Adaptation to a novel multi-force environment.

Humans display accurate limb behavior when they move in familiar environments composed of many simultaneously-acting forces. Little is known about how multi-force environments are represented and whether this process partitions between the underlying force components, reflects the net forces present, or is cued to the force-context. We tested between these three main alternatives by examining how reaching movements adapt to a novel multi-force field composed of a velocity-dependent force and a constant force. These hypotheses were dissociated first by making the constant force larger and oppositely-oriented to the velocity-dependent force; thereby, the net force was always opposite the velocity-dependent component. Second, we tested adaptation with all novel forces removed to eliminate any potential cues for the force-context. In two experiments that used forces perpendicular or parallel to the forward movement direction, we found adaptation aftereffects consistent with a mechanism that partitioned the velocity-dependent component from the net force field. Specifically, we found aftereffects opposite the rightward or resistive velocity-dependent component of the multi-force field, even though the net force imposed was leftward or assistive, respectively. An additional experiment suggested that the velocity-dependent component is partitioned relative to the background load in a limb-based coordinate frame.