Deficient internal models for planning hand-object interactions in apraxia

Motor imagery (MI) has been associated with planning stages of motor production, and in particular, with internal models that predict the sensory consequences of motor commands and specify the motor commands required to achieve a given outcome. In this study we investigated several predictions derived from the hypothesis that ideomotor apraxia (IM), a deficit in pantomime and imitation of skilled actions, may be attributable in part to deficits in internal models for planning object-related actions, in the face of relatively intact on-line, feedback-driven control of action. This hypothesis predicts that in IM, motor imagery should be (a) strongly correlated with other motor tasks not providing strong visual, tactile, and proprioceptive feedback from objects, i.e., object-related pantomime and imitation; (b) poorly correlated with performance tasks providing strong environmental feedback about the locations of effectors and targets, i.e., actual interaction with objects; and (c) particularly deficient in conditions that are computationally difficult for the motor planning system. Eight left fronto-parietal stroke patients with IM, five stroke patients without IM, and six healthy matched controls imagined grasping dowels and widgets presented at varying orientations, and actually grasped the same objects. The experimental predictions were confirmed. In addition, patients with IM and motor imagery deficits were significantly more likely than the non-apraxic group to have lesions in the intraparietal sulcus, a region previously implicated in imagery for hand-object interactions. The findings suggest a principled explanation for the deficits of IM patients in object-related gesture pantomime, imitation, and learning of new object-related gestures.