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  • Author: Mark L. Latash x
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Mark L. Latash and Irina L. Mikaelian

We explored the relations between task difficulty and speech time in picture description tasks. Six native speakers of Mandarin Chinese (CH group) and six native speakers or Indo-European languages (IE group) produced quick and accurate verbal descriptions of pictures in a self-paced manner. The pictures always involved two objects, a plate and one of the three objects (a stick, a fork, or a knife) located and oriented differently with respect to the plate in different trials. An index of difficulty was assigned to each picture. CH group showed lower reaction time and much lower speech time. Speech time scaled linearly with the log-transformed index of difficulty in all subjects. The results suggest generality of Fitts’ law for movement and speech tasks, and possibly for other cognitive tasks as well. The differences between the CH and IE groups may be due to specific task features, differences in the grammatical rules of CH and IE languages, and possible use of tone for information transmission.

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Israel M. Gelfand and Mark L. Latash

An adequate language is a prerequisite for progress in any area of science, including movement science. Notions of structural units and synergies and the principle of minimal interaction are revisited, discussed, and illustrated with a few examples from recent studies. Equilibrium-point hypothesis is considered an example of identifying significant variables in the control of a voluntary movement.

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Momoko Yamagata, Ali Falaki and Mark L. Latash

We explored the effects of voluntary coactivation of agonist–antagonist leg and trunk muscles on stability of vertical posture. Young healthy subjects performed several tasks while standing with no additional muscle coactivation, low coactivation, and high coactivation. Postural stability was estimated using indices of postural sway and of intertrial variance in the space of muscle groups with parallel scaling of activation levels (M-modes). An increase in coactivation led to a significant increase in the postural sway speed reflected in faster rambling and trembling trajectories. Coactivation also led to a relative drop in the variance component that had no effects on the center of pressure coordinate and an increase in the component that shifted the center of pressure. We conclude that additional muscle coactivation does not help to stabilize vertical posture and is more likely to lead to postural destabilization. The results are consistent with an earlier hypothesis on muscle coactivation ensuring abundance (excessive degrees of freedom) at the level of control variables.

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Daniel M. Corcos and Mark L. Latash

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Stacey L. Gorniak, Marcos Duarte and Mark L. Latash

We explored possible effects of negative covariation among finger forces in multifinger accurate force production tasks on the classical Fitts’s speed-accuracy trade-off. Healthy subjects performed cyclic force changes between pairs of targets “as quickly and accurately as possible.” Tasks with two force amplitudes and six ratios of force amplitude to target size were performed by each of the four fingers of the right hand and four finger combinations. There was a close to linear relation between movement time and the log-transformed ratio of target amplitude to target size across all finger combinations. There was a close to linear relation between standard deviation of force amplitude and movement time. There were no differences between the performance of either of the two “radial” fingers (index and middle) and the multifinger tasks. The “ulnar” fingers (little and ring) showed higher indices of variability and longer movement times as compared with both “radial” fingers and multifinger combinations. We conclude that potential effects of the negative covariation and also of the task-sharing across a set of fingers are counterbalanced by an increase in individual finger force variability in multifinger tasks as compared with single-finger tasks. The results speak in favor of a feed-forward model of multifinger synergies. They corroborate a hypothesis that multifinger synergies are created not to improve overall accuracy, but to allow the system larger flexibility, for example to deal with unexpected perturbations and concomitant tasks.

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Joel R. Martin, Mark L. Latash and Vladimir M. Zatsiorsky

This study investigated the effects of modifying contact finger forces in one direction—normal or tangential—on the entire set of the contact forces, while statically holding an object. Subjects grasped a handle instrumented with finger force-moment sensors, maintained it at rest in the air, and then slowly: (1) increased the grasping force, (2) tried to spread fingers apart, and (3) tried to squeeze fingers together. Analysis was mostly performed at the virtual finger (VF) level (the VF is an imaginable finger that generates the same force and moment as the four fingers combined). For all three tasks there were statistically significant changes in the VF normal and tangential forces. For finger spreading/squeezing the tangential force neutral point was located between the index and middle fingers. We conclude that the internal forces are regulated as a whole, including adjustments in both normal and tangential force, instead of only a subset of forces (normal or tangential). The effects of such factors as EFFORT and TORQUE were additive; their interaction was not statistically significant, thus supporting the principle of superposition in human prehension.

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Mark L. Latash, Fan Gao and Vladimir M. Zatsiorsky

The method of multidimensional scaling was applied to matrices of finger interaction (IFM) computed for individual participants for finger force production tasks. When IFMs for young controls, elderly, and persons with Down syndrome were pooled, only two dimensions described interpersonal differences; these were related to total force and to the total amount of enslaving. When IFMs for each group were analyzed separately, subpopulation-specific dimensions were found. Potentially, this analysis can be applied to discover meaningful dimensions that reflect differences in indices of finger interaction across and within subpopulations which differ in their apparent ability to use the hand. It may also be useful for tracking changes in finger interaction that occur in the process of specialized training or motor rehabilitation.

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Gregory P. Slota, Mark L. Latash and Vladimir M. Zatsiorsky

When grasping and manipulating objects, the central controller utilizes the mechanical advantage of the normal forces of the fingers for torque production. Whether the same is valid for tangential forces is unknown. The main purpose of this study was to determine the patterns of finger tangential forces and the use of mechanical advantage as a control mechanism when dealing with objects of nonuniform finger positioning. A complementary goal was to explore the interaction of mechanical advantage (moment arm) and the role a finger has as a torque agonist/antagonist with respect to external torques (±0.4 N m). Five 6-df force/torque transducers measured finger forces while subjects held a prism handle (6 cm width × 9 cm height) with and without a single finger displaced 2 cm (handle width). The effect of increasing the tangential moment arm was significant (p < .01) for increasing tangential forces (in >70% of trials) and hence creating greater moments. Thus, the data provides evidence that the grasping system as a rule utilizes mechanical advantage for generating tangential forces. The increase in tangential force was independent of whether the finger was acting as a torque agonist or antagonist, revealing their effects to be additive.