Dalia De Santis

Robot-assisted training of the kinesthetic sense: enhancing proprioception after stroke

Proprioception has a crucial role in promoting or hindering motor learning. In particular, an intact position sense strongly correlates with the chances of recovery after stroke. A great majority of neurological patients present both motor dysfunctions and impairments in kinesthesia, but traditional robot and virtual reality training techniques focus either in recovering motor functions or in assessing proprioceptive deficits. An open challenge is to implement effective and reliable tests and training protocols for proprioception that go beyond the mere position sense evaluation and exploit the intrinsic bidirectionality of the kinesthetic sense, which refers to both sense of position and sense of movement. Modulated haptic interaction has a leading role in promoting sensorimotor integration, and it is a natural way to enhance volitional effort. Therefore, we designed a preliminary clinical study to test a new proprioception-based motor training technique for augmenting kinesthetic awareness via haptic feedback. The feedback was provided by a robotic manipulandum and the test involved seven chronic hemiparetic subjects over 3 weeks. The protocol included evaluation sessions that consisted of a psychometric estimate of the subject’s kinesthetic sensation, and training sessions, in which the subject executed planar reaching movements in the absence of vision and under a minimally assistive haptic guidance made by sequences of graded force pulses. The bidirectional haptic interaction between the subject and the robot was optimally adapted to each participant in order to achieve a uniform task difficulty over the workspace. All the subjects consistently improved in the perceptual scores as a consequence of training. Moreover, they could minimize the level of haptic guidance in time. Results suggest that the proposed method is effective in enhancing kinesthetic acuity, but the level of impairment may affect the ability of subjects to retain their improvement in time.

Strategy Switching in the Stabilization of Unstable Dynamics

In order to understand mechanisms of strategy switching in the stabilization of unstable dynamics, this work investigates how human subjects learn to become skilled users of an underactuated bimanual tool in an unstable environment. The tool, which consists of a mass and two hand-held non-linear springs, is affected by a saddle-like force-field. The non-linearity of the springs allows the users to determine size and orientation of the tool stiffness ellipse, by using different patterns of bimanual coordination: minimal stiffness occurs when the two spring terminals are aligned and stiffness size grows by stretching them apart. Tool parameters were set such that minimal stiffness is insufficient to provide stable equilibrium whereas asymptotic stability can be achieved with sufficient stretching, although at the expense of greater effort. As a consequence, tool users have two possible strategies for stabilizing the mass in different regions of the workspace: 1) high stiffness feedforward strategy, aiming at asymptotic stability and 2) low stiffness positional feedback strategy aiming at bounded stability. The tool was simulated by a bimanual haptic robot with direct torque control of the motors. In a previous study we analyzed the behavior of naïve users and we found that they spontaneously clustered into two groups of approximately equal size. In this study we trained subjects to become expert users of both strategies in a discrete reaching task. Then we tested generalization capabilities and mechanism of strategy-switching by means of stabilization tasks which consist of tracking moving targets in the workspace. The uniqueness of the experimental setup is that it addresses the general problem of strategy-switching in an unstable environment, suggesting that complex behaviors cannot be explained in terms of a global optimization criterion but rather require the ability to switch between different sub-optimal mechanisms.

Proprioceptive Bimanual Test in Intrinsic and Extrinsic Coordinates

Is there any difference between matching the position of the hands by asking the subjects to move them to the same spatial location or to mirror-symmetric locations with respect to the body midline? If the motion of the hands were planned in the extrinsic space, the mirror-symmetric task would imply an additional challenge, because we would need to flip the coordinates of the target on the other side of the workspace. Conversely, if the planning were done in intrinsic coordinates, in order to move both hands to the same spot in the workspace, we should compute different joint angles for each arm. Even if both representations were available to the subjects, the two tasks might lead to different results, providing some cue on the organization of the “body schema”. In order to answer such questions, the middle fingertip of the non-dominant hand of a population of healthy subjects was passively moved by a manipulandum to 20 different target locations. Subjects matched these positions with the middle fingertip of their dominant hand. For most subjects, the matching accuracy was higher in the extrinsic modality both in terms of systematic error and variability, even for the target locations in which the configuration of the arms was the same for both modalities. This suggests that the matching performance of the subjects could be determined not only by proprioceptive information but also by the cognitive representation of the task: expressing the goal as reaching for the physical location of the hand in space is apparently more effective than requiring to match the proprioceptive representation of joint angles.

Testing proprioception in intrinsic and extrinsic coordinate systems: is there a difference?

An intact position sense is considered important for neuromotor recovery, but the available methods and protocols for its assessment are still limited. In the clinical practice it is generally tested trough a bimanual position matching test, that consists of replicating with one arm the angular positions of the other arm in space (intrinsic coordinates matching). However, the same test could be carried out by matching the hand location in space (extrinsic coordinates matching). Is there any difference between the procedures that may be relevant to the evaluation of position sense deficits? In this study we compared the performance of eight right handed subjects and two stroke survivors with left hemiparesis performing the test in the two conditions. A robotic manipulandum passively moved the left arm of the participants in twenty-four positions in the workspace. Subjects had to match the left arm position with their right arm either in intrinsic or extrinsic coordinates. The results show that all the subjects (impaired and controls) performed better when using the extrinsic paradigm.

