Neurocognition and ­Action - Biomechanics

Cognitive mechanisms of motor planning - how cognitive and mechanical costs of a motor task affect the fractions of motor plan reuse and novel planning (CogMech)

Project duration: 10/2017 - 08/2019
Project funding: 207.428 Euro

Even the simplest voluntary movements require the creation of a motor plan. In sequential reaching tasks, motor plans are not created from scratch for every movement, but the former plan is partially reused. This reuse manifests in a persistence on the former posture, termed motor hysteresis. When a column of drawers is opened in sequential order, for example, people persist on a more pronated posture in the descending and on a more supinated posture in the ascending sequence. The motor hysteresis effect is explained by the cost-optimization hypothesis, which states that motor plans in a sequential task are created by a modification of the former plan, but with a varying fraction of reuse. A larger fraction of reuse reduces the cognitive cost of motor planning. At the same time, the larger persistence on the former, less optimal posture increases the mechanical cost of motor execution. The summed cost of the movement is minimized at an intermediate, optimal fraction of reuse.

The optimal fraction of reuse can be determined by a mathematical model. It depends on the relative weights assigned to the cognitive and mechanical cost of the task. Based on the mathematical model, one can predict how changes in the cognitive or mechanical cost of the task affect the fraction of reuse. An increase of the mechanical cost should decrease the optimal fraction of reuse and, in turn, the size of the hysteresis effect. This prediction has already been confirmed in a previous study of ours. In the proposed research program, we want to test whether the cost-optimization hypothesis also holds true for changes in the cognitive cost of a task.

To this end, we conduct two lines of experiments. In the first line, we experimentally increase the cognitive cost by depleting the available cognitive resources with a concurrent memory task. In the second line, we exploit intrinsic differences in the cognitive or mechanical cost, which are a result of hemispheric lateralization. Studies suggest that (1) the left hemisphere is dominant in motor planning and (2) the dominant hemisphere has superior control of limb dynamics. We therefore expect less cognitive cost of motor planning in the right limb and less mechanical cost of motor execution in the dominant limb, which should be reflected by according differences in hysteresis effect size between limbs.

If the cost-optimization hypothesis holds true, the underlying mathematical model can be used to estimate the relative weights assigned to the cognitive and mechanical cost during motor planning. To this end, we vary the mechanical cost of a two-parts movement sequence and measure the corresponding fractions of reuse. Based on this function, the weights of the two cost factors are determined by a model fit. This third line of experiments ultimately provides an estimate of how, during motor planning, 1 Nm of mechanical work is rated against the cognitive cost for the creation of a grasping movement plan from scratch.