Learning of Dexterous Object Manipulation in a Virtual Reality Environment.

Humans have unrivalled abilities to perform dexterous object manipulation. This requires the sensorimotor system to quickly adapt to environmental changes and predictively counter act the external disturbances. Many studies have focused on the anticipatory control of digits with real-world experiments. However, examining manipulation using virtual reality with haptic devices expands the possibilities of investigation. In this work, participants grasped and lifted an inverted T-shaped object in a virtual reality setup. The graspable surface of the object was either constrained to a small area or unconstrained. The position of the object's center of mass changed between blocks, and the participants were asked to minimize the rotation of the object during the lift. Our results show that, consistent with the results of real-world experiments, participants gradually learn to adjust the digit positions and forces to predictively compensate for the torque due to the shifted center of mass prior to liftoff. The only major difference found was that the length of trials needed during the adaptation phase to each condition increased from 3 in real-world to 5 in virtual environment.

Bimanual Manipulation of a Complex Object with Internal Dynamics.

Object manipulation often requires coordination between hands and adaption to the dynamic characteristics of the object. When manipulating the same object, the two hands can have either symmetric or asymmetric impact on the object's trajectory. In this work, we used a bimanual manipulation task of a complex object with internal dynamics to examine how symmetric or scaled-down control of one of the hands affects the coordination between hands. Our result shows that participants are able to quickly adapt to different conditions but the coordination between the two hands changes very little.

Force Control During the Precision Grip Translates to Virtual Reality.

When grasping and manipulating objects we implicitly adapt grip forces according to the physical parameters of the object. We integrate visual, cutaneous, and force feedback to estimate these parameters and adapt our control accordingly. Using virtual reality, both feedback integration and control can be investigated in ways that are not possible using real-life objects. Here, we present our custom-built virtual reality setup and show its validity for use in human studies of fine motor control. Participants grasped and lifted virtual objects with different weights. We show that, consistent with lifting real objects, all participants adapt their grip forces to the object mass, and do so on a trial-by-trial basis. Compared to similar studies with real objects and full feedback, grip forces were increased, and adaptation required more trials. This study successfully demonstrated that grip force control in the precision grip translates to virtual reality. While our setup can be used for similar work in the future, subsequent virtual reality experiments should include a longer adaptation phase compared to classic setups.

Correction to: Safety of long-term electrical peripheral nerve stimulation: review of the state of the art.

An amendment to this paper has been published and can be accessed via the original article.

Safety of long-term electrical peripheral nerve stimulation: review of the state of the art.

BACKGROUND: Electrical stimulation of peripheral nerves is used in a variety of applications such as restoring motor function in paralyzed limbs, and more recently, as means to provide intuitive sensory feedback in limb prostheses. However, literature on the safety requirements for stimulation is scarce, particularly for chronic applications. Some aspects of nerve interfacing such as the effect of stimulation parameters on electrochemical processes and charge limitations have been reviewed, but often only for applications in the central nervous system. This review focuses on the safety of electrical stimulation of peripheral nerve in humans. METHODS: We analyzed early animal studies evaluating damage thresholds, as well as more recent investigations in humans. Safety requirements were divided into two main categories: passive and active safety. We made the distinction between short-term (< 30 days) and chronic (> 30 days) applications, as well as between electrode preservation (biostability) and body tissue healthy survival (harmlessness). In addition, transferability of experimental results between different tissues and species was considered. RESULTS: At present, extraneural electrodes have shown superior long-term stability in comparison to intraneural electrodes. Safety limitations on pulse amplitude (and consequently, charge injection) are dependent on geometrical factors such as electrode placement, size, and proximity to the stimulated fiber. In contrast, other parameters such as stimulation frequency and percentage of effective stimulation time are more generally applicable. Currently, chronic stimulation at frequencies below 30 Hz and percentages of effective stimulation time below 50% is considered safe, but more precise data drawn from large databases are necessary. Unfortunately, stimulation protocols are not systematically documented in the literature, which limits the feasibility of meta-analysis and impedes the generalization of conclusions. We therefore propose a standardized list of parameters necessary to define electrical stimulation and allow future studies to contribute to meta-analyses. CONCLUSION: The safety of chronic continuous peripheral nerve stimulation at frequencies higher than 30 Hz has yet to be documented. Precise parameter values leading to stimulation-induced depression of neuronal excitability (SIDNE) and neuronal damage, as well as the transition between the two, are still lacking. At present, neural damage mechanisms through electrical stimulation remain obscure.