Coordination of grip configurations as a function of force output

In this investigation, the authors examined the coordination and control of force production by the digits of the hand as a function of criterion force level and grip configuration. Each adult participant (N = 6: 3 men and 3 women) was required to place the thumb and a finger (or fingers) upon load cells that were fixed to a grasping apparatus that was clamped to a table. In the task, participants had to match a criterion continuous constant total force level displayed on a computer screen. There were 10 trials at each grip configuration and criterion force level combination on each of 3 consecutive days. The results showed that (a) different grip configurations minimized error at each force level; (b) there was a specific digit pairing within a given grip configuration that produced the highest correlation of force output; (c) the correlation between the force output of digits generally increased at higher force levels; (d) error was reduced at each force level and grip configuration over the practice period; and (e) the organization of the force output of each digit varied as a function of digit, force level, grip configuration, and practice. The findings are consistent with the hypothesis that coordination of the digits in prehension is reflective of an adaptive, task-specific solution that is modified with practice.     

The stability of precision grip force in older adults

The exceedingly large grip forces that many older adults employ when lifting objects with a precision pinch grip (Cole, 1991) may compensate for a reduced capability to produce a stable isometric force. That is, their grip force may fluctuate enough from moment to moment to yield grip forces that approach the force at which the object would slip from grasp. We examined the within-trial variability of isometric force in old (68-85 years, n = 13) and young (n = 11) human subjects (a) when they were asked to produce a constant pinch force at three target levels (0.49, 2.25, and 10.5 N) with external support of the arm, hand, and force transducer and (b) when they were asked to grasp, lift, and hold a small test object with a precision grip. Pinch force produced in the first task was equally stable across the two subject groups during analysis intervals that lasted 4 s. The elderly subjects produced grip forces when lifting objects that averaged twice as much as those produced by the young subjects. The force variability during the static (hold) phase of the lift for the old subjects was comparable with that used by the young subjects, after adjusting for the difference in grip force. The failure to observe less stable grip force in older adults contradicts a similar recent study. Differences in task (isometric grip force versus isometric abduction torque of a single digit) may account for this conflict, however. Thumb and finger forces for grip are produced through coactivation of many muscles and thus promote smooth force output through temporal summation of twitches. We conclude that peripheral reorganization of muscle in older adults does not yield increased instability of precision grip force and therefore does not contribute directly to increased grip forces in this population. However, force instability may affect other grip configurations (e.g., lateral pinch) or manipulation involving digit abduction or adduction forces.     

Grip width and the organization of force output

The authors investigated the structure of force production and variability as a function of grip configuration and width during precision grasping. Variability was studied in absolute (standard deviation) and relative (coefficient of variation) terms; in addition, the authors used approximate entropy to examine regularity. In Experiment 1, the participants (N = 14) used a 2-digit grasp (thumb, index), whereas in Experiment 2, the participants (N = 11) used a 3-digit grasp (thumb, index, middle). The level and regularity of force increased with grip width. The amount of variability was least at narrow grip widths for 2-digit grasping and greatest at narrow grip widths for 3-digit grasping. That pattern of findings is not necessitated by the mechanical equilibrium of grasping; thus, it also reflected adaptive neural reorganization of force output to task demands.     

Effect of Sensory Deprivation on Maximal Force Abilities from Local to Non-local Digits

Abstract

The present study investigates the effect of sensory deprivation of the index and middle finger on motor function of all digits during maximal voluntary force production tasks. A total of 27 subjects performed maximal isometric pressing tasks by using different instructed finger combinations. Subjects completed the same tasks in two visits: a control visit when they had normal sensory feedback in all fingers, and an anesthesia visit when digital nerve blocks were performed on their right index and middle fingers. We evaluated three aspects of motor adaptation on both local (anesthetized) and non-local (non-anesthetized) digits during maximal force production: (1) task-relevant and overall force magnitude, (2) force directional application, and (3) digital individuation and force sharing. Our results indicate that selective digital anesthesia resulted in decreased maximal force magnitude, changed direction of force production, and significant changes extended to non-local digits. The motor weakness and inefficiency revealed in the non-local digits implies that sensory information from each digit can be shared across the digits to assist motor execution within the same hand.     

