Figure 3.
Consider a task of producing a constant sum of two variables, e.g., pressing with two fingers and producing a constant total force level. The dashed lines show solution spaces (uncontrolled manifolds, UCMs) for three different force levels, C1, C2, and C3. Across repetitive trials, clouds of data points form ellipses elongated along the corresponding UCM. This shape reflects lower stability along the UCM as compared to the orthogonal direction relevant to the task-imposed constraints. Note that the three data clouds are centered not randomly along the UCMs but reflect a certain preferred sharing of the task between the two effectors. This preference may reflect an optimization principle.