Grasping tools: effects of task and apraxia

In patients with apraxia the ability to access tool-use-knowledge and to integrate it into a movement plan is frequently disturbed. The present study investigated potential differences between a 'transport'- and a 'use'-task as well as the influence of apraxia on the preceding grasping movement, when the tool is presented with its handle pointing away from the participant. 20 controls, 10 patients with right brain damage (RBD-group) and 17 patients with left brain damage (LBD-group) were tested. 10 LBD-patients were classified as moderately to severely apraxic according to erroneous demonstration of tool-use. Participants were asked to grasp the tool and either to demonstrate its typical 'use' or to 'transport' it into a container. Functional grasp, which enables comfortable use, was distinguished from non-functional grasp. The results showed that task-content influences the way to grasp an object. Apart from some LBD-patients, nearly all participants produced 100% functional grasps during the 'use'-task. Additionally inappropriate non-functional grasping has always been followed by apraxic tool-use behaviour. Compared to the 'use'-task in the 'transport'-task, functional grasping was significantly reduced in all groups. Our results imply that grasping a tool is specific to a task. Selection of the grasp type seems to be determined by individual weighting of several factors, including knowledge about the function of the object, structural tool characteristics, biomechanical costs of the movement, and previous experience. In general perception and integration of tool-structure plus specific tool-related functional knowledge are necessary for its use and might be processed synchronously. Patients with apraxic tool-use may fail to access any of these information resources.