Changes in the structure of children's isometric force variability with practice

This study examined the effect of age and practice on the structure of children's force variability to test the information processing hypothesis that a reduction of sensorimotor system noise accounts in large part for age-related reductions in perceptual-motor performance variability. In the study, 6-year-olds, 10-year-olds, and young adults practiced on 5 consecutive days (15 trials/day), maintaining for 15-s trials a constant level of force (5 or 25% of maximum voluntary contraction) against an object using a pinch grip (thumb and index finger). With increasing age, the amount of force error and variability decreased, but the sequential structure of variability increased in irregularity. With practice, children reduced the amount of variability by changing the structure of the force output so as to be more similar to that of their older counterparts. The findings provide further evidence that practice-driven changes in the structure of force output, rather than a decline in the amount of white noise, largely account for age-related reductions in the amount of force variability.     

Coming to grips with weight perception: effects of grasp configuration on perceived heaviness

We investigated how changes in grasp configuration affect perceived heaviness in a weight discrimination task in which participants compared the weights of a series of test objects with the weight of a reference object. In different experiments, we varied the width of the grasp, the number of digits employed, the angle of the grasp surface, and the size of the contact area between the digits and the object. We show that objects are perceived to be lighter when lifting with (1) a wide grip in comparison with a narrow grip, (2) five digits in comparison with two digits, and (3) a large contact area in comparison with a small contact area. However, the angle of the contact surfaces did not influence perceived weight. We suggest that changes in central motor commands associated with grasp differences may influence perceived weight, at least under some conditions.     

Serial and parallel processes in working memory after practice

Six young adults practiced for 36 sessions on a working-memory updating task in which 2 digits and 2 spatial positions were continuously updated. Participants either did 1 updating operation at a time, or attempted 1 numerical and 1 spatial operation at the same time. In contrast to previous research using the same paradigm with a single digit and a single dot, dual-task costs were not eliminated with practice. Costs of switching between digits and between spatial positions were found throughout practice, supporting the existence of a focus of attention in working memory that can hold 1 digit and 1 spatial position simultaneously, but is not expanded to hold 2 elements of the same kind. The results can be understood by assuming that observed limits on parallel processing, as well as on the capacity of the focus of attention, arise not from structural constraints but rather reflect the optimal configuration of the cognitive system for avoiding information cross-talk in a given task.     

Grasping numbers: evidence for automatic influence of numerical magnitude on grip aperture

Previous research has shown that the fingers’ aperture during grasp is affected by the numerical values of numbers embedded in the grasped objects: Numerically larger digits lead to larger grip apertures than do numerically smaller digits during the initial stages of the grasp. The relationship between numerical magnitude and visuomotor control has been taken to support the idea of a common underlying neural system mediating the processing of magnitude and the computation of object size for motor control. The purpose of the present study was to test whether the effect of magnitude on motor preparation is automatic. During grasping, we asked participants to attend to the colors of the digit while ignoring numerical magnitude. The results showed that numerical magnitude affected grip aperture during the initial stages of the grasp, even when magnitude information was irrelevant to the task at hand. These findings suggest that magnitude affects grasping preparation in an automatic fashion.     

Articulatory complexity at item boundaries in serial recall: the case of Welsh and English digit span

The proposition that the difference in memory span between Welsh digits and English digits is accounted for by the longer articulatory duration of Welsh digits is critically reexamined. Two methods of measuring digit duration are contrasted. One is derived from digits spoken in isolation; the other is based on digits spoken in list format. Duration of Welsh digits was greater only when spoken in lists; with isolated production Welsh digits were significantly shorter than English digits. Also, span was shorter for Welsh digits. The results are interpreted in the light of the different articulatory demands made at the junctures between words in the English and Welsh lists. A supplementary experiment, using English words, illustrated that articulatory complexity at item boundaries increased serial recall error.     

Internal Forces During Static Prehension: Effects of Age and Grasp Configuration

The authors studied effects of healthy aging on 3 components of the internal force vector during static prehensile tasks. Young and older subjects held an instrumented handle using a 5-digit prismatic grasp under different digit configurations and external torques. Across digit configurations, older subjects showed larger internal normal (grip) and tangential (load-resisting) digit force components and larger internal moment of force. In contrast to earlier reports, safety margin values were not higher in the older subjects. The results show that the increased grip force in older persons is a specific example of a more general age-related problem reflected in the generation of large internal force vectors in prehensile tasks. It is possible that the higher internal forces increase the apparent stiffness of the hand+handle system and, hence, contribute to its stability. This strategy, however, may be maladaptive, energetically wasteful, and inefficient in ensuring safety of hand-held objects.     

