Influence of pain on human sensorimotor control and learning
Creators: name: Bagnato, Carlo
Publisher: name: Imperial College London
Year: 2017

description: Through pain, contact area can be minimised when handling a hot stove or the hands shifted away from sharp edges whilst carrying a heavy piece of furniture. In light of the importance of pain for humans in the context of haptic exploration and manipulation of objects, this thesis will address fundamental questions on how painful stimuli are processed by humans to produce adequate motor reactions. To attain this end, I have used a robotics framework, as (i) I have developed instrumentation for the investigation of the neural correlates of pain and its effect on human motor behaviour and (ii) examined the control problems solved by the human motor system to prevent or minimise pain during accomplishment of a motor task, from a robotic perspective. In the first part of the thesis, I describe two robotic systems that were developed to provide computer controlled mechanical stimulation while guaranteeing safe and robust stimulation time-locked with data acquisition. An fMRI compatible somatosensory stimulator to investi- gate the brain activity in response to mechanical pain and innocuous touch was developed to investigate the neural correlates of touch and pain. A second robotic stimulator was designed and fabricated to study the influence of pain on motor control and learning. The device can be coupled with robotic interfaces typically employed to investigate human motor control and learning, and it is synchronised with the acquisition of relevant physiological measures like electromyography. The second part of the thesis presents the results from three experiments where the latter robotic stimulator was employed to examine the influence of pain on human motor behaviour. The first study investigated how humans approach a potentially painful object. I found that humans modulate the velocity of their approach towards a potentially painful object based on the probability that the object may be noxious. The higher the probability of the presence of a painful object, the slower they move towards the object, and vice versa. This behaviour suggests that humans find a trade-off between the cost of time and that of pain. In the second experiment, I looked at how humans regulate painful contact forces over time when a total displacement needs to be produced in order to attain a manipulation task. The participants tended to minimise painful contact forces, even if this meant being subjected to pain for longer. With respect to non-painful trials, participants increased their force more slowly, and plateaued at a lower force. Crucially, they were free to take pauses and distribute the painful forces with intermittent pushes, but only exhibited continuous force profiles. In the last experiment, I investigated the effect of pain on the immediate, consolidation and retention stages of motor learning. I tested how subjects learn to modulate their force in a tracking task where physical pain provided feedback of their instantaneous tracking error. While pain feedback did not deteriorate nor improve the early stages of motor learning, pain feedback elicited overnight improvement in sensorimotor task performance, which was retained after two weeks. These results shed light on optimisation mechanisms of human motor control in the context of exploration and manipulation of noxious objects, and further our understanding of how pain influences motor learning. The findings also suggest several motor strategies that could be implemented in robots and prostheses with the aim of minimising or avoiding damage, paving the way for the foundation of the novel field of Pain Robotics.