Laterality of the command center in relation to handedness and simple reaction time: a clinical perspective

Evidence for right-hand feeding biases in a left-handed population

We have recently shown that actions with similar kinematic requirements, but different end-state goals may be supported by distinct neural networks. Specifically, we demonstrated that when right-handed individuals reach-to-grasp food items with intent to eat, they produce smaller maximum grip apertures (MGAs) than when they grasp the same item with intent to place it in a location near the mouth. This effect was restricted to right-handed movements; left-handed movements showed no difference between tasks. The current study investigates whether (and to which side) the effect may be lateralized in left-handed individuals. Twenty-one self-identified left-handed participants grasped food items of three different sizes while grasp kinematics were captured via an Optotrak Certus motion capture array. A main effect of task was identified wherein the grasp-to-eat action generated significantly smaller MGAs than did the grasp-to-place action. Further analysis revealed that similar to the findings in right-handed individuals, this effect was significant only during right-handed movements. Upon further inspection however, we found individual differences in the magnitude and direction of the observed lateralization. These results underscore the evolutionary significance of the grasp-to-eat movement in producing population-level right-handedness in humans as well as highlighting the heterogeneity of the left-handed population.

Manual asymmetries in the kinematics of a reach-to-grasp action

In the present study, we manipulated the perceived demand of an ecologically valid task to investigate the possible presence of manual asymmetries in a reach-to-grasp action. Participants reached, grasped and sipped from a water glass under low (nearly empty) and high (nearly full) demand conditions. Participants reached to grasp in closed-loop, open-loop and delay visual conditions. Manual asymmetries were found in movement time, peak velocity and maximum grip aperture variability. Consistent with reach-to-point literature: (1) right-handed actions were completed in less time than left-handed actions in visually and memory-guided conditions; (2) right-handed movements were more accurate (i.e., produced more consistent maximum grip apertures) than left-handed movements in visually guided conditions. The results support a theory of left-hemisphere specialization for visual control of action.

Does bimanual grasping of the Müller-Lyer illusion provide evidence for a functional segregation of dorsal and ventral streams?

Studies claiming a differential processing of visual illusions for perception and action have been subjected to many challenges. One criticism is that attentional demands were mismatched between the perception and action tasks. Dewar and Carey (2006) reexamined this argument by comparing bimanual grasping to bimanual size estimation and concluded that manual size estimation (ManEst) was affected by the illusion to a greater extent than grasping, supporting the case for two functionally distinct streams of visual processing. We tested whether this result may be due to their use of closed loop visual conditions by replicating their study under both closed and open loop conditions. We found that the difference in illusion effects between grasping and ManEst disappeared under open loop conditions, indicating that Dewar and Carey's findings can be explained by the availability of visual feedback and not a perception/action dissociation. We also discuss potential shortcomings of bimanual designs.

Grasping for parsimony: do some motor actions escape dorsal processing?

It is an open question whether the visual transformations guiding human actions are similar to those generating visual perception. The Action-Perception model assumes a strict division of labor: the ventral cortical stream generates perception while the dorsal stream guides actions. However, only skilled and natural actions are assumed to be under dorsal control, while awkward and left-handed actions should be under ventral control in the same way as perception. Here, we used a combination of Garner-Interference and the psychological refractory period (PRP) paradigm to test this notion. We found that all types of grasping (left-handed, awkward, using a tool) behave in a way similar to skilled right-handed grasping: other than perception they show no Garner-Interference, but similar to perception they show a limitation of processing capacities as indicated by the PRP paradigm. This behavior suggests that similar processes guide all these actions.

Movement structure in young and elderly adults during goal-directed movements of the left and right arm

Elderly adults often exhibit performance deficits during goal-directed movements of the dominant arm compared with young adults. Recent studies involving hemispheric lateralization have provided evidence that the dominant and non-dominant hemisphere-arm systems are specialized for controlling different movement parameters and that hemispheric specialization may be reduced during normal aging. The purpose was to examine age-related differences in the movement structure for the dominant (right) and non-dominant (left) during goal-directed movements. Young and elderly adults performed 72 aiming movements as fast and as accurately as possible to visual targets with both arms. The findings suggest that previous research utilizing the dominant arm can be generalized to the non-dominant arm because performance was similar for the two arms. However, as expected, the elderly adults showed shorter relative primary submovement lengths and longer relative primary submovement durations, reaction times, movement durations, and normalized jerk scores compared to the young adults.

Differentiating between two models of motor lateralization

This study was designed to differentiate between two models of motor lateralization: "feedback corrections" and dynamic dominance. Whereas the feedback correction hypothesis suggests that handedness reflects a dominant hemisphere advantage for visual-mediated correction processes, dynamic dominance proposes that each hemisphere has become specialized for distinct aspects of control. This model suggests that the dominant hemisphere is specialized for controlling task dynamics, as required for coordinating efficient trajectories, and the nondominant hemisphere is specialized for controlling limb impedance, as required for maintaining stable postures. To differentiate between these two models, we examined whether visuomotor corrections are mediated differently for the nondominant and dominant arms. Participants performed targeted reaches in a virtual reality environment in which visuomotor rotations occurred in two directions that elicited corrections with different coordination requirements. The feedback correction model predicts a dominant arm advantage for the timing and accuracy of corrections in both directions. Dynamic dominance predicts that correction timing and accuracy will be similar for both arms, but that interlimb differences in the quality of corrections will depend on the coordination requirements, and thus, direction of corrections. Our results indicated that correction time and accuracy did not depend on arm. However, correction quality, as reflected by trajectory curvature, depended on both arm and rotation direction. Nondominant trajectories were systematically more curvilinear than dominant trajectories for corrections with the highest coordination requirement. These results support the dynamic dominance hypothesis.