Memory skills mediating superior memory in a world-class memorist

Laboratory studies have investigated how individuals with normal memory spans attained digit spans over 80 digits after hundreds of hours of practice. Experimental analyses of their memory skills suggested that their attained memory spans were constrained by the encoding time, for the time needed will increase if the length of digit sequences to be memorised becomes longer. These constraints seemed to be violated by a world-class memorist, Feng Wang (FW), who won the World Memory Championship by recalling 300 digits presented at 1 digit/s. In several studies we examined FW’s memory skills underlying his exceptional performance. First FW reproduced his superior memory span of 200 digits under laboratory condition, and we obtained his retrospective reports describing his encoding/retrieval processes (Experiment 1). Further experiments used self-paced memorisation to identify temporal characteristics of encoding of digits in 4-digit clusters (Experiment 2), and explored memory encoding at presentation speeds much faster than 1 digit/s (Experiment 3). FW’s superiority over previous digit span experts is explained by his acquisition of well-known mnemonic techniques and his training that focused on rapid memorisation. His memory performance supports the feasibility of acquiring memory skills for improved working memory based on storage in long-term memory.     

Body-scaled transitions in human grip configurations

This article reports two experiments that were set up to examine the preferred human grip configuration used to displace cubes that varied in length (Lc), mass (Mc), and density (ML3). In particular, the authors sought to provide a more precise test of a dimensional relation between the object and the hand that had previously been shown to predict the grip configuration used to transport an object from one location to another. The experiments examined 2 grip transitions (from 3 digits to 4 digits and from 1 hand to 2 hands) within 2 sets of object conditions. In Experiment 1, cubes with a low density and a small increment in size (1 mm) were used, whereas in Experiment 2, cubes with 2 fixed sizes and small increments in mass were used. The results showed that the body-scaled equation K = logLc + (logMc/a + bMh + cLh), where Mh and Lh are the anthropometric measures of the hand mass and length and a, b, and c are empirical constants, is the body-scaled information that predicts the grip configurations used to displace objects.     

Holistic or compositional representation of two-digit numbers? Evidence from the distance, magnitude, and SNARC effects in a number-matching task

Whether two-digit numbers are represented holistically (each digit pair processed as one number) or compositionally (each digit pair processed separately as a decade digit and a unit digit) remains unresolved. Two experiments were conducted to examine the distance, magnitude, and SNARC effects in a number-matching task involving two-digit numbers. Forty undergraduates were asked to judge whether two two-digit numbers (presented serially in Experiment 1 and simultaneously in Experiment 2) were the same or not. Results showed that, when numbers were presented serially, unit digits did not make unique contributions to the magnitude and distance effects, supporting the holistic model. When numbers were presented simultaneously, unit digits made unique contributions, supporting the compositional model. The SNARC (Spatial-Numerical Association of Response Codes) effect was evident for the whole numbers and the decade digits, but not for the unit digits in both experiments, which indicates that two-digit numbers are represented on one mental number line. Taken together, these results suggested that the representation of two-digit numbers is on a single mental number line, but it depends on the stage of processing whether they are processed holistically or compositionally.     

Adjustments to Local Friction in Multifinger Prehension

The authors studied the effects of surface friction at the digit-object interface on digit forces and moments when 12 participants statically held an object in a 5-digit grasp. The authors changed low-friction contact (LFC) with rayon and high-friction contact (HFC) with sandpaper independently for each digit in all 32 possible combinations. Normal forces of the thumb and virtual finger (VF), an imagined finger with a mechanical effect equal to that of the 4 fingers, increased with the thumb at LFC or with an increase in the number of fingers at LFC. When the thumb was at LFC, the thumb tangential force decreased. The VF tangential force decreased when the number of fingers at LFC increased. The interaction of the local responses to friction and the synergic responses necessary to maintain the equilibrium explain the coordination of individual digit forces.     

Adjustments to local friction in multifinger prehension

The authors studied the effects of surface friction at the digit-object interface on digit forces and moments when 12 participants statically held an object in a 5-digit grasp. The authors changed low-friction contact (LFC) with rayon and high-friction contact (HFC) with sandpaper independently for each digit in all 32 possible combinations. Normal forces of the thumb and virtual finger (VF), an imagined finger with a mechanical effect equal to that of the 4 fingers, increased with the thumb at LFC or with an increase in the number of fingers at LFC. When the thumb was at LFC, the thumb tangential force decreased. The VF tangential force decreased when the number of fingers at LFC increased. The interaction of the local responses to friction and the synergic responses necessary to maintain the equilibrium explain the coordination of individual digit forces.     

