Learning vs. minding: How subjective costs can mask motor learning

Abstract

When learning new movements older adults tend to make larger kinematic errors than younger adults, interpreted as an age-related decline in motor-learning ability. However, this conclusion assumes that both older and younger adults use the same error-canceling strategy. Alternatively, it could be that older adults' higher errors can be explained by a difference in strategy, rather than a reduction in learning ability. Consider that error-cancelling strategies can incur higher effort costs. Older adults may be choosing to sacrifice error reduction in favor of a lower effort movement. We test this hypothesis using trajectories where subjects reached to targets in a force field. Utilizing the framework of optimal control theory, we infer subjective costs (i.e., strategies) and internal model accuracy (i.e., proportion of the novel dynamics learned) by fitting a model to each population's trajectory data. Our results demonstrate that the trajectories are not uniquely specified by a precise amount of learning, but rather through a combination of the amount learned and strategic differences represented by relative cost weights. Based on the model fits, younger adults learn between 60-85% of the novel dynamics, and older adults learn between 55-80%. Each model fit produces trajectories which match the experimentally observed data, where a lower proportion learned in the model is compensated for by increasing costs on kinematic errors relative to effort. This finding supports our hypothesis that older and younger adults could be learning to the same extent, but older adults have a higher relative cost on effort compared to younger adults. These results call into question the proposition that older adults learn less than younger adults and suggest that the metrics commonly used to probe motor learning paint an incomplete picture. Importantly, to accurately quantify the learning process the subjective costs of movements should be considered.