Planning while moving: when two hands are better than one

Humans routinely prepare their next movement while performing the current one. Because accumulating evidence suggests that movement planning and execution depend on partially overlapping neural substrates, this study examined whether motor preparation is as efficient when a limb is in motion as compared to static. In Experiment 1 (12 females, 12 males), participants performed a two-choice reaction time (RT) task in which they initiated a leftward or rightward reach following the appearance of a visual target. On some trials, the upcoming target direction was precued a few seconds in advance. When the hand was at rest, the directional precue reduced RTs by 44ms, indicating effective preprogramming of the reaching movement. Interestingly, when the same hand was producing a rhythmic movement, the precue only reduced RTs by 16ms, suggesting interference. In Experiment 2 (13 females, 11 males), the same task was performed with the exception that the rhythmic movement was performed with the contralateral hand. Under these conditions, the benefit of the precue remained strong (37ms), suggesting that the interference arose from hemisphere-dependent competition rather than general attentional or biomechanical factors. Taken together, these findings constrain the behavioral conditions under which ongoing execution interferes with planning a forthcoming action while highlighting a fundamental limit in the brain's capacity for parallel motor control. We propose that planning and execution compete for neural resources within the same hemisphere, revealing a form of motoric bottleneck whose strength depends upon the degree of overlap between neural populations engaged by the ongoing and upcoming actions.Significance statement Many activities such as playing an instrument or typing require preparing a movement while one is ongoing. This study investigated the efficiency with which an action can be planned when the same limb is already moving. Using a reach reaction time task with directional precues, we found that advance information sped up responses when the hand was stationary but much less so when that hand was already moving. Notably, this interference was greatly reduced when reach planning and background movement were distributed across hands. These results are consistent with a hemisphere-dependent motoric bottleneck and open up the possibility that the balance between ongoing execution and upcoming preparation is constrained by the degree of overlap in neural representation.