People have long been fascinated by the opportunities given by augmentation. This vision not only depends on technological innovations, but also critically depends on the ability of our brain to learn, adapt and interface with augmentation devices. Here we investigated whether successful motor enlargement with an additional robotic thumb can be achieved and what its implications are on the neural representation and function of the biological hand. Able body participants were trained to use an extra robotic thumb (called the Third Thumb) for 5 days, including daily use in a laboratory and unstructured. We challenged participants to perform normally dual-handed tasks using only the augmented hand and examined their ability to develop hand-robot interactions. Participants were tested on various behavioral and brain imaging tests, designed to interrogate the representation of the augmented hand before and after the training. Training improved Third Thumb motor control, agility, and hand-robot coordination, even when cognitive load increased or when vision was closed. It also resulted in an increased sense of incarnation over the Third Thumb. Therefore, an increase affected key aspects of manual representation and motor control. Third thumb use weakened natural cinematic synergies of the biological hand. In addition, brain decoding revealed a mild collapse of the motor representation of the augmented hand after training, even while the Third Thumb was not worn out. Together, our findings demonstrate that motor augmentation can be easily achieved, with the option for flexible use, reduced cognitive dependence, and increased sense of incorporation. However an increase may occur changes to the biological representation of hands. Such neurocognitive consequences are crucial for successful implementation of future augmentation technologies.
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