Mental rotation of congenitally absent hands.

We compared motor imagery performance of normally limbed individuals with that of individuals with one or both hands missing since birth (i.e., hand amelia). To this aim, 14 unilaterally and 2 bilaterally amelic participants performed a task requiring the classification of hands depicted in different degrees of rotation as either a left or a right hand. On the same task, 24 normally limbed participants recapitulated previously reported effects; that is, that the hand motor dominance and, more generally, a lifelong use of hands are important determinants of left-right decisions. Unilaterally amelic participants responded slower to hands corresponding to their absent, compared with their existing, hand. Moreover, left and right hand amelic participants showed prolonged reaction times to hands (whether left or right) depicted in unnatural orientations compared with natural orientations. Among the bilateral amelics, the individual with phantom sensations, but not the one without, showed similar differentiation. These findings demonstrate that the visual recognition of a hand never physically developed is prolonged, but still modulated by different rotation angles. They are further compatible with the view that phantom limbs in hand amelia may constrain motor imagery as much as do amputation phantoms.