Apraxia and the parietal lobes.
The widely held belief in a central role of left parietal lesions for apraxia can be traced back to Liepmann's model of a posterior to anterior stream converting mental images of intended action into motor execution. Although this model has undergone significant changes, its modern descendants still attribute the parietal contribution to the existence of mental representations of intended movements which precede and direct their motor execution. They predict that pantomime of tool use should be particularly vulnerable to parietal lesions. A review of clinical studies contradicts these assumptions: The impact of parietal lobe damage on pantomime of tool use is inconstant if not absent altogether. The domains of action which are most affected by left parietal damage are the imitation of meaningless gestures and, although probably only in the context of additional more widespread brain damage, actual use of tools and objects. I hypothesize that imitation of meaningless gestures and use of tool and objects depend on left parietal lobe integrity because of their demands on categorical apprehension of spatial relationships between multiple objects or between multiple parts of objects. For use of tools and objects the spatial relationships are between the hand, the tool, its recipient, and the material it acts upon. Categorical apprehension concentrates on features of these relations which determine mechanical interactions. For imitation of meaningless gestures, categorical apprehension of demonstrated gesture results in "body part coding" which reduces the visual appearance of the demonstrated gestures to simple spatial relationships between a limited set of discrete body parts. The hypothesis that the role of the left parietal lobe in apraxia concerns categorical apprehension of spatial relationships fits well with more general theories of parietal lobe function and hemisphere asymmetries.