Adapting relative phase of bimanual isometric force coordination through scaling visual information intermittency

Visual information plays an adaptive role in the relation between bimanual force coupling and error corrective processes of isometric force control. In the present study, the evolving distribution of the relative phase properties of bimanual isometric force coupling was examined by scaling within a trial the temporal feedback rate of visual intermittency (short to long presentation intervals and vice versa). The force error (RMSE) was reduced, and time-dependent irregularity (SampEn) of the force output was increased with greater amounts of visual information (shorter intermittency). Multi-stable coordination patterns of bimanual isometric force control were differentially shifted toward and away from the intrinsic dynamics by the changing the intermittency of visual information. The distribution of Hilbert transformed relative phase values showed progressively a predominantly anti-phase mode under less intermittent visual information to predominantly an in-phase mode with limited (almost no) visual information. Correlation between the hands showed a continuous reduction, rather than abrupt “transition,” with increase in visual information, although no mean negative correlation was realized, despite the tendency towards an anti-phase distribution. Lastly, changes in both the performance outcome and bimanual isometric force coordination occurred at visual feedback rates faster than the minimal visual processing times established from single limb movement and isometric force protocols.     

Short term retention of sentences

A series of experiments was performed on the interaction between the short-term retention of sentences and of digits. In Experiment I a digit span method was used whereby subjects were presented with a sentence followed by a sequence of digits and were required either (a) to recall the sentence first and then the digits or (b) to recall the digits followed by the sentence. Under condition (a) prior recall of the sentence reduced the percentage of digit sequences correctly recalled, while under condition (b) retention of the sentence appeared to have no effect on digit recall. This last finding was confirmed in Experiment II, where the sentences varied both in grammatical complexity and length.

In Experiment III the effect of prior recall of a sentence on the recall of digits was found to depend on the type of sentence used. A correlation was observed between the size of this effect and the time taken to recall a sentence. The rate of forgetting suggested by this observation was comparable to that obtained in Experiment IV, where subjects performed an intervening task that did not involve immediate memory for sentences in the interval between the presentation and recall of a six-digit sequence.

It was concluded from these results that the short-term retention of sentences and of lists of items cannot be explained in terms of some general store of limited capacity.     

Changing the focus of attention: The interacting effect of valence and arousal

We examined how valence and arousal of an image influence visual attention. “Spotlight of attention” theory suggests that positive affect broadens, and negative affect narrows, one's aperture of attention, whereas the arousal theory literature suggests that arousal level is what modulates attentional focus, with highly arousing affect capturing attention, regardless of valence. In two experiments, a digit parity task was used to index the influence of valence, and arousal, on visual attention. Positive or negative images were displayed centrally on each trial, with single digits presented more peripherally (Experiment 1) or more centrally (Experiment 2) to the image. In both Experiments participants were slower, and less accurate at making parity decisions (e.g., both digits odd or both even) when the image was negative relative to positive, and of high arousal. For low arousal images, positive, relative to negative, valence images led to greater impairment of the digit parity task. Findings suggest that arousal level of images modulates the influence of valence on distribution of visual attention. Highly negative emotional images may command or capture attention, but there are other factors that can lead to attention capture, even in low arousing positive stimuli.     

Subclinical depression modulates the impact of emotion on force control

The primary goals of this project were to examine whether (1) the impact of emotional state on force control varies as a function of target force level, (2) self reports of emotional state covary with force control, and (3) emotional state and trait levels of depression interact to alter force control. Subjects varying in self-reported depression performed a sustained pinch grip for 20 s at low, moderately low, and moderate target force levels. Each trial began with 8 s of visual feedback, which was replaced with an emotional or neutral image for 12 s. Subjects reported valence and arousal ratings for each image. Across the entire sample, self-reported arousal predicted constant error (CE) during low and moderately low target force trials. Depression significantly predicted the relationship between self-reported valence and CE during moderate target force trials. Theoretical explanations, implications, and future research directions are discussed.     

Motor prediction at the edge of instability: alteration of grip force control during changes in bimanual coordination

Predicting the consequences of actions is fundamental for skilled motor behavior. We investigated whether motor prediction is influenced by the fact that some movements are easier to perform and stabilize than others. Twelve subjects performed a bimanual rhythmical task either symmetrically or asymmetrically (the latter being more difficult and less stable) while oscillating in each hand an object attached to an elastic cord. Motor prediction was monitored through the adequacy of anticipatory grip force adjustments with respect to the elastic resisting force. Results showed less adequate predictive control during asymmetrical movements (compared with symmetrical ones). Furthermore, switching between modes of coordination induced even larger alterations. An interesting finding was that grip force control did not always stabilize around the expected value after voluntary transition. We conclude that motor prediction is affected by the degree of coordination between the upper limbs and by phase transitions and is prone to carryover effects.