TOOL USE AND MECHANICAL PROBLEM-SOLVING IN APRAXIA

Moorlaas (1928) proposed that apraxic patients can identify objects an d can remember the purpose they have been made for but do not know the way in which they must be used to achieve that purpose. Knowledge abo ut the use of objects and tools can have two sources: II can be based on retrieval of instructions of use from semantic memory or on a direc t inference of function from structure. The ability to infer function from structure enables subjects to use unfamiliar tools and to detect alternative uses of familiar tools. It is the basis of mechanical prob lem solving. The purpose of the present study was to analyze retrieval of instruction of use, mechanical problem solving, and actual tool us e in patients with apraxia due to circumscribed lesions of the left he misphere. For assessing mechanical problem solving we developed a test of selection and application of novel tools. Access to instruction of use was tested by pantomime of tool use. Actual tool use was examined for the same familiar tools. Forty two patients with left brain damag e (LBD) and aphasia, 22 patients with right brain damage (RBD) and 22 controls were examined. Only LED patients differed from controls on al l tests. RED patients had difficulties with the use but not with the s election of novel tools. In LED patients there was a significant corre lation between pantomime of tool use and novel tool selection but ther e were single cases who scored in the defective range on one of these tests and normally on the other. Analysis of LED patients' lesions sug gested that frontal lobe damage does not disturb novel tool selection. Only LED patients who failed on pantomime of object use and on novel tool selection committed errors in actual use of familiar tools. The f inding that mechanical problem solving is invariably defective in apra xic patients who commit errors with familiar tools is in good accord w ith clinical observations, as the gravity of their errors goes beyond what one would expect as a mere sequel of loss of access to instructio n of use. (C) 1998 Published by Elsevier Science Ltd. All rights reser ved.