An equilibrium-point model for fast, single-joint movement: II. Similarity of single-joint isometric and isotonic descending commands

The model for isotonic movements introduced in the preceding article in this issue is used to account for isometric contractions. Isotonic movements and isometric contractions are analyzed as consequences of one motor program acting under different peripheral conditions. Differences in isotonic and isometric EMG patterns are analyzed theoretically. Computer simulation of the EMG patterns was performed both with and without the inclusion of possible effects of reciprocal inhibition. A series of experiments was performed to test the model. The subjects made fast isotonic movements that were unexpectedly blocked at the very beginning in some of the trials. The observed differences in the EMG patterns between blocked and unblocked trials corresponded to the model's predictions. The results suggest that these differences are due to the action of a tonic stretch reflex rather than to preprogrammed reactions. The experimental and simulation findings, and also the data from the literature, are discussed in the framework of the model and the dual-strategy hypothesis. They support the hypothesis that the motor control system uses one of a few standardized subprograms, specifying a small number of parameters to match a specific task.     

Orienting reaction,organizing for action,and emotional processes

In the first half of the sixties, general notions were formulated concerning the functional role of the orienting reaction (OR) in adaptive activity, its elicitation and habituation. These notions included the following: a) The OR is elicited only by significant changes in a situation. This implies that OR elicitation is preceded by brain processes (usually unconscious) pertaining to the evaluation of the significance of changes according to an existing hierarchy of motivations, attitudes, and goals. Therefore, the OR is of an active (vs. reactive) nature, i.e., is inevitably determined by internal factors of brain activity. b) The OR is not a unitary reaction, but a complex polyfunctional activity, different aspects of which are reflected in different OR components which can be modified rather independently. c) The OR represents the processes of organizing new (non-standard) actions: sensory, motor, or intellectual. OR habituation is a manifestation of attenuation of the active control of an action, and an increase in its automation. Thus, the emphasis in understanding the OR has shifted from a predominantly “sensualistic” platform to a predominantly “actualistic” one. In recent experiments, the role of emotional processes in the elicitation and habituation of components of the OR has been analyzed. Complex relations between the GSR and anxiety were found in a study of patients with acute alcohol withdrawal syndrome treated with different psychopharmacological agents. The study of auditory evoked potential habituation in depressive patients has shown the emotional state influence on sensory aspects of the OR with the participation of the OR brain mechanisms in perceptual defense. The possible significance of some new ideas from the field of non-linear dynamics for perspectives on future investigations of the OR are discussed.     

Multifinger prehension: an overview

The authors review the available experimental evidence on what people do when they grasp an object with several digits and then manipulate it. The article includes three parts, each addressing a specific aspect of multifinger prehension. In the first part, the authors discuss manipulation forces (i.e., the resultant force and moment of force exerted on the object) and the digits' contribution to such forces' production. The second part deals with internal forces defined as forces that cancel each other and do not disturb object equilibrium. The authors discuss the role of the internal forces in maintaining the object stability, with respect to such issues as slip prevention, tilt prevention, and resistance to perturbations. The third part is devoted to the motor control of prehension. It covers such topics as prehension synergies, chain effects, the principle of superposition, interfinger connection matrices and reconstruction of neural commands, mechanical advantage of the fingers, and the simultaneous digit adjustment to several mutually reinforcing or conflicting demands.     

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.     

Characteristics of finger force production during one- and two-hand tasks

Relations among finger forces were studied during one-hand and two-hand isometric maximal force production tasks in right- and left-handers. We particularly focused on the phenomena of force deficit during one-hand multi-finger tasks and of bilateral force deficit during two-hand tasks. Ten healthy subjects (five of them left-handed) performed maximal voluntary force production tasks with different finger combinations involving fingers of one of the hands or of both hands together. In one-hand tasks, finger enslaving (forces produced by fingers that were not instructed to produce force) was larger in the dominant hand, while force deficit (drop in individual finger peak force during multi-finger tasks) showed no differences between the hands. An additional drop in finger forces was seen in two-hand tests (bilateral deficit). The magnitude of the bilateral deficit for a hand was larger for tasks involving fewer fingers within the hand and more fingers in the other hand, with a ceiling effect. Smaller bilateral deficit was seen in tasks involving symmetrical finger combinations. In two-hand tasks that could potentially lead to the generation of large total moments in the frontal plane, the hand that was expected to generate larger moments showed larger bilateral deficit, so that the magnitude of the total moment was reduced. These observations suggest that force deficit within a hand and bilateral deficit have different origins but their effects are combined at a certain level of the multi-finger control hierarchy. Bilateral deficit may display task dependence reflecting, in particular, the principle of minimization of secondary moments. A double-representation, mirror-image hypothesis is suggested to provide a neurophysiological basis for the observed patterns of bilateral deficit.     

