Load-dependent and load-independent effects on longitudinal motor training in human continuous hand movements

Motor learning involves complex interactions between the cognitive and sensorimotor systems, which is susceptible to different levels of task load. While the mechanism underlying load-dependent regulations in cognitive functions has been extensively investigated, their influence on downstream execution in motor skill learning remains less understood. The current study extends the understanding of how load levels affect motor learning by a longitudinal functional near-infrared spectroscopy (fNIRS) study in which 30 participants (15 females) engaged in extensive practice on a two-dimensional continuous hand tracking task with varying task difficulties. We propose the index of difficulty (ID) as a quantitative estimate of task difficulty, which is positively correlated with psychometric measure of subjective workload level. Results shows that as behavioral performance improved over time, participants adopted a direction-specific and load-independent (i.e., consistent across different load levels) control strategy, shifting from feedback-dominant to feedforward-dominant control in the vertical direction as training progressed. Crucially, we provide robust evidence of the learning-induced alteration in load-dependent cortical activation patterns, suggesting that effective motor skill learning may lead to shift towards an inverted-U relationship between activation and load level in the pre-motor and supplementary motor areas. In addition, brain-behavior relationship in the frontoparietal network was strengthened after training. Taken together, our findings provide new insights into the learning-induced plasticity in brain and behavior associated with load-dependent and load-independent contributions to motor skill learning.