Abstract
Do movement plans, like representations in working memory, share a limited pool of resources? If so, the precision with which each individual movement plan is specified should decrease as the total number of movement plans increases. To explore this, human participants made speeded reaching movements toward visual targets. We examined if preparing one movement resulted in less variability than preparing two movements. The number of planned movements was manipulated in a delayed response cueing procedure that limited planning to a single target (experiment 1) or hand (experiment 2) or required planning of movements toward two targets (or with two hands). For both experiments, initial movement direction variability was higher in the two-plan condition than in the one-plan condition, demonstrating a cost associated with planning multiple movements, consistent with the limited resource hypothesis. In experiment 3, we showed that the advantage in initial variability of preparing a single movement was present only when the trajectory could be fully specified. This indicates that the difference in variability between one and two plans reflects the specification of full motor plans, not a general preparedness to move. The precision cost related to concurrent plans represents a novel constraint on motor preparation, indicating that multiple movements cannot be planned independently, even if they involve different limbs.
Highlights
Various lines of evidence indicate that multiple movements can be prepared in parallel
Behavioral evidence comes from studies that examined the effects of noncued targets on movement selection (Gallivan et al 2015), external perturbations that required rapid switches to an alternative plan (Nashed et al 2014), and tasks in which participants were forced to move before the target was fully specified (Chapman et al 2010; Gallivan et al 2016; Ghez et al 1997; Haith et al 2015; Stewart et al 2014)
We found that movement plans at both levels of competition were more variable if two movements were prepared at the same time than if only one movement was prepared, consistent with the concept of a limited resource for movement planning
Summary
Various lines of evidence indicate that multiple movements can be prepared in parallel. Neurophysiology studies with nonhuman primates have indicated that choosing between two possible actions is not a serial process, whereby one of the two actions is selected and the selected action is planned and executed, but rather a parallel process in which both actions are prepared simultaneously (Cisek 2006; Cisek and Kalaska 2005) These studies suggest that the final movement is the outcome of a competition between the motor plans. Work on visual working memory has highlighted the idea that resource sharing has consequences for the precision with which information is internally represented (Bays and Husain 2008; Ma et al 2014; Palmer 1990; van den Berg et al 2012; Zhang and Luck 2008) These models describe the variability with which visual items are represented in memory by a power law: as the number of items increases, the variability associated with the representation of each item increases
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