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Hand-specific specialization of grip force control in bimanual prehension
Ninety percent of humans are right-handed, and this is often construed to mean that the right hand is better than the left at all tasks, in the right-handed individuals. However, we often ignore the important role played by the left-hand when performing certain bimanual tasks. For instance, when slicing a bread loaf, stabilizing the bread with the left hand is as important as slicing it with the right hand. This implies that each hand is dominant in different types of tasks. The influential dynamic dominance theory posits that each hand’s specific dominance arises from the contralateral hemisphere specialization. The dominant (right) arm produces well-coordinated movements because the left hemisphere is superior at predictive control, and the non-dominant (left) arm shows better stabilizing performance because the right hemisphere is superior at impedance control. This theory has been proposed by studying arm movements. However, it is unclear whether the features of this theory extend to grip force control during object manipulation. This is an important gap in our knowledge; identifying the differences in hand-specific control of grip force would improve our understanding of sensorimotor control of skilled bimanual manipulations. Therefore, the goal of my dissertation was to investigate whether the features of dynamic dominance extend to the control of grip forces in bimanual object manipulation.
In this dissertation, I performed two studies to investigate the control of complex, bimanual object manipulation tasks. Participants held an object in each hand. The two objects were connected by a spring. The grip forces of each hand accounted not only for the dynamics of the object that the hand manipulated, but also for the destabilizing forces that arose from the actions of the other hand that were transmitted by the spring. This experimental design mimics ecological tasks like slicing bread to the first degree of approximation. The goals were to determine whether the features of hand-specific specialization of control observed in wrist movements is also evident in the movement of hand-held objects and in the control of grip forces during movement execution (study 1) and planning (study 2). Furthermore, I investigated how task uncertainty interacts with hemispheric specialization in modes of grip forces control in both studies.
In study 1, I investigated whether grip force characteristics differ between hands during an ongoing bimanual manipulation. The right hand produced accurate object movement performance accompanied by stronger modulations in grip force in response to dynamics of loads associated with object movement compared to the left hand. In contrast, the left hand stabilized the object’s position better by exerting a higher grip force on the object. The main contribution of this study is that it suggests that the left hemisphere specializes in predicting dynamics of loads associated with object movements whereas the right hemisphere specializes in ensuring object’s stability by increasing its impedance.
In study 2, I investigated anticipatory modulations in grip force before an impending bimanual object manipulation task. I studied how each hand prepared grip force for the expected increase in load force, thereby uncovering differences in underlying motor planning processes of each hand. Grip force increased in both hands; however, this increase was greater in the left hand. This result indicates that the right hemisphere relies more on impedance control during planning. The main contribution of this study is that it suggests that even the motor planning processes that prepare each hand for an impending motor action are specialized in a way consistent with the predictions of dynamic dominance theory.
This dissertation adds to the existing knowledge about hemispheric specialization of arm movement control and extends it to grip force control. Future studies should focus on how the control of arm movements and grip force interact in each hemisphere. Furthermore, building on this dissertation, future work on pathology- and age-related dexterity loss could potentially lead to better interventions that improve the quality of life in these populations.
- Doctor of Philosophy
- Health and Kinesiology
- West Lafayette