Redefining the Human Body Boundaries: The Impact of Assistive Devices on Body Representation

Introduction: Exoskeletons and other assistive devices have shown great potential in improving the quality of life of patients with spinal cord injury by enabling their independence, promoting plastic reorganisation of body representations, and improving body ownership and sense of agency. In SCI patients who participated in EXO training, our study aimed to explore: the functional embodiment of wheelchair and exoskeleton devices; and how loss of sensory sensitivity and motor function affects the representation of body parts affected (lower limbs) and spared (upper limbs) by the injury. Methods: The study involved 20 male participants with SCI lesions ranging from T4 to L2 who underwent an EXO training protocol. Measurements were taken at baseline (T0) and after 2 months (T1, 10/14 training sessions). 20 matched healthy participants were recruited as a control group. A mental rotation task of biological (hands/feet) and non-biological (wheelchair/exoskeleton) stimuli presented on a screen in 4 different orientations (0°, 90°, 180°, 270°) was administered. Subjects had to verbally indicate the laterality (right or left) of the presented stimulus. Accuracy and Reaction times (RTs) were collected. Participants were asked to complete the Mental Rotation Questionnaire to inspect strategies adopted during task execution and subjective opinions on the presented stimuli. Results: Mixed ANOVAs on RTs considering between-group differences and within factors (type of stimulus, measurement time) were conducted. General RTs did not differ significantly between groups (p=0.42). There was a significant group x stimulus interaction (p=0.02). Healthy controls performed slower RTs than SCI in wheelchair rotation (p=0.03). Within-group differences in SCI highlighted a significant effect of time (p=0.047) in the exoskeleton mental rotation, with faster RTs at T1 compared to T0 (PRE=1068, POST=968). The significant effect of PRE/POST (p=0.02) indicates that the time required to rotate exoskeleton images is faster immediately after the training, indicating a brief-term plasticity. Strategies adopted in mental rotation task did not differ significantly between groups (p=0.42). Discussion:The integration of assistive devices into the users' body schema can be reflected in the mental rotation of non-biological stimuli. If stimuli are related to familiar or 'embodied' objects, they may be processed as bodily parts, leading to faster responses. Lower response times after training suggest short-term adaptation of body representation promoting the need for new research efforts in efficacy and acceptance of assistive devices.