In the typically developed brain, numerous networks and regions are involved in sensing, actuating and understanding hand functions. In this thesis I take a neurocognitive approach to provide a better understating of how prosthesis users harness hand-specific resources to support their prosthetic limb (embodiment). In a series of cognitive, motor and neural studies, I test two distinct populations: congenital one-handers who are born without a hand and acquired amputees who lost their hand in adulthood, to investigate the processes underlying upper-limb prosthesis use.
First, available hand-related resources were assessed in each of the tested populations using a visual hand laterality task. Congenital one-handers were less efficient than amputees in processing visual hand information, possibly due to atypical available motor hand resources. This finding reaffirms the notion of 'embodied cognition' – that our motor repertoire shapes visual perception and cognition. This was further supported by a significant relationship between amputees’ ability to move their phantom hand and their visual hand processing efficiency. Within the embodiment framework presented in this thesis, having different hand resources could potentially lead to different capabilities in prosthetic-limb use.
Next, using fMRI brain decoding analysis, I probed the neural representation of prostheses in relation to hands and tools, within the visual cortex. Increased levels of prosthesis use were associated with a formation of a novel neural visual ‘prosthesis’ category. This experience-dependent plasticity was observed in both congenital one-handers and amputees, pointing towards high levels of flexibility in occipitotemporal cortex. These findings challenge the naïve notion of neural embodiment, since prosthesis representation did not become more similar to hand representation.
Finally, the control dynamics of prosthetic-arms were assessed using several reaching and localisation tasks. Motor reaching accuracy with a prosthesis was found to depend on early life experience with a hand (either prosthetic or biological). Specifically, the ability to efficiently perform visually guided reaches with a prosthesis is impaired in the absence of early experience. These findings point towards a possible reliance of prosthesis use on neural hand-specific processes and provide evidence for a sensitive developmental period for sensorimotor control.
Taken together, this thesis provides a comprehensive overview of the multifaceted phenomenon of embodiment, shedding light on the underlying processes supporting human-machine interactions, and expanding our understanding of the development and plasticity of body representation.</p
