Title: Transfer and Generalization of Learned Manipulation between Unimanual and Bimanual Tasks
Description: Successful grasping and dexterous object manipulation relies on the ability to form internal representations of object properties that can be used to control digit kinetics and kinematics. Sensory cues and sensorimotor experience enable the updating of these internal representations. Aside from the weight of the object, the center of mass of the object results in object torque that needs to be represented and compensated for. In order to counter object torque, digit forces and centers of pressure are modulated to generate a compensatory moment to prevent object roll. Generalization studies can be used to examine whether this learning is represented on a low effector-specific level or a high task-specific level. Previous studies have shown that the internal representation of object torque does not generalize after object rotation or contralateral hand switch suggesting an effector level of representation. However, it has been shown that switching from two to three digits and vice versa does lead to full generalization suggesting a high level representation in certain circumstances.

Thus, an understanding of whether learned manipulation would generalize when adding or removing the number of degrees of freedom and effectors would provide more information on these levels of representation. We asked 30 participants to lift a visual symmetrical object with an asymmetrical center of mass. Participants lifted the object 10 times in one grasp type (right hand unimanual, bimanual, or left hand unimanual). Following that, they switched to another grasp type and lifted the object another 10 times. Through various different orders of these transfer blocks, we examined their ability to generalize between unimanual and bimanual grasping by comparing the pre- and post-transfer trials. Our results show the partial generalization of learned manipulation when switching between unimanual and bimanual grasps. This is shown from the reduction in peak roll after transfer compared to novel trials and the generation of compensatory moments in the appropriate direction (but insufficient magnitude) after transfer.

Moreover, after transfer to the right hand unimanual and bimanual grasps, moment generation was driven by digit center of pressure modulation while transfer for left hand unimanual grasps was driven by load force modulation. In addition, we also show failed generalization after contralateral hand switch as evidenced by large post-transfer rolls and minimal moments. We suggest that learned manipulation of object torque is a high level of representation but that this representation can only be accessed by either digit kinematics or kinetics, depending on the hand used.