Although tremendous resources are being devoted to the development of ground-breaking WR technologies, little attention has been afforded to how the human brain might support it. To address the question of technological embodiment empirically and rigorously, we took a combined approach for substitution and augmentation.
Prosthetic limbs provide a strong test case for researching the embodiment of WR technologies. In our project we tested the relationship between successful prosthesis usage in individuals with congenital and acquired hand loss and a range of experimental parameters, aimed at probing hand representation at multiple experiential, behavioural and neural levels (embodiment). On average, we find that prosthesis do not feel like a body part (Maimon-Mor et al., 2020). Nevertheless, those individuals who use their prosthesis more productively in daily life also tended to report experiencing stronger sense of embodiment. Moreover, individuals who use their prosthesis more in daily-life show greater activity in hand-selective visual areas, as well as greater functional coupling between the visual and sensorimotor hand areas (van den Heiligenberg et al., 2018). However, despite benefiting from hand-selective cortical resources, the prosthesis is not necessarily ‘embodied’ into people’s body representation. Instead, prosthesis users represented their own prosthesis more dissimilarly to hands, compared to controls, challenging the traditional view of prosthesis embodiment (Maimon-Mor & Makin, 2020). This result highlight new and exciting opportunities for recycling neural resources devoted to the body to enable engineering solutions that are not strictly biomimetic (known as soft embodiment; Makin, de Vignemont, & Micera, 2020).
We next studied how to best merge augmentation technology with the human body and mind. For this purpose, we teamed up with Dani Clode, the Designer of the Third Thumb. We investigated whether successful and intuitive motor augmentation with an extra robotic thumb can be achieved, and what its implications are for the representation and function of the biological hand. We ran a series of longitudinal studies where individuals were trained to develop hand-robot interactions, including both lab-based and unstructured daily usage. Our studies demonstrate that successful integration of motor augmentation can be readily achieved in healthy participants, with the potential of flexible use, reduced cognitive reliance and an increased sense of embodiment of the device (Kieliba et al., 2021). This was, in part, due to surrogate somatosensory information being recruited from the controlling toes to inform the motor command to move the Third Thumb (Amoruso et al., 2022). Importantly, we showed that motor integration of the device resulted in a changed natural hand use at the behavioural level and an altered representation of the biological hand at the neural level (Kieliba et al., 2021). This latter finding is of major significance as it indicates that successful human-robot integration may have consequences for certain aspects of biological body representation and motor control.
Together, our research demonstrates that embodiment of an artificial limb is not trivial. Although, in principle, opportunities exist for harnessing hand neural and cognitive resources to control artificial limbs, the brain does not assimilate neural representations for the artificial limb with those for the biological body, creating opportunities for nonbiomimetic technological interfaces.
