Estimating the minimum grip force required when grasping objects under impulsive loading conditions

Direct Shear Stress Mapping Using a Gallium Nitride LED-Based Tactile Sensor

An experiment was performed to calibrate the capability of a tactile sensor, which is based on gallium nitride (GaN) nanopillars, to measure the absolute magnitude and direction of an applied shear force without the need for any post-processing of data. The force's magnitude was deduced from monitoring the nanopillars' light emission intensity. Calibration of the tactile sensor used a commercial force/torque (F/T) sensor. Numerical simulations were carried out to translate the F/T sensor's reading to the shear force applied to each nanopillar's tip. The results confirmed the direct measurement of shear stress from 3.71 to 50 kPa, which is in the range of interest for completing robotic tasks such as grasping, pose estimation, and item discovery.

Neurophysiology of slip sensation and grip reaction: insights for hand prosthesis control of slippage

Sensory feedback is pivotal for a proficient dexterity of the hand. By modulating the grip force in function of the quick and not completely predictable change of the load force, grabbed objects are prevented to slip from the hand. Slippage control is an enabling achievement to all manipulation abilities. However, in hand prosthetics, the performance of even the most innovative research solutions proposed so far to control slippage remain distant from the human physiology. Indeed, slippage control involves parallel and compensatory activation of multiple mechanoceptors, spinal and supraspinal reflexes, and higher-order voluntary behavioral adjustments. In this work, we reviewed the literature on physiological correlates of slippage to propose a three-phases model for the slip sensation and reaction. Furthermore, we discuss the main strategies employed so far in the research studies that tried to restore slippage control in amputees. In the light of the proposed three-phase slippage model and from the weaknesses of already implemented solutions, we proposed several physiology-inspired solutions for slippage control to be implemented in the future hand prostheses. Understanding the physiological basis of slip detection and perception and implementing them in novel hand feedback system would make prosthesis manipulation more efficient and would boost its perceived naturalness, fostering the sense of agency for the hand movements.

A review of the neurobiomechanical processes underlying secure gripping in object manipulation

O'SHEA, H. and S. J. Redmond. A review of the neurobiomechanical processes underlying secure gripping in object manipulation. NEUROSCI BIOBEHAV REV 286-300, 2021. Humans display skilful control over the objects they manipulate, so much so that biomimetic systems have yet to emulate this remarkable behaviour. Two key control processes are assumed to facilitate such dexterity: predictive cognitive-motor processes that guide manipulation procedures by anticipating action outcomes; and reactive sensorimotor processes that provide important error-based information for movement adaptation. Notwithstanding increased interdisciplinary research interest in object manipulation behaviour, the complexity of the perceptual-sensorimotor-cognitive processes involved and the theoretical divide regarding the fundamentality of control mean that the essential mechanisms underlying manipulative action remain undetermined. In this paper, following a detailed discussion of the theoretical and empirical bases for understanding human dexterous movement, we emphasise the role of tactile-related sensory events in secure object handling, and consider the contribution of certain biophysical and biomechanical phenomena. We aim to provide an integrated account of the current state-of-art in skilled human-object interaction that bridges the literature in neuroscience, cognitive psychology, and biophysics. We also propose novel directions for future research exploration in this area.

Linguistic initiation signals increase auditory feedback error correction

Previous research has shown that speakers can adapt their speech in a flexible manner as a function of a variety of contextual and task factors. While it is known that speech tasks may play a role in speech motor behavior, it remains to be explored if the manner in which the speaking action is initiated can modify low-level, automatic control of vocal motor action. In this study, the nature (linguistic vs non-linguistic) and modality (auditory vs visual) of the go signal (i.e., the prompts) was manipulated in an otherwise identical vocal production task. Participants were instructed to produce the word "head" when prompted, and the auditory feedback they were receiving was altered by systematically changing the first formants of the vowel /ε/ in real time using a custom signal processing system. Linguistic prompts induced greater corrective behaviors to the acoustic perturbations than non-linguistic prompts. This suggests that the accepted variance for the intended speech sound decreases when external linguistic templates are provided to the speaker. Overall, this result shows that the automatic correction of vocal errors is influenced by flexible, context-dependant mechanisms.

A simple technique to study embodied language processes: the grip force sensor

Research in cognitive neuroscience has shown that brain structures serving perceptual, emotional, and motor processes are also recruited during the understanding of language when it refers to emotion, perception, and action. However, the exact linguistic and extralinguistic conditions under which such language-induced activity in modality-specific cortex is triggered are not yet well understood. The purpose of this study is to introduce a simple experimental technique that allows for the online measure of language-induced activity in motor structures of the brain. This technique consists in the use of a grip force sensor that captures subtle grip force variations while participants listen to words and sentences. Since grip force reflects activity in motor brain structures, the continuous monitoring of force fluctuations provides a fine-grained estimation of motor activity across time. In other terms, this method allows for both localization of the source of language-induced activity to motor brain structures and high temporal resolution of the recorded data. To facilitate comparison of the data to be collected with this tool, we present two experiments that describe in detail the technical setup, the nature of the recorded data, and the analyses (including justification about the data filtering and artifact rejection) that we applied. We also discuss how the tool could be used in other domains of behavioral research.

