Goal-Selection and Movement-Related Conflict during Bimanual Reaching Movements

0 Department of Psychology, University of California , Berkeley , Tolman Hall 1 Department of Psychological and Brain Sciences, Dartmouth College , 6207 Moore Hall, Hanover, NH 03755, USA 2 Department of Biomedical Engineering, Johns Hopkins University , 720 Rutland Avenue, 419 Traylor Building, Baltimore, MD 21205, USA 3 The Author 2006. Published by Oxford University Press. All rights reserved. For permissions , please 4 Department of Psychology, University of Iowa , 11 Seashore hall E, Iowa City, IA 52242, USA Conflict during bimanual movements can arise during the selection of movement goals or during movement planning and execution. We demonstrate a behavioral and neural dissociation of these 2 types of conflict. During functional magnetic resonance imaging scanning, participants performed bimanual reaching movements with symmetric (congruent) or orthogonal (incongruent) trajectories. The required movements were indicated either spatially, by illuminating the targets, or symbolically, using centrally presented letters. The processing of symbolic cues led to increased activation in a left hemisphere network including the intraparietal sulcus, premotor cortex, and inferior frontal gyrus. Reaction time cost for incongruent movements was substantially larger for symbolic than for spatial cues, indicating that the cost was primarily associated with the selection and assignment of movement goals, demands that are minimized when goals are directly specified by spatial cues. This goal-selection conflict increased activity in the pre--supplementary motor area and cingulate motor areas. Both cueing conditions led to larger activation for incongruent movements in the convexity of the superior parietal cortex, bilaterally, making this region a likely neural site for conflict that arises during the planning and execution of bimanual movements. These results suggest distinct neural loci for 2 forms of constraint on our ability to perform bimanual reaching movements. Introduction Many skilled behaviors involve the coordination of both hands (Guiard 1987). For example, opening a jar requires that 1 hand grasp the jar while the other twists the lid. This action demands tightly integrated control of the 2 hands. In other behaviors, efficient performance requires that the 2 hands perform with relative independence such as when we pick through cherries at the marketplace. Limitations in producing bimanual movements can illuminate the underlying functional architecture of the action system (Kelso 1984; Franz and others 1996; Heuer and others 2001; Swinnen 2002). Compared with symmetric movements, people are slower to initiate asymmetric movements and exhibit spatial assimilation effects between the 2 trajectories (Heuer and others 2001). The source of these constraints has been extensively debated in the motor control literature. One hypothesis states that processes involved in movement planning and execution are facilitated for spatially symmetric movements through interhemispheric communication (Franz and others 1996; Kennerley and others 2002) or ipsilateral corticospinal pathways (Carson 2005). An underlying assumption here is that the tendency to mirror symmetric movements has evolutionary roots in phylogenetic older behaviors, such as locomotion, and that additional neural processes are needed to modify synergistic tendencies for asymmetric movements. We describe this kind of interference as movement-related conflict. However, the preference for symmetric movements is dependent on the manner in which the actions are cued (Mechsner and others 2001; Swinnen 2002). Diedrichsen and others (2001) had participants perform bimanual reaching movements, with each hand reaching forward or sideways (Fig. 1A). The resulting trajectories could be congruent, in mirror-symmetric directions, or incongruent, in orthogonal directions. Movements were cued either symbolically (a letter indicated the target for each hand) or spatially (the stimuli appeared directly at the target locations). Reaction times (RTs) were considerably slower for incongruent than for congruent movements but only for symbolic cues; with spatial cues, minimal difference in RT was observed. Given that movement planning and execution are similar for both cueing conditions, this dissociation suggests that the RT cost associated with asymmetric movements is mainly due to the conflict related to the translation of the symbolic cues into their associated responses. When 2 different symbolic cues are presented, this translation process is required for each cue and the responses must be assigned to the correct hand. We refer to interference arising at this level as goal-selection conflict. With spatial cues, the movement goals are directly and externally specified, eliminating or minimizing the translation and assignment operations. We used functional magnetic resonance imaging (MRI) to identify neural regions related to goal-selection and movementrelated conflict. Movement-related conflict reflects interactions of movement parameters during planning or execution (Heuer and others 2001). This form of conflict would therefore be associated with higher activation for incongruent than for congruent movements for both symbolic and spatial cues. In particular, we predicted that this form of activity would be observed in the supplementary motor area (SMA), implicated by numerous studies in the production of nonsymmetric bimanual movements (Brinkman 1984; Sadato and others 1997; Ja ncke and others 2000; Debaere and others 2001; Steyvers and others 2003). In contrast, areas required to resolve goal-selection conflict should be more active during incongruent movements compared with congruent movements but only with symbolic cues. This form of conflict may arise in areas associated with the mapping of arbitrary stimuli onto their associated responses. Previous evidence suggests involvement of left parietal (Rushworth and others 2003) and premotor cortex (Grafton and others 1998; Eliassen and others 2003) in this translation process. We will be able to characterize this network for our task by Symbolic Cues ) 800 s m ( e700 m i T ion600 t c a eR500 Left Right Congruent Incongruent unimanual bimanual S + Symbolic cues comparing unimanual and bimanual congruent movements in the symbolic cueing condition with those in the spatial cueing condition. We can then ask whether areas that show goalselection conflict lie within this network. Alternatively, goalselection conflict may engage additional regions, not involved in the translation process itself, such as pre-SMA or the anterior cingulate, which are associated with response conflict in a wide range of tasks (MacDonald and others 2000; Garavan and others 2003; Nachev and others 2005). Materials and Methods Participants Nineteen right-handed participants (Oldfield 1971) (age 18--31 years, 6 men and 13 women) were recruited through advertisements. The data from 4 participants were (...truncated)