Objective: To assess whether a supernumerary robotic digit can enter into the user’s body schema, thereby altering bodily self-representation and proprioceptive awareness.

Background: Stroke is a leading global cause of permanent grasping impairment, with traditional rehabilitation showing limited efficacy. Robotic extra-fingers to compensate for hand motor impairment represent an emerging, promising field bridging neuroscience, robotics, and rehabilitation. A patented wearable oft Sixth Finger (SSF) [1-2]  has helped chronic stroke patients compensate for paretic hand thumb function [3-4]. Despite successful pilot studies, it remains unclear whether the SSF might be actually embodied into the user’s body schema–a fact that would favour either the acceptance of the device and, likely, its daily use.

Method: To measure SSF embodiment, we adapted a proprioceptive drift paradigm typically used to assess embodiment effects in the rubber hand illusion research [5]. Blindfolded participants (n= 40) were asked to locate their hand, which was placed on a table with the forearm perpendicular to its surface, by pointing its position on a millimeter grid paper placed on top of a container that covered the hand. Proprioceptive drift was measured before and immediately after a motor task. Participants repeatedly reached for and grasped an object (e.g. a water bottle) by opening and closing the SSF. The SSF could be worn in such a way as to grasp the object with either the palm or the back of the hand. In a control condition, participants simply raised their hands while opening and closing the SSF, which was worn either on the palm or the back of the hand.

Results: Preliminary results indicate that acting with the SSF, but not simply moving the hand while wearing it, impacts body schema significantly modifying how the hand is represented. Interestingly, the proprioceptive drift was consistent with whether the SSF was worn on the palm or the back of the hand.

Conclusion: If these results were corroborated, our study would provide the first evidence of a hard embodiment of the SSF [6], with a significant impact on self-representation and proprioceptive awareness.. Potential follow-ups to our research concern the possibility of measuring the degree of embodiment in patients during different phases of SSF learning and the possibility of investigating the neural processes and mechanisms that make such embodiment possible.

References: Prattichizzo D et al. Brevetto n. 102016000013321, 2016
Prattichizzo, Domenico, et al. “Human augmentation by wearable supernumerary robotic limbs: review and perspectives.” Progress in Biomedical Engineering 3.4 (2021): 042005.
Hussain, Irfan, et al. “The soft-sixthfinger: a wearable emg controlled robotic extra-finger for grasp compensation in chronic stroke patients.” IEEE Robotics and Automation Letters 1.2 (2016): 1000-1006.
Hussain, Irfan, et al. “Toward wearable supernumerary robotic fingers to compensate missing grasping abilities in hemiparetic upper limb.” The International Journal of Robotics Research 36.13-14 (2017): 1414-1436.
Tosi, G., Mentesana, B., & Romano, D. (2023). The correlation between proprioceptive drift and subjective embodiment during the rubber hand illusion: A meta-analytic approach. Quarterly Journal of Experimental Psychology, 76(10), 2197-2207. https://doi.org/10.1177/17470218231156849 (Original work published 2023).
Makin, Tamar R., Frederique de Vignemont, and Silvestro Micera. “Soft embodiment for engineering artificial limbs.” Trends in Cognitive Sciences 24.12 (2020): 965-968.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/embodying-a-sixth-soft-finger-how-wearing-a-supernumerary-robotic-digit-impacts-our-body-schema/