Evaluation of an Action's Effectiveness by the Motor System in a Dynamic Environment

An important model for explaining humans' feeling of agency-the Comparator model-draws on ideas used to explain effective motor control. The model describes how our brain estimates the degree of control over the environment offered by a specific motor program (in short, an action's effectiveness). However, given its current level of specification, the model is at best vague on how (or even whether) the prediction of effectiveness of an action is dynamically updated. To test the issue empirically, our participants performed multiple experimental blocks of a task (reliably shown to measure reinforcement from effectiveness) in which blocks with and without action-effects (or with spatially unpredictable feedback) were interlaced. This design creates a sinusoidal-like objective increase or decrease in effectiveness (quantified as the n-trials back probability of receiving feedback), which participants were unable to report. As previously found, response speed indexed reinforcement from effectiveness. The results suggest that reinforcement from effectiveness is sensitive to both the degree and trend of effectiveness; that is, reinforcement is sensitive to whether it is increasing, decreasing, or is unchanged. Given the previous links made between reinforcement from effectiveness and the computation of effectiveness by the motor-system, the results are the first to show an online, dynamic and complex sensitivity to a motor-programs' effectiveness that is directly translated to its production. The importance of testing the so-called sense of agency in a dynamic environment and the implications of the current findings for a dominant model of the sense of agency are discussed.