Movement imitation depends on an abstract trajectory representation in dorsal premotor cortex

Humans are particularly good at copying novel and meaningless gestures. The mechanistic and anatomical basis for this specialized imitation ability remains largely unknown. One idea is that imitation depends on a representation of the relative configurations of body parts (body schema). Here we propose an alternative route to imitation that depends on an abstract representation of the trajectory path of the end-effector. We studied a group of patients with strokes that encompassed the left premotor and posterior parietal cortices. We found that they were equally impaired at imitating movement trajectories using the ipsilateral limb (i.e., the non-paretic side) that were cued either by an actor using their whole arm or just by a cursor, suggesting that body configuration is not always critical for imitation and that a representation of abstract trajectory shape may suffice. In addition, imitation ability was uncorrelated to the ability to identify the trajectory shape, suggesting a dissociation between producing trajectory shapes and perceiving their paths. Finally, a lesion-symptom mapping analysis found that imitation deficits were associated with lesions in left dorsal premotor but not parietal cortex. Together, these findings suggest a novel body-independent route to imitation that relies on the ability to plan abstract movement paths within dorsal premotor cortex.