SPATIAL PLANNING DEFICITS IN LIMB APRAXIA

Geschwind (1975) proposed a disconnection model in which an apraxic su bject is unable to carry out movements to command because the left hem isphere that comprehended the verbal command is disconnected from the right premotor and motor areas which controls the left hand. An altern ate model, however, proposes that apraxia results from destruction of spatiotemporal representations of learned movement stored in the left hemisphere (Heilman, 1979). The disconnection hypothesis would predict that apraxic subjects should be able to correctly imitate gestures an d correctly use actual tools since these tasks do nor require language . The movement representation model predicts that imitation and actual tool use would also be impaired Motion analyses were performed on the trajectories of repetitive 'slicing' gestures made in a series of con ditions in which contextual cues were introduced in a graded fashion. Four cite conditions were presented: no cues (verbal command), object present, tool present and both object and tool present. Positions of t he hand, wrist, elbow, and shoulder were digitized from neighbouring v iews, reconstructed in three dimensions and analysed with respect to s pecific spatiotemporal features of the trajectories. Three subjects wi th limb apraxia, who had lesions that included left parietal cortex, a nd four neurologically intact subjects participated The apraxic subjec ts showed disturbances in planning the movement of the hand in space a cross the cue conditions. For example, they showed deficits in the pla ne of motion, the shape of the trajectory and in the coupling of hand speed and trajectory shape even when given fill contextual cues. These data support the hypothesis that apraxia can result from the destruct ion of spatiotemporal representations of learned movement, rather than from a disconnection between the receptive language areas in the left hemisphere and the contralateral motor cortices.