Misuse of tools and objects by patients with left brain damage is generally recognized as a manifestation of apraxia, caused by parietal lobe damage. The use of tools and objects can, however, be subdivided in several components. The purpose of our study was to find out which of these are dependent on parietal lobe function. Thirty-eight patients with left brain damage and aphasia were examined using tests to assess the retrieval of functional knowledge from semantic memory (Functional Associations), mechanical problem solving (Novel Tools) and use of everyday tools and objects (Common Tools). Voxel-wise analysis of magnetic resonance images revealed two regions where lesions had a significant impact on the test results. One extended rostrally from the central region and ventrally through the middle frontal cortex to the dorsal margin of the inferior frontal gyrus. The other reached dorsally and caudally from the supramarginal gyrus, through the inferior, to superior parietal lobe. Whereas the frontal lesions had an adverse influence on all experimental tests as well as on the subtests of the Aachen Aphasia test, parietal lesions impaired Novel and Common Tools, but did not have an adverse effect on the Functional Associates. An association between Functional Associations and temporal lesions became apparent when patients with only a selective deficit in the test were considered, but did not show up in the whole group analysis. The parietal influence was as strong for the selection as for the use of either novel or common tools, although choice of appropriate manual configuration and movements was more important for use than for selection. We conclude that the contribution of the parietal lobe to tool use concerns general principles of tool use rather than knowledge about the prototypical use of common tools and objects, and the comprehension of mechanical interactions of the tool with other tools, recipients or material rather than the selection of grip formation and manual movements.

It has been reported that patients with semantic dementia function well in everyday life and sometimes show striking preservation of the ability to use objects, even those specific objects for which the patient has degraded conceptual information. To explore this phenomenon in nine cases of semantic dementia, we designed a set of semantic tests regarding 20 everyday objects and compared performance on these with the patients' ability to demonstrate the correct use of the same items. We also administered a test of mechanical problem solving utilizing novel tools, on which the patients had completely normal ability. All but the mildest affected patient showed significant deficits of naming and on the visually based semantic matching tasks. Object use was markedly impaired and, most importantly, correlated strongly with naming and semantic knowledge. In a small number of instances, there was appropriate use of an object for which the patient's knowledge on the semantic matching tasks was no better than chance; but this typically applied to objects with a rather obvious relationship between appearance and use, or was achieved by trial and error. The results suggest that object use is heavily dependent upon object-specific conceptual knowledge, supplemented to some degree by a combination of visual affordances and mechanical problem solving.

Patients with profound semantic deterioration resulting from temporal lobe atrophy have been reported to use many real objects appropriately. Does this preserved ability ref lect (i) a separate component of the conceptual knowledge system (''action semantics'') or (ii) the operation of a system that is independent of conceptual knowledge of specific objects, and rather is responsible for general mechanical problem-solving skills, triggered by object affordances? We contrast the performance of three patients-two with semantic dementia and focal temporal lobe atrophy and the third with corticobasal degeneration and biparietal atrophy-on tests of real object identification and usage, picturebased tests of functional semantic knowledge, and a task requiring selection and use of novel tools. The patient with corticobasal degeneration showed poor novel tool selection and impaired use of real objects, despite near normal semantic knowledge of the same objects' functions. The patients with semantic dementia had the expected deficit in object identification and functional semantics, but achieved f lawless and effortless performance on the novel tool task. Their attempts to use this same mechanical problem-solving ability to deduce (sometimes successfully but often incorrectly) the use of the real objects provide no support for the hypothesis of a separate action-semantic system. Although the temporal lobe system clearly is necessary to identify ''what'' an object is, we suggest that sensory inputs to a parietal ''how'' system can trigger the use of objects without reference to object-specific conceptual knowledge.