1 3 Exp Brain Res DOI 10.1007/s00221-014-4077-8 RESEARCH ARTICLE Symmetries in action: on the interactive nature of planning constraints for bimanual object manipulation John M. Huhn III · Kimberly A. Schimpf · Robrecht P. van der Wel Received: 10 December 2013 / Accepted: 18 August 2014 © Springer-Verlag Berlin Heidelberg 2014 Keywords Object manipulation · Action planning · Bimanual · Symmetry Introduction Many of the actions people carry out reflect the outcome of a complex decision process in which one action alternative is instantiated while others are weeded out. Unlike other everyday decisions, such as what to eat and which car to buy, deciding which action to perform is a largely implicit process. Indeed, people have relatively little insight into what guides their physical behaviors, or what shape such behaviors take (e.g., Fourneret and Jeannerod 1998; Kno- blich and Kircher 2004). Thus, asking people why they per- form actions in a particular way is unlikely to yield accu- rate insights into the cognitive processes that govern action planning. How then could we derive a comprehensive theory on action planning? One way to think about action planning is to consider whether certain factors consistently influ- ence the actions people perform. We will use the term con- straints to refer to such factors. When cast this way, two key issues arise; first, which constraints do people plan for, and second, how do multiple constraints get com- bined in the planning process. Here, we turn to the study of bimanual object manipulation to examine these issues. We examined how different forms of bimanually symmetrical actions influence action planning, and how they combine. In the context of bimanual object manipulation, previ- ous research has examined how two plausible constraints for action planning behave when pitted against one another. These constraints are end-state planning and bimanual sym- metry. End-state planning refers to the tendency for people to manipulate objects in ways that result in comfortable Abstract An important functional question for under- standing how people perform physical actions is to under- stand how they manipulate objects. Previous research sug- gests that people prefer to move their hands symmetrically. For bimanual object manipulation, such symmetry may take on several forms, however. Actions may be symmetri- cal when objects are grasped (start symmetry), when they are placed on their target locations (target symmetry), and/ or relative to the objects being moved (object symmetry). We studied how these forms of symmetry influenced grasp selection when participants moved two plungers from two start locations to two target locations. We varied the heights of these locations across conditions. The grasp locations participants adopted indicated a preference for object symmetry. This preference was even stronger when initial symmetry coincided with object symmetry. These results provide a tractable illustration of how multiple planning constraints may interact to give rise to both regularity and flexibility in motor behavior. John M. Huhn and Robrecht P. van der Wel have equal contributions. J. M. Huhn III · K. A. Schimpf · R. P. van der Wel (*) Department of Psychology, Rutgers University, Camden, NJ, USA e-mail: r.vanderwel@rutgers.edu J. M. Huhn III e-mail: jmh734@psu.edu K. A. Schimpf e-mail: kaschimpf@gmail.com J. M. Huhn III Department of Psychology, The Pennsylvania State University, University Park, PA, USA