Body and action in patients with central nervous system injuries

The central nervous system supports a wide range of sensory, motor, and cognitive abilities that are inextricably linked to our body - a body that we inhabit, perceive and control and that allows us to interact with the world around us, primarily through movement. Spinal cord injury or brain damage can severely impair our ability to act, the 'glue' that binds body and mind, profoundly affecting our bodily self and sensorimotor interactions. Through four different experimental studies, this thesis aims to deepen our understanding of the mechanisms underlying body representation, motor control and action perception, particularly in relation to emotional content. The first study examines the role of interoceptive and exteroceptive signals in maintaining body awareness following thoracic spinal cord injury using the rubber hand illusion paradigm and a heartbeat monitoring task. Results reveal that individuals with high thoracic injuries exhibit greater malleability of body ownership, reduced accuracy in perceiving interoceptive signals, and a greater sense of detachment from their body compared to those with lower injuries. The second and third studies focus on apraxia. A systematic review of lesion studies in stroke patients reveals that insular damage is associated with deficits in imitation and pantomime. This finding can be explained by the role of the insula in integrating sensorimotor information, which is crucial for body representation and motor awareness processes. The third study investigates the influence of emotions expressed by specific body parts on action recognition in patients with apraxia. Results indicate that emotional content improves the detection of actions performed with the face and body in patients with bucco-facial and limb apraxia, respectively. Furthermore, voxel-based lesion symptom mapping reveals that emotional processing can occur independently of motor perception within the frontal areas of the action observation network. Finally, the fourth study assesses pupil dilation as a physiological response to the observation and imitation of gestures based on their meaning and spatial direction (towards or away from the body) in a healthy population. Results indicate that observing meaningful gestures modulates autonomic activation, with increased sympathetic activity for gestures associated with mental states or self-care. Individual differences in empathy and prosocial tendencies further influence physiological responses. Interestingly, the imitation condition did not produce significant differences compared to passive observation. These findings provide new insights into the neurocognitive mechanisms underlying body awareness and motor cognition, with potential implications for developing targeted therapeutic strategies following central nervous system injury. Specifically, the results underscore the importance of interoceptive and visual information for maintaining a stable body representation after spinal cord injury. They also highlight that insular damage is associated with symptoms of apraxia and not as a mere consequence of middle cerebral artery infarction. The last two studies demonstrate the complex interaction between actions and their communicative (symbolic or emotional) value in gesture processing.