A new method for evaluating kinesthetic acuity during haptic interaction

SUMMARY Although proprioceptive impairment is likely to affect in a significant manner the capacity of stroke patients to recover functionality of upper limb, clinical assessment methods currently in use are rather crude, with a low level of reliability and a limited capacity to discriminate the relevant features of this severe deficit. In the present paper, we describe a new technique based on robot technology, with the goal of providing a reliable, accurate, and quantitative evaluation of kinesthetic acuity, which can be integrated in robot therapy. The proposed technique, based on a pulsed assistance paradigm, has been evaluated on a group of healthy subjects.

Pulsed assistance: A new paradigm of robot training

In this preliminary study we compare continuous with pulsed robot assistance in five chronic stroke survivors with a mild degree of spasticity, with the aim of promoting volitional effort and reducing assistance during a reaching task. The protocol consists of one familiarization session and a single training session during which a manipulandum provides subjects with pulsed or continuous assistance in random order. The basic level of assistive force is calibrated for each subject and is the same for both modalities; however, the average force during continuous assistance is about twice the average force in pulsed assistance. In spite of this, the results show that pulsed assistance allows subjects to reach similar performance levels as compared to continuous assistance after a single training session. Moreover, we introduce a novel kinematic-based measure to assess voluntary participation of subjects during the rehabilitation task, which is only applicable with pulsed assistance.

Skill Learning and Skill Transfer Mediated by Cooperative Haptic Interaction

It is known that physical coupling between two subjects may be advantageous in joint tasks. However, little is known about how two people mutually exchange information to exploit the coupling. Therefore, we adopted a reversed, novel perspective to the standard one that focuses on the ability of physically coupled subjects to adapt to cooperative contexts that require negotiating a common plan: we investigated how training in pairs on a novel task affects the development of motor skills of each of the interacting partners. The task involved reaching movements in an unstable dynamic environment using a bilateral non-linear elastic tool that could be used bimanually or dyadically. The main result is that training with an expert leads to the greatest performance in the joint task. However, the performance in the individual test is strongly affected by the initial skill level of the partner. Moreover, practicing with a peer rather than an expert appears to be more advantageous for a naive; and motor skills can be transferred to a bimanual context, after training with an expert, only if the non-expert subject had prior experience of the dynamics of the novel task.

Human-human physical interaction in the joint control of an underactuated virtual object

Human-human physical interaction has proven to be advantageous especially in contexts with high coordination requirements. But under which conditions can haptic communication bring to performance benefits in a challenging cooperative environment? In this work we investigate which are the dynamics that intervene when two subjects are required to switch from a bimanual to a dyadic configuration in order to solve a complex reaching and stabilization task of a virtual tool in the presence of an unstable dynamics. Results show that dyadic cooperation can improve the performance respect to the individual condition, while minimizing the effort. However, in the joint task, when the stiffness of the system becomes harder to manipulate the feedback delays appear to be critical in determining the maximum achievable level of performance.

Exploiting the link between action and perception: Minimally assisted robotic training of the kinesthetic sense

Since action and perception are tightly coupled and the dysfunction of one of the two channels necessary give rise to different degrees of impairment in the other, we believe that the recovery process would significantly benefit from training protocols able to evaluate and consistently recruit both motor aspects and proprioception concurrently. Therefore, we propose a novel assistive protocol for kinesthetic training of reaching movements that is able to adaptively regulate the level of haptic guidance according to the level of proprioceptive performance along specific directions. Preliminary results show that our adaptive procedure is able to finely tune the level of guidance to the desired level of kinesthetic performance in all the target directions within the duration of the training session. Moreover, the algorithm is able to compensate for perceptual anisotropies that depend on the force direction and its parameters are sensitive to modulations of the kinesthetic sensitivity that may arise as a consequence of practice.

Enhancing Recovery of Sensorimotor Functions: The Role of Robot Generated Haptic Feedback in the Re-learning Process

The term Robotic Rehabilitation defines a class of machines employed for different scenarios, ranging from therapeutic and assistive applications to robots devoted to neuroscience, behavioral research, and cognitive aspects. The first use of such technology dates back to early 1990s, with a relatively long history and it remains linked to the idea that robots, even with a certain degree of autonomy, must be directly controlled by humans while the interaction must be opportunely regulated in order to promote motor recovery or independent living. These devices are designed for individuals with neuromotor and cognitive disabilities to provide rehabilitative exercises or assistance for activity of daily living. They are also measurement systems i.e. they can incorporate sensors for monitoring kinematic and kinetic interaction with subjects such as movement, force or inertial sensors, or for detecting EMG signals to trigger the assistance or to provide – in more complex architectures – Functional Electric Stimulation (FES) to promote motor activity. In this chapter we will focus on therapeutic robots, which are usually employed to perform rehabilitation protocol, describing in details the most widely used control architectures, the implementation of rehabilitation exercises to restore specific motor functions and the measures of the corresponding performance.