"What" and "how": Evidence for the dissociation of object knowledge and mechanical problem-solving skills in the human brain

Patients with profound semantic deterioration resulting from temporal lobe atrophy have been reported to use many real objects appropriately. Does thi s preserved ability reflect (i) a separate component of the conceptual know ledge system ("action semantics") or (ii) the operation of a system that is independent of conceptual knowledge of specific objects, and rather is res ponsible for general mechanical problem-solving skills, triggered by object affordances? We contrast the performance of three patients-two with semant ic dementia and focal temporal lobe atrophy and the third with corticobasal degeneration and biparietal atrophy-on tests of real object identification and usage, picture-based tests of functional semantic know-ledge, and a ta sk requiring selection and use of novel tools. The patient with corticobasa l degeneration showed poor novel tool selection and impaired use of real ob jects, despite near normal semantic knowledge of the same objects' function s. The patients with semantic dementia had the expected deficit in object i dentification and functional semantics, but achieved flawless and effortles s performance on the novel tool task Their attempts to use this same mechan ical problem-solving ability to deduce (sometimes successfully but often in correctly) the use of the real objects provide no support for the hypothesi s of a separate action-semantic system. Although the temporal lobe system c learly is necessary to identify "what" an object is, we suggest that sensor y inputs to a parietal "how" system can trigger the use of objects without reference to object-specific conceptual knowledge.