Unilateral basal ganglia damage causes contralesional force control deficits: a case study.

When grasping to lift an object, the grip force is usually scaled to the mass of the object. However, it has been shown that lifting objects of different sizes but equal masses results in the generation of higher forces for larger compared to smaller objects. The objective of this study was to investigate whether a similar effect is present in an individual (RI) with a unilateral lesion to the basal ganglia (BG). It was hypothesized that if the BG have an influence on the use of visual information in updating of the internal model used to anticipate the forces required for grasping, damage to these structures should result in the inability of RI's contralesional hand to anticipate object mass based on size cues. To test this hypothesis three objects of equal mass but different sizes were grasped and lifted by RI and six control individuals. The forces that were generated during these lifts were quantified. The controls showed the expected increases in peak grip force as object size increased. RI showed no effect of object size for his contralesional hand, but did show force scaling with his ipsilesional hand. In conclusion, RI's BG damage affected the on-line control of grip forces and the inability to integrate visual and tactile information in the programming of finger forces.