Alan Wing

Impaired anticipatory finger grip-force adjustments in a case of cerebellar degeneration

Abstract We describe adjustments in grip force as a consequence of fluctuations in inertial load force during vertical movements of the upper limb in a patient with cerebellar degeneration. Normally grip force is adapted to load-force fluctuations, in particular to the maximum load force, which occurs early in upward movements and late in downward movements. Increased grip force during movement was observed in the patient, but the timing of maximum grip force was not different between upward and downward movements. This suggests impaired cerebellar prediction of the dynamic consequences of voluntary movement.

Grip force dynamics in the approach to a collision

Abstract This experiment investigated the prediction of load force (LF) in impulsive collisions inferred from anticipatory adjustments of grip force (GF) used to stabilise a hand-held object. Subjects used a precision grip to hold the object between thumb and index finger of their right hand and used the arm either: (1) to move the object to produce a collision by hitting the lower end of a pendulum, causing it to swing to one of three target angles, or (2) to hold the object still while receiving a collision produced by the experimenter releasing the pendulum from one of three angles. Visual feedback of the pendulum’s trajectory was available in the production task only. In all conditions, subjects increased GF in advance of the collision. In receiving the collision without advance information, subjects set GF levels to the mid-range of the experienced forces. When subjects possessed knowledge about the maximum angle of pendulum swing – either because they were going to produce it or because they were verbally informed – magnitude of the anticipatory-GF magnitude response was scaled to the predicted LF magnitude. Furthermore, GF was scaled to LF with a higher gain when producing compared to receiving the collision. This suggests that updating forward models through a semantic route is not as powerful as when the updating is achieved through the more direct route of dynamic exploration.

Stroke-related differences in axial body segment coordination during preplanned and reactive changes in walking direction

This study quantitatively describes differences between participants with hemiparetic stroke and age-matched healthy participants in axial body segment and gait kinematics during a direction change task. Participants were required to change walking direction by 45°, either to their left or right, at the midpoint of a 6-m path. Participants were visually cued either at the start of the walk (pre-planned) or one stride before they reached the turn point (reactive). The sequence and inter-segmental timing of axial orientation onset was preserved in participants with stroke. Analysis of a subgroup of stroke survivors indicated that participants with lesions affecting the basal ganglia (BG) took significantly longer time than control participants to initiate the reorientation synergy when making turns to their non-paretic side. We hypothesize that these differences are a product of asymmetrical activity of dopaminergic pathways in the brain resulting from compromised BG function.

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(

Attentional focus of feedback for improving performance of reach-to-grasp after stroke: a randomised crossover study.

OBJECTIVE To investigate whether feedback inducing an external focus (EF) of attention (about movement effects) was more effective for retraining reach-to-grasp after stroke compared with feedback inducing an internal focus (IF) of attention (about body movement). It was predicted that inducing an EF of attention would be more beneficial to motor performance. DESIGN Crossover trial where participants were assigned at random to two feedback order groups: IF followed by EF or EF followed by IF. SETTING Research laboratory. PARTICIPANTS Forty-two people with upper limb impairment after stroke. INTERVENTION Participants performed three reaching tasks: (A) reaching to grasp a jar; (B) placing a jar forwards on to a table; and (C) placing a jar on to a shelf. Ninety-six reaches were performed in total over one training session. MAIN OUTCOME MEASURES Kinematic measures were collected using motion analysis. Primary outcome measures were movement duration, peak velocity of the wrist, size of peak aperture and peak elbow extension. RESULTS Feedback inducing an EF of attention produced shorter movement durations {first feedback order group: IF mean 2.53 seconds [standard deviation (SD) 1.85]; EF mean 2.12 seconds (SD 1.63), mean difference 0.41 seconds; 95% confidence interval -0.68 to 1.5; P=0.008}, an increased percentage time to peak deceleration (P=0.01) when performing Task B, and an increased percentage time to peak velocity (P=0.039) when performing Task A compared with feedback inducing an IF of attention. However, an order effect was present whereby performance was improved if an EF of attention was preceded by an IF of attention. CONCLUSIONS Feedback inducing an EF of attention may be of some benefit for improving motor performance of reaching in people with stroke in the short term; however, these results should be interpreted with caution. Further research using a randomised design is recommended to enable effects on motor learning to be assessed.

Knowing your nose better than your thumb : measures of over-grasp reveal that face-parts are special for grasping.

Typically, when a grasping response is made, the hand opens wider than the target object. We show that this “over-grasp” response is reduced when we reach to parts of our own face, relative to when we reach to other body parts or to neutral objects. This is not due to reaching to different parts of body space, as over-grasp responses are indifferent to whether or not other body parts or neutral objects are placed close to the face. It is also not due to differences in perceptual knowledge of the size of the target object. We conclude instead that the familiarity of face parts influences the grasping response directly. Subsequent experiments demonstrate that the movement representation determining any effect is not based on a torso-centred frame, and not abstracted from the specific hand used for grasping. We discuss the implications of the results for understanding and measuring motor representations for familiar actions.

