Intact motor imagery in chronic upper limb hemiplegics: evidence for activity-independent action representations.

Four experiments were undertaken to examine the effects of chronic hemiplegia on the ability to internally represent actions involving either the paralyzed (contralesional) or relatively unaffected (ipsilesional) limb. An experimental group of chronic, densely hemiplegic patients was compared with controls who experienced nearly full recovery from an initially dense hemiparesis. All participants suffered cerebral vascular accidents that spared sites in premotor and parietal areas directly involved in representing upper limb actions. Despite chronic limb immobility, hemiplegic patients performed all four tasks at a high level of accuracy and showed no differences in their ability to represent actions of the contralesional versus ipsilesional limbs. On tasks that involved representing actions of the hands and lower arms, hemiplegic patients were as accurate as recovered controls. Hemiplegic patients were, however, less accurate than controls on a task that involved representing actions of either upper arm. Overall, chronic hemiplegics performed more accurately for decisions based on their contralesional limbs: a "hemiplegic advantage" that may be related to an ongoing focus on planning and/or imagining currently impossible movements. These findings reveal a dissociation between the ability to internally "represent" versus "produce" manual actions. Further, they demonstrate that internal action representations can be robust to even years of limb disuse.