Bimanual coordination in children: manipulation of object distance.

The performance of many everyday activities requires the coordination of the two upper limbs to achieve the goal of the task. Although bimanual performance has been studied in detail in adults, few studies have examined how children coordinate the movements of the two hands during symmetric and asymmetric bimanual prehension. With the current study, we asked younger (4-6 years, n = 14) and older (7-10 years, n = 16) children to complete a discrete bimanual task. Specifically, they reached to grasp cylinders located at near and far positions in either unimanual or bimanual condition. During bimanual symmetric conditions, participants performed movements with both hands toward two objects located at the same distance (both near or both far), while in the bimanual asymmetric conditions, they reached for objects at different distances. Results of the kinematic analyses indicated that the young children consistently experienced the "two target" effect, whereby bimanual movements were executed more slowly than unimanual movements to the same distance. Older children employed a hybrid strategy, exhibiting slower movements in bimanual congruent conditions, but larger non-dominant apertures in bimanual incongruent conditions. This hybrid strategy was hypothesized to stem from developmental changes occurring in the integration of sensory information around 8 years of age. While older children exhibited temporal and spatial coordination patterns that were similar to patterns reported in adults, large relative timing differences at the start and end of bimanual movements and considerably weaker spatial coupling were seen in the younger children.

Bimanual coordination in children: manipulation of object size.

An experiment was designed to investigate the temporal and spatial couplings of the transport and grasp components for bimanual movements performed by children. Thirty-one participants aged 4-6 (younger) and 7-10 (older) performed the unimanual task of reaching for, grasping, and lifting a small or large cylinder with the right or left hands or the bimanual task of reaching for, grasping and lifting two small cylinders, two large cylinders, or one small and one large cylinder with the right and left hands. Kinematic measures, relative timing differences between the hands, spatial plots and cluster analysis were used to quantify both temporal and spatial couplings of the limbs. While average kinematic results indicated that children in the 4-6 and 7-10 age range performed bimanual movements similarly to each other, spatio-temporal coupling measures indicated that the younger children performed the bimanual movements in a more sequential (serial) fashion. Kinematic results also indicated that the cost of the increase in task complexity normally seen in adults when grasping two targets bimanually compared to a single target unimanually are not consistently present for children. Instead, the cost associated with increases in task complexity appear to be mediated by whether the bimanual task imposes significantly greater demands on attentional processes. These results indicate that attention demands of the task as well as the intrinsic dynamics of the individual determine the degree of interlimb coupling of children during bimanual reach-to-grasp of different-sized objects.

Temporal coordination during bimanual reach-to-grasp movements: the role of vision.

The performance of bimanual movements involving separate objects presents an obvious challenge to the visuo-motor system: Visual feedback can only be obtained from one target at a time. To overcome this challenge overt shifts in visual attention may occur so that visual feedback from both movements may be used directly (Bingham, Hughes, & Mon-Williams, 2008; Riek, Tresilian, Mon-Williams, Coppard, & Carson, 2003). Alternatively, visual feedback from both movements may be obtained in the absence of eye movements, presumably by covert shifts in attention (Diedrichsen, Nambisan, Kennerley, & Ivry, 2004). Given that the quality of information falls with increasing distance from the fixated point, can we obtain the level of information required to accurately guide each hand for precision grasping of separate objects without moving our eyes to fixate each target separately? The purpose of the current study was to examine how the temporal coordination between the upper limbs is affected by the quality of visual information available during the performance of a bimanual task. A total of 11 participants performed congruent and incongruent movements towards near and/or far objects. Movements were performed in natural, fixate-centre, fixate-left, and fixate-right vision conditions. Analyses revealed that the transport phase of incongruent movements was similar across vision conditions for the temporal aspects of both the transport and grasp, whereas the spatial aspects of grasp formation were influenced by the quality of visual feedback. We suggest that bimanual coordination of the temporal aspects of reach-to-grasp movements are not influenced solely by overt shifts in visual attention but instead are influenced by a combination of factors in a task-constrained way.

Manual asymmetries in bimanual prehension tasks: manipulation of object size and object distance.

Two experiments were designed to investigate the temporal and spatial couplings of the transport and grasp components for bimanual movements to both congruent and incongruent targets. We studied conditions where task requirements were largely different for the two hands. Ten participants performed Experiment 1 and were required to reach for, grasp, and lift two small (1 mm) cylinders, two large (70 mm) cylinders, or one small and one large cylinder with the right and left hands. In Experiment 2, 10 participants were required to reach for, grasp, and lift two objects that were positioned either near (50mm) the start mark, far (maximum comfortable reaching distance) from the start mark, or one near and one far from the start mark. Kinematic measures, relative timing differences between the hands and spatial plots were used to quantify both temporal and spatial couplings of the limbs. For temporal coupling, the results from both experiments indicated that the upper limbs were controlled independently with some execution-level interference occurring for the transport component only. In terms of spatial coupling our results indicated weak coupling of the grasp component regardless of task parameters (i.e., congruent or incongruent movements) and a dependence on task parameters in determining the level of spatial coupling for the transport component. These results can be collectively interpreted as evidence for a functional coupling of the upper limbs. That is, the movements of the hands may be coupled during tasks in which temporal and spatial synchronizations are beneficial for performance. However, if the coupling of the upper limbs is either unimportant or perhaps even detrimental to the coordination of the overall movement, then the upper limbs may perform the desired movements independently of one another.