P174. Effects of left and right brain damage on anticipatory motor behavior during grasping and lifting with the ipsilesional hand

Abstract

Behavioral studies in patients with left and right brain damage revealed the specialized function of both hemispheres in motor control. Recently, we investigated anticipatory grip force control in patients with stroke affecting the left or the right hemisphere during object lifting with the ipsilesional non-paretic hand. We found imprecise anticipation of grip force to the weight of everyday objects (like a carton with milk or a package of paper tissues) in patients with left brain damage while patients with right brain damage performed closer to normal. This finding suggests a particular role of the motor-dominant left brain in predicting object properties based on knowledge about the object identity. By contrast, we found no deficit of anticipatory grip force control in stroke patients when the initial weight information was provided by the size of the object. Also the perceptual size-weight-illusion was the same in patients and control subject. In the present study we tested material as the critical variable determining object weight. We hypothesized that the anticipation of material information as well as the so-called material-weight illusion may be reduced in patients with left brain damage, as material information about object characteristics may be a less direct cue to weight compared to size. Twelve patients with left brain damage (LBD), ten patients with right brain damage (RBD) and fourteen control subjects (CTR) lifted two pairs of identically-weighted objects which differed in size in the size-weight-illusion paradigm and in apparent material (polystyrene and aluminum) in the material-weight-illusion paradigm. In an additional control pair, the size and weight of two wooden blocks differed in the natural way. Participants’ grip forces and the lift forces were measured with a sensor (170 g) attached to the objects’ top surfaces. Subjects had to indicate which object in a pair is felt heavier before and after 6 lifts of each cube in a pair. Participants of all three groups experienced the size–weight–illusion. The material–weight–illusion was perceived less clearly without a clear difference between patients and control subjects. During the first lift of each object, the steepness of the grip force increase (quantified as the peak grip force rate) reflected the objects’ apparent weight as indicated by size. However variability was substantial in both patients groups. Different materials induced less clear differences in grip force rate. In particular, patients with left brain damage failed to scale their grip force profile according to the material. Also for the wooden object pair, data of LBD patients revealed a weaker anticipation of object properties than observed in the other groups. Our results confirm the normal processing of object size information for the scaling of grip forces and for the perception of the size-weight illusion in patients with brain damage, indicating that size processing is resistant to a wide range of brain damage. LBD patients in particular showed no consistent scaling of grip forces according to variations in material properties. Performance variability was however large in patients, suggesting that deficits depend on the specific lesion pattern which varied a lot across patients. In conclusion, the motor-dominant left hemisphere seems to play a prominent role in the processing of material information for object manipulation. However the responsibility seems to be less selective than when information is derived from object identity.