To lift an object, the motor system must predict the weight of the object and use this information to program appropriate lifting forces. If this prediction is erroneous, people may assign blame for the error to either themselves or an external source-a process called credit assignment. In the present study, we explored the role of credit assignment on weight predictions during a lifting task. Participants were told that the EMG surface electrodes attached to their lifting hand were either part of a "passive" system that recorded muscular activity, or part of an "active" system that would apply energy to the muscle, influencing weight perception. Participants performed 90 lifts of the training blocks, followed by 10 lifts of a newly encountered larger test block. In between training and test trials, the experimenter turned off the recording system and removed the surface electrodes for participants in the "active" group. For each lift, we determined the initial peak rate of change of vertical load force rate and load-phase duration, estimates of predicted object weight. Analysis of the first 10 training lifts and the last 10 training lifts revealed no effect of Active versus Passive EMG on weight predictions. However, after removing the EMG equipment, participants in the "active" group failed to scale their predictive load forces in the same manner as those in the "passive" condition when lifting a novel block. We conclude that cognitive information may play a role in credit assignment, influencing weight prediction when lifting novel objects.
Keywords: Credit assignment; Load force; Motor learning; Object lifting; Weight prediction.