Interference between flexible and adaptive reaching control

Humans rapidly update the control of an ongoing movement following changes in contextual parameters. This involves adjusting the controller to exploit redundancy in the movement goal, such as when reaching for a narrow or wide target, and adapting to dynamic changes such as velocity-dependent force fields (FFs). Although flexible control and motor adaptation are computationally distinct, the fact that both unfold within the same movement suggests they may share common neural resources for task-specific adjustments. To test this hypothesis, we conducted a series of experiments combining changes in the target structure and a force field presented separately or in combination. Seventy-six human participants (both sexes) took part in this study, with each experiment involving different participants. They were asked to reach for a target that could change from a narrow square to a wide rectangle between or during trials. Step loads were used to assess whether participants exploited target redundancy. In a separate experiment, we added a force field in addition to target changes and step loads. Our results revealed a reduced ability to exploit target redundancy when sudden target changes occurred concurrently with FF adaptation. Furthermore, the magnitude of adaptation was reduced when step loads were added to the FF. Crucially, this interference emerged specifically when all perturbations impacted motor execution simultaneously. These results indicate that flexible control and motor adaptation interact in a non-trivial manner, suggesting possible overlap between their underlying neural mechanisms, and a clear identification of the timescale at which they are engaged - namely, during movement. Significant statementHumans rapidly adapt to changes in task demands, such as target structure changes or exposure to force fields (FFs). These two types of adjustments occur within a single movement, suggesting potential interactions between them. Our experiments revealed that the combination of FF exposure with online target shape changes selectively reduced participants ability to exploit target redundancy, while the combination of FF and step loads led to a reduced extent of motor adaptation. These findings confirm that motor adaptation occurs not only between trials but also during movement. The selective nature of the observed interference highlights an interplay between flexible control and motor adaptation, underscoring the importance of understanding the timing of these processes to better characterise their underlying neural circuits.