Impairment of grasping movements following a bilateral posterior parietal lesion.

The observation of a patient (A.T.) with a bilateral posterior parietal lesion of vascular origin is reported. A.T. presented a bilateral (more marked on the right) deficit in grasping simple objects (neutral cylindrical dowels) without deficit in reaching toward the location of these objects. The major symptom was an exaggerated anticipatory opening of the fingers with poor correlation with object size, resulting in awkward grasps. It was present both when the hand was visible to the subject and when it was not. This deficit was much less marked if the neutral objects were replaced by usual objects of the same sizes. Finally, in tasks where she had to indicate with her fingers the size of visual objects presented as virtual images through a mirror, or the size of imagined usual objects, A.T. performed normally. These results are discussed within the framework of a dual representation of objects. Only the "pragmatic" representation for steering object-oriented actions would be impaired in this patient as a result of posterior parietal damage. By contrast the semantic representation for object identification would be intact.