Neuroprostheses for Restoring Sensation

Benefits

Embodiment or incorporation is the experience of incorporating the prosthesis into one’s body image and actually perceive the prosthesis as part of themselves ( ). Without incorporation, a prosthesis is an artificial tool attached to the end of the residual limb. When embodied, the prosthesis feels either like it is their hand or that their phantom is directly in the prosthesis ( ). Embodiment provides the individual the sense of being whole again. Practically, the perception of their hand performing a task is easier to control than an attached, disembodied tool ( ). Techniques for promoting embodiment include mirror box therapy ( ), robotic therapies ( ), and task-oriented focus on use of the prosthetic rather than the prosthesis itself ( ). These techniques demonstrated improvement, but without sensation, the visual experience of the prosthesis interacting with the environment is incongruous with the perceived experience, thereby disrupting the sense of embodiment ( ). Even one’s own body will feel disembodied and foreign with the loss of sensation, such as in hemi-neglect following stroke ( ). Functional magnetic resonance imaging (fMRI) studies, show that sensation is related to restoring a sense of embodiment following limb loss ( ).

Somatosensation is necessary for arbitrary control of sophisticated and complex prostheses, especially with the additional goal of reducing, not increasing, reliance on vision. Somatosensation is important to normal motor function ( ). Study of a single degree of freedom to perform a task with able-bodied subjects shows that proprioception is necessary to improve performance even when vision is present and is more important as the task difficulty increases ( ). The importance of somatosensation and motor function is evident in other disorders ( ) such as stroke ( ) and large axon neuropathies resulting in lost somatosensation ( ). Even with a normally functioning motor system of an intact hand, patients with large axon neuropathies have difficulty controlling their otherwise intact limb. This results in high cognitive load and poor performance. Even the cortical representations of motor commands are degraded with the lack of somatosensory feedback ( ). Therefore, it is not very likely that complex prostheses with highly anthropomorphic mechatronics will function well without somatosensation. Given that increasing task complexity has increasing reliance on somatosensation, the significant investment in mechatronics would paradoxically provide less value and function to the user as the added degrees of freedom become exceedingly difficult for the user to control. Even the most complex hands will essentially be preprogrammed, patterned movement.

The psychological impact of limb loss is debilitating. Depression and posttraumatic stress are common following traumatic limb loss ( ), correlated with long-term psychosocial adjustment ( ), and more prevalent following upper extremity amputation than lower extremity amputation ( ). Psychotherapy is recommended care following limb loss ( ). Depression can lead to self-destructive behavior ( ), disengagement from society ( ), lost productivity ( ), pharmacological dependence ( ), and stress on family units ( ). Depending on the cultural background, limb loss makes one feel defective or less of a human. The lack of tactile interaction with a family member results in a feeling of disconnection and lack of intimacy ( ). Use of a prosthesis provides a sense of normalcy and improvement in psychosocial interaction as users reported a reduced awareness of being different ( ). Since restoring sensation is expected to increase embodiment and connection, it is expected to further alleviate psychosocial effects of traumatic limb loss.

Pain is often associated with limb loss ( ). The physical and neurological cause of pain is complex and an active area of research. There is evidence that underlying dysfunction of somatosensation is related to the chronic pain of complex regional pain syndrome ( ). Pain, however, is a complex phenomenon with many interrelated causes that can range from being neurogenic from activity of pain fibers within a sensitive neuroma and other peripheral activity ( ), to changes in central connectivity resulting from loss of sensory input ( ) ( Fig. 103.1 ). Pain can result in reduced quality of life ( ) including loss of productivity, further deepening depression, and additional medication costs with associated risks of dependency ( ).