Assessment and enhancement of proprioception

In daily living activities, proprioception is fundamental to interact with the environment and rapidly react to changing circumstances. The ability to coordinate force and position in bimanual tasks is essential for manipulating objects out of view and preventing their slippage. The work carried out in my Ph.D. research project aims to highlight the importance of assessing proprioception in both people with sensorimotor deficits and unimpaired and provide guidelines on how to design an effective supplementary vibrotactile feedback to enhance proprioception and the associated motor outcomes. In the usual formulation of assessment protocols, either in research or clinical environments, position and force sense are mainly evaluated separately while their possible interactions and interference have received less attention. In my Ph.D. research project, I did a step toward filling this gap, identifying the reciprocal interaction between position sense and force control in bimanual tasks performed by unimpaired participants. I found that position sense is influenced by the symmetry of the loading condition, while force control is mostly affected by the position of the non-dominant hand. I also found that this latter result was not determined by handedness, but more likely by the specialization of the brain hemispheres. However, handedness influenced the overall proprioceptive performance since left-handers had a more asymmetrical performance than right-handers. In the neurological assessment protocols commonly used in the clinical practice, proprioceptive functions are mainly assessed subjectively by clinicians referring to qualitative clinical scales. However, reliable methods to quantify proprioceptive deficits are crucial for better enhancing the detection of early symptoms, developing effective neuro-rehabilitative treatments, and monitor the progress of both disease and treatments. Furthermore, after stroke the main focus of clinical assessment is on the contralesional side of the body. Less attention is dedicated to the ipsilesional side and to the bimanual coordination. To this end, in my Ph.D. project, I investigated the position sense deficits of the two upper limbs taking into account also the location of the lesion. I found that the ipsilesional arm of stroke survivors had similar matching accuracy but higher precision than the contralesional arm. The accuracy of the two arms inter-correlated in the left and central regions of the peripersonal space for all the stroke survivors independently of the location of the brain lesion. This findings highlight that after stroke the two arms have different proprioception and motor capabilities. As results, one of the main consequences is a defective bimanual coordination, which impacts the ability to perform many daily living activities. Despite its importance, the current formulation of the neurological assessment and rehabilitative protocol is more focused on unimanual task, limiting the possibility to investigate the interaction and interference that arise from the inter-limb coordination. This could be due to the limited availability of devices to assess the bimanual proprioception. In this context, in my Ph.D. research project I optimized a device to assess proprioception and the reciprocal interaction between position sense and force control in bimanual tasks. Its usability has been tested on stroke survivors, which performed force matching tasks and a lifting task. In the matching task, I found that the stroke survivors had higher difficulty to match a level of force required, even when it was tailored on their capability, while their ability to maintain the force was not affected. In the lifting task, I found that stroke survivors applied more force than age-matched unimpaired participants to lift the device. However, the timing in which the force was applied was not significantly different between the two populations. Due to impact of the proprioceptive deficits, several solutions have been proposed to mitigate them and enhance the related motor outcome. Among all, the application of supplementary somatosensory feedback has been shown to be an effective resource to enhance sensorimotor ability in unimpaired participants as well as in people with sensorimotor deficits. This type of feedback is an strong modulator of plasticity, enhances motor (re-)learning and control, and can also temporally reduce position sense disorders. However, how to convey the proprioceptive information in the supplementary feedback (i.e., encoding method) and the importance of the information conveyed by the feedback (i.e., informational content) are not well investigated. In this context, my Ph.D. research project aims at deepening the actual knowledge on how to encode information in supplementary vibrotactile feedback to enhance proprioception and related motor outcomes. To this end, I compared the effects on postural control of two methods for encoding the amplitude and direction of the acceleration of the body center of mass in the activation of two vibration motors placed on the back and on the abdomen of the participants. I also evaluated the importance of the informational content of the feedback by applying a vibrotactile feedback that was uncorrelated with the actual oscillations of the center of mass. The results show that synchronized vibrotactile feedback significantly reduced the sway amplitude while increasing the frequency in anterior-posterior and medial-lateral directions. The presence of uncorrelated vibration, instead, increased the sway amplitude, highlighting the importance of the informational content. In a second study, I tested the effects of applying two types of supplemental vibrotactile feedback on the ipsilesional arm in stroke survivors while they performed goal-directed movements with their contralesional arm. I found that all the three stroke survivors were able to perceive the vibrotactile feedback and to perform the motor tasks (i.e., reaching and stabilization) when it was applied, but they reached various levels of capability in distinguishing and using it during the motor tasks. Indeed, all of them improved their performance in the stabilization task using one encoding scheme. The stroke survivor with the better sensory assessment score also improved in the reaching performance when one supplemental feedback was applied. These preliminary results encourage investigating the effects of a longer multi-session training with a personalized vibrotactile feedback design. The findings of my Ph.D. research project enlarge the actual knowledge on interaction between the upper limb position sense and force control, and its asymmetries related to handedness and how it is affected after stroke. My Ph.D. research project also provide evidences to support the need to a assess both ipsilesional and contralesional proprioceptive deficits separately and concurrently during bimanual tasks.