed a significant biomechanical effect when judging backs. Thus, males and females seem to differently exploit motor simulation processes during mental transformation of hand images depending on a specific familiarity with body parts portrayed from different views. This result might be taken into account when tailoring motor imagery tasks in applied contexts, as motor rehabilitation.