Integration of the hand in postural reactions to sustained sideways force at the pelvis

Abstract In order to investigate the potential hand contribution to sideways balance, sideways pushes to the right, which subjects resisted using either the lower limbs (”hip only”) or the lower limbs assisted by the right upper limb (”hand and hip”), were delivered to the pelvis. Analysis of force and electromyogram recordings from the legs, arm and hand in the hand-and-hip condition showed a close co-ordination of upper and lower limbs in terms of mean latencies and amplitudes. However, trial to trial fluctuations of forces generated by the hand and leg did not correlate, suggesting parallel pathways under central co-ordinative control.

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

As an aid to studying the efficiency of grip force scaling in the context of collisions, we present a simple cost-effective approach to estimating the slip ratio—that is, the minimum grip-to-load-force ratio needed to prevent object slippage. The grip apparatus comprises a sturdy load cell to measure grip force and two linear potentiometers to provide detailed description of finger movements. The slip ratio was estimated by plotting the magnitude of finger movement against the grip-to-load-force ratio at the time of impact. The slip ratio was dependent on the direction of loading, which stresses the importance of estimating slip ratios in a context similar to that of the experiment in which the efficiency of subjects’ behavior is to be assessed.

FAST INdiCATE Trial protocol. Clinical efficacy of functional strength training for upper limb motor recovery early after stroke: Neural correlates and prognostic indicators

Rationale Functional strength training in addition to conventional physical therapy could enhance upper limb recovery early after stroke more than movement performance therapy plus conventional physical therapy. Aims To determine (a) the relative clinical efficacy of conventional physical therapy combined with functional strength training and conventional physical therapy combined with movement performance therapy for upper limb recovery; (b) the neural correlates of response to conventional physical therapy combined with functional strength training and conventional physical therapy combined with movement performance therapy; (c) whether any one or combination of baseline measures predict motor improvement in response to conventional physical therapy combined with functional strength training or conventional physical therapy combined with movement performance therapy. Design Randomized, controlled, observer-blind trial. Study The sample will consist of 288 participants with upper limb paresis resulting from a stroke that occurred within the previous 60 days. All will be allocated to conventional physical therapy combined with functional strength training or conventional physical therapy combined with movement performance therapy. Functional strength training and movement performance therapy will be undertaken for up to 1·5 h/day, five-days/week for six-weeks. Outcomes and Analysis Measurements will be undertaken before randomization, six-weeks thereafter, and six-months after stroke. Primary efficacy outcome will be the Action Research Arm Test. Explanatory measurements will include voxel-wise estimates of brain activity during hand movement, brain white matter integrity (fractional anisotropy), and brain–muscle connectivity (e.g. latency of motor evoked potentials). The primary clinical efficacy analysis will compare treatment groups using a multilevel normal linear model adjusting for stratification variables and for which therapist administered the treatment. Effect of conventional physical therapy combined with functional strength training versus conventional physical therapy combined with movement performance therapy will be summarized using the adjusted mean difference and 95% confidence interval. To identify the neural correlates of improvement in both groups, we will investigate associations between change from baseline in clinical outcomes and each explanatory measure. To identify baseline measurements that independently predict motor improvement, we will develop a multiple regression model.

Humans adjust their grip force when passing an object according to the observed speed of the partner’s reaching out movement

The way an object is released by the passer to a partner is fundamental for the success of the handover and for the experienced fluency and quality of the interaction. Nonetheless, although its apparent simplicity, object handover involves a complex combination of predictive and reactive control mechanisms that were not fully investigated so far. Here, we show that passers use visual-feedback based anticipatory control to trigger the beginning of the release, to launch the appropriate motor program, and adapt such predictions to different speeds of the receiver’s reaching out movements. In particular, the passer starts releasing the object in synchrony with the collision with the receiver, regardless of the receiver’s speed, but the passer’s speed of grip force release is correlated with receiver speed. When visual feedback is removed, the beginning of the passer’s release is delayed proportionally with the receiver’s reaching out speed; however, the correlation between the passer’s peak rate of change of grip force is maintained. In a second study with 11 participants receiving an object from a robotic hand programmed to release following stereotypical biomimetic profiles, we found that handovers are experienced as more fluent when they exhibit more reactive release behaviours, shorter release durations, and shorter handover durations. The outcomes from the two studies contribute understanding of the roles of sensory input in the strategy that empower humans to perform smooth and safe handovers, and they suggest methods for programming controllers that would enable artificial hands to hand over objects with humans in an easy, natural and efficient way.