Is voluntary control of natural postural sway possible?

The authors explored whether standing human participants could voluntarily decrease the amplitude of their natural postural sway when presented with explicit visual feedback and a target. Participants (N = 9) stood quietly, without any feedback and with feedback on the center of pressure coordinate or the head orientation. They were unable to decrease sway amplitude when presented with visual feedback and a target. Decreasing target size led to contrasting effects on the 2 fractions of sway: rambling and trembling. The smaller target was associated with a decrease in rambling and an increase in trembling. Those observations suggest that sway represents a superposition of at least 2 independent processes. They also suggest that providing visual feedback on a variable tied to body sway may not be an effective way to decrease postural sway in young healthy people.     

Approaches to analysis of handwriting as a task of coordinating a redundant motor system

We consider problems of motor redundancy associated with handwriting using the framework of the uncontrolled manifold (UCM) hypothesis. Recent studies of finger coordination during force production tasks have demonstrated that the UCM-hypothesis provides a fruitful framework for analysis of multi-finger actions. In particular, it has been shown that during relatively fast force changes, finger force variance across trials is structured such that a time pattern of total moment produced by the fingers with respect to a point between the two most lateral fingers involved in the task is stabilized while the time pattern of total force may be destabilized. The findings of selective moment stabilization have been interpreted as being conditioned by the experience with everyday motor tasks that commonly pose more strict requirements to stabilization of total moment than to stabilization of total force. We discuss implications of these findings for certain features of handwriting seen in elderly, children, patients with neurological disorders, and forgers.     

Interaction of Afferent and Efferent Signals Underlying Joint Position Sense

A model of joint position sense is considered based on the concept that central motor commands are adequately expressed in terms of shifts of the so-called invariant length-tension characteristics for agonist and antagonist muscles. The main points of this concept are discussed to clarify them and to prevent misunderstanding. The basic idea of the model is that the efferent copy of the central motor commands plays the role of a reference frame for evaluation of afferent proprioceptive discharges. An experiment with reproduction of constrained movements was performed in order to investigate the influence of voluntary flexor or extensor muscle tension on position sense in the elbow joint. The results demonstrate adequate perception of joint position on the background of voluntary muscle tension and thus are quite consistent with the model. The role of afferent and efferent signals in position sense is discussed with special reference to views expressed recently by different authors. The interrelation between position sense and sense of effort is considered, based on a concept of senso-motor space.     

THE FUNDAMENTALIST MINDSET: PSYCHOLOGICAL PERSPECTIVES ON RELIGION, VIOLENCE, AND HISTORY. Edited by Charles B. Strozier, David M. Terman, and James W. Jones, with Katharine A. Boyd. Foreword by Martin E. Marty. 274?pp. Oxford: Oxford University Press, 2010. $19.95

The following review essay explores the nature and characteristics of the fundamentalist mindset. The editors note that the fundamentalist mindset, wherever if occurs, is composed of distinct traits such as dualistic thinking, paranoia, and rage in a group context; an apocalyptic orientation that incorporates distinct perspectives on time, death, and violence; a relationship to charismatic leadership; and a totalized conversion experience. The review essay will explore these characteristics in brief, underscoring the major themes found in the authors?? exploration of fundamentalism and the psychological and historical underpinning of the pervading mindset.     

Nonsuicidal Self‐Injury among Adolescents: A Training Priority for Primary Care Providers

Primary care providers were surveyed to determine how prepared they feel to address nonsuicidal self‐injury (NSSI) among adolescents, their interest in training on NSSI, and factors associated with routinely asking about NSSI when providing health supervision. Participants included family medicine physicians (n = 260), pediatricians (n = 127), family nurse practitioners (n = 96), and pediatric nurse practitioners (n = 54). Almost 50% felt unprepared to address NSSI, and over 70% wanted training in this area. Overall, relative to other areas of mental health care, clinicians felt least prepared to address and wanted more training on NSSI. Just 27% reported they routinely inquired about NSSI during health supervision. Factors associated with routinely asking about NSSI were identifying as female (OR = 2.37; 95% CI = 1.25–4.49), feeling better prepared to address NSSI (OR = 1.51; 95% CI = 1.04–2.20), and more frequently using a psychosocial interview to identify adolescents in distress (OR = 1.23; 95% CI = 1.02–1.48). Teaching clinicians to assess NSSI within a psychosocial interview may increase screening for and identification of the behavior among adolescents in primary care.