The field of robotics has long served society in the development of prosthetics to compensate for the absence of body parts. Now, scientists are turning their attention to how the human body may be enhanced, rather than simply maintained, through the use of robotic limbs. These enhancements could result in increased abilities such as strength, dexterity, and the reduction of fatigue.
Meraz et al. investigated the possibility of adding a sixth robotic digit to the human hand as an enhancement. The addition of this second thumb is intended to increase the grasping range of the hand, as well as increase the ability of the user to perform activities with only one hand. The robotic thumb not only works as an extra digit, but also gives somatosensory feedback to the user. Somatosensory feedback (a type of feedback received by the brain concerning pain, touch, pressure, temperature, position, vibration, and movement) increases the sense of embodiment for the user, as well as performance levels of the prosthetic.
The robotic thumb is controlled via the thumb of the opposite hand. Contact information is conveyed through electrostimulation to the opposite thumb tip. The researchers compared users’ abilities to touch specified digits with the robotic thumb on the left hand, with and without the ability to see that hand.
These experiments tested the level of embodiment of the sixth digit as well as the self-perception of the thumb controlling the system. The users were also asked to point to the tip of the right thumb before and after the experiment. This was done in order to assess how the users’ self-perception of their existing body schema may have been affected.
The results of these experiments indicate that the sixth finger is incorporated into the body schema, and that the body schema of the controlling thumb is also modified. This may imply that the human brain is able to adapt to these additional robotic enhancements.
The users with no visual feedback had a slightly better performance, indicating that their reliance on tactile information helped them to visualize what needed to be done before executing the movement. It was found that the plasticity of the brain modified the existing body schema through the course of the experiment, as users improved their task results a day at a time.
The results of this research have potential medical implications. It is indicated that it may be possible to help certain patients, such as those recovering from cervical injuries or stroke, to achieve a new body schema through sacrificing the body schema that is no longer in use. Supernumerary limbs may also be used to extend the body schema of the user.