A pre-reflective indicator of an impaired sense of agency in patients with Schizophrenia

In schizophrenia, passivity phenomenon are clinically related to an abnormal sense of agency, which has been experimentally studied through self-recognition tasks. However, Tsakiris et al. (Cognition 96(3):215-231, 2005) have recently shown in healthy controls that the sense of agency is distinct from self-recognition abilities. We propose a simple motor task to obtain an implicit indicator of the working status of the pre-reflective sense of agency in schizophrenia. Collision dynamics gave us the means to further dissociate agency from motor prediction. Twenty-four patients and a group of matched controls used a hand-held object to stop the fall of a pendulum that was released either by the Subject (task S) or by the Experimenter (task E). The objective indicator of the sense of agency was taken as the efficiency difference between tasks S and E, before the availability of afferent information from collision. Qualitative feedback was provided to assess the top-down effect of explicit information. Motor prediction was as accurate in patients as in controls in tasks E and S. Controls were more efficient in S than in E. Patients revealed similar efficiency levels in both tasks. Qualitative feedback helped but did not affect the efficiency difference between tasks. Our results suggest an impairment of a pure efferent-driven sense of agency in schizophrenia, which is (1) distinct from motor prediction and (2) not under voluntary control. The abnormal judgments previously reported in schizophrenia for self-recognition abilities might be the consequence of a low order deficit of a pre-reflective sense of agency.

Efficiency of grip force adjustments for impulsive loading during imposed and actively produced collisions

During object manipulation, both predictive feedforward and reactive feedback mechanisms are available to adjust grip force (GF) levels to compensate for the destabilizing effects of load force changes. During collisions, load force increases impulsively (< 20 ms). Thus, only predictive control of GF can be used to ensure grasp stabilization. A collision paradigm is here used to investigate the effects of practice and vision on the efficiency of the predictive control of GF. Subjects actively produced or received an imposed collision with a pendulum. Subjects were more efficient (used smaller GF for identical loads) when producing than when receiving the collisions. Effects of practice were evident in the active producing task only, with GF levels reducing over repetitions, suggesting that sensorimotor memory for the task was used to adjust GF more efficiently. With imposed collisions, GF levels did not reduce with repetition, which suggests that a direct relation between motor action and sensory feedback may be necessary to improve efficiency. Nevertheless, in this condition GF was lower with visual feedback, indicating potential for more efficient grip possibly associated with subjects degree of confidence. We discuss the implications of these results for accounts of the predictive and the reactive control of movement.

A deficit in the adjustment of grip force responses in schizophrenia

Delusions of control in schizophrenia may be due to a deficit in the generation of an efference copy, used to distinguish between self generated and externally imposed changes in the environment. This hypothesis was tested using a framework that differentiated automatic and controlled levels of motor behavior. Subjects resisted collisions that were either self or externally imposed. The grip to load force correlation (response accuracy) and the overall grip force level used (response efficiency) were measured. Controls improved both accuracy and efficiency of their grip force responses in self compared to externally imposed collisions. Patients improved accuracy but not efficiency of motor response. There was no difference between patients with and without delusions of control. These results refute the hypothesis of a perturbed efference copy in patients with delusions of control. We rather propose that schizophrenia globally preserves the automatic level but affects the controlled, more voluntary level of motor behavior.

Use of 'chalk' in rock climbing: sine qua non or myth?

Magnesium carbonate, or 'chalk', is used by rock climbers to dry their hands to increase the coefficient of friction, thereby improving the grip of the holds. To date, no scientific research supports this practice; indeed, some evidence suggests that magnesium carbonate could decrease the coefficient of friction. Fifteen participants were asked to apply a force with the tip of their fingers to hold a flattened rock (normal force), while a tangential force pulled the rock away. The coefficient of friction--that is, the ratio between the tangential force (pulling the rock) and the normal force (applied by the participants)--was calculated. Coating (chalk vs no chalk), dampness (water vs no water) and rock (sandstone, granite and slate) were manipulated. The results showed that chalk decreased the coefficient of friction. Sandstone was found to be less slippery than granite and slate. Finally, water had no significant effect on the coefficient of friction. The counter-intuitive effect of chalk appears to be caused by two independent factors. First, magnesium carbonate dries the skin, decreasing its compliance and hence reducing the coefficient of friction. Secondly, magnesium carbonate creates a slippery granular layer. We conclude that, to improve the coefficient of friction in rock climbing, an effort should be made to remove all particles of chalk; alternative methods for drying the fingers are preferable.