Dissociation between manipulation and conceptual knowledge of object use in the supramarginalis gyrus

Because tool naming activates motor-related areas in the posterior parietal cortex, it has been suggested that conceptual knowledge of tools relies on their unique manipulation patterns. However, this view is questioned by the finding that some patients impaired in retrieving manipulation knowledge of man-made objects are still able to perform conceptual judgments on them. To address this issue, we used repetitive transcranial magnetic stimulation (rTMS) to interfere with the functioning of the anterior part of the right or left supramarginalis gyrus (SMG), a region critically involved in object-directed actions. rTMS was delivered in healthy participants performing four judgment tasks designed to explore different aspects of manipulation and conceptual knowledge of man-made objects. The two manipulation judgment tasks consisted in determining whether (1) two objects displayed on a computer screen are normally used by adopting a comparable hand posture, or (2) a given hand posture is appropriate to use an object. In the two conceptual judgment tasks, subjects had to decide whether (1) two objects displayed on the computer screen are normally used in the same context or (2) they are functionally related. We found that virtual lesions of left SMG interfere only with the performance of the manipulation judgment task in which subjects had to decide whether two different objects are used by adopting the same hand posture, all the other tasks being unaltered. rTMS applied over the right SMG had no effect. These results challenge the assumption that conceptual knowledge of tools is grounded upon motor representations.