Efference copy in kinesthetic perception: A copy of what is it?

A number of notions in the fields of motor control and kinesthetic perception have been used without clear definitions. In this review, we consider definitions for efference copy, percept, and sense of effort based on recent studies within the physical approach, which assumes that the neural control of movement is based on principles of parametric control and involves defining time-varying profiles of spatial referent coordinates for the effectors. The apparent redundancy in both motor and perceptual processes is reconsidered based on the principle of abundance. Abundance of efferent and afferent signals is viewed as the means of stabilizing both salient action characteristics and salient percepts formalized as stable manifolds in high-dimensional spaces of relevant elemental variables. This theoretical scheme has led recently to a number of novel predictions and findings. These include, in particular, lower accuracy in perception of variables produced by elements involved in a multi-element task compared to the same elements in single-element tasks, dissociation between motor and perceptual effects of muscle co-activation, force illusions induced by muscle vibration, and errors in perception of unintentional drifts in performance. Taken together, these results suggest that participation of efferent signals in perception frequently involves distorted copies of actual neural commands, particularly those to antagonist muscles. Sense of effort is associated with such distorted efferent signals. Distortions in efference copy happen spontaneously and can also be caused by changes in sensory signals, e.g., those produced by muscle vibration.

The Sense of Effort: a Cost-Benefit Theory of the Phenomenology of Mental Effort

In the current paper, we articulate a theory to explain the phenomenology of mental effort. The theory provides a working definition of mental effort, explains in what sense mental effort is a limited resource, and specifies the factors that determine whether or not mental effort is experienced as aversive. The core of our theory is the conjecture that the sense of effort is the output of a cost-benefit analysis. This cost-benefit analysis employs heuristics to weigh the current and anticipated costs of mental effort for a particular activity against the anticipated benefits. This provides a basis for spelling out testable predictions to structure future research on the phenomenology of mental effort.

Effort, Uncertainty, and the Sense of Agency

Orthodox neurocognitive accounts of the bodily sense of agency suggest that the experience of agency arises when action-effects are anticipated accurately. In this paper, I argue that while successful anticipation is crucial for the sense of agency, the role of unsuccessful prediction has been neglected, and that inefficacy and uncertainty are no less central to the sense of agency. I will argue that this is reflected in the phenomenology of agency, which can be characterized both as the experience of (1) efficacy and (2) effort. Specifically, the “sense of efficacy” refers to the perceptual experience of an action unfolding as anticipated. The “sense of effort”, in contrast, arises when an action has an uncertain trajectory, feels difficult, and demands the exertion of control. In this case, actions do not unfold as anticipated and require continuing adaptation if they are to be efficacious. I propose that, taken individually, the experience of efficacy and effort are insufficient for the sense of agency and that these experiences can even disrupt the sense of agency when they occur in isolation from each other. I further argue that a fully-fledged sense of agency depends on the temporally extensive process of prediction error-cancelation. This way, a comparator account can accommodate both the role of accurate prediction and prediction error and thus efficacy and effort.

Interaction between position sense and force control in bimanual tasks

Background Several daily living activities require people to coordinate the motion and the force produced by both arms, using their position sense and sense of effort. However, to date, the interaction in bimanual tasks has not been extensively investigated. Methods We focused on bimanual tasks where subjects were required: (Experiment 1) to move their hands until reaching the same position – equal hand position implied identical arm configurations in joint space - under different loading conditions;(Experiment 2) to produce the same amount of isometric force by pushing upward, with their hands placed in symmetric or asymmetric positions. The arm motions and forces required for accomplishing these tasks were in the vertical direction. We enrolled a healthy population of 20 subjects for Experiment 1 and 25 for Experiment 2. Our primary outcome was the systematic difference between the two hands at the end of each trial in terms of position for Experiment 1 and force for Experiment 2. In both experiments using repeated measure ANOVA we evaluated the effect of each specific condition, namely loading in the former case and hand configuration in the latter. Results In the first experiment, the difference between the hands’ positions was greater when they were concurrently loaded with different weights. Conversely, in the second experiment, when subjects were asked to exert equal forces with both arms, the systematic difference between left and right force was not influenced by symmetric or asymmetric arm configurations, but by the position of the left hand, regardless of the right hand position. The performance was better when the left hand was in the higher position. Conclusions The experiments report the reciprocal interaction between position sense and sense of effort inbimanual tasks performed by healthy subjects. Apart for the intrinsic interest for a better understanding of basic sensorimotor processes, the results are also relevant to clinical applications, for defining functional evaluation and rehabilitative protocols for people with neurological diseases or conditions that impair the ability to sense and control concurrently position and force.

Adherence to a Dysphagia Exercise Program by Oropharyngeal Cancer Survivors

Introduction: Adherence to a swallowing exercise protocol and a common compliance barrier, oral pain, was evaluated and described. Methods: A four-week dysphagia exercise program was completed by 12 individuals with a history of base of tongue cancer who were experiencing latent dysphagia. Adherence to a dysphagia exercise program was quantified. Focused outcome measures on oral pain related to dysphagia exercises and exercise related sense of effort were also included. Results: Moderate to strong adherence was reported by 75% of participants. Overall, 78.9% of exercise sessions were completed. Individuals reported little to no pain associated with dysphagia exercises throughout protocol participation. Conclusions: Routine reminders and establishment of a tracking method supported adherence with a dysphagia exercise protocol. Oral pain and sense of effort associated with completing oral and dysphagia exercises were not demonstrated to be barriers to participation in a dysphagia exercise program in people who are several years post radiation therapy completion. Keywords: dysphagia, oropharyngeal cancer, latent dysphagia, swallowing exercises

Effort matching between arms depends on relative limb geometry and personal control

Proprioception encompasses our sense of position and movement of our limbs, as well as the effort with which we engage in voluntary actions. Historically, sense of effort has been linked to centrally generated signals that elicit voluntary movements. We were interested in determining the effect of differences in limb geometry and personal control on sense of effort. In experiment 1, subjects exerted either extension or flexion torques to resist a torque applied by a robot exoskeleton to their reference elbow. They attempted to match this torque by exerting an equal effort torque (in a congruent direction with the reference arm) with their opposite (matching) arm in different limb positions (±15°). Subjects produced greater matching torque when their matching arm exerted effort toward the mirrored position of the reference (e.g., reference/matching arms at 90°/105° elbow flexion) vs. away (e.g., 90°/75° flexion). In experiment 2, a larger angular difference between arms (30°) resulted in a larger discrepancy in matched torques. Furthermore, in both experiments 1 and 2, subjects tended to overestimate the reference arm torque. This motivated a third experiment to determine whether providing more personal control might influence perceived effort and reduce the overestimation of the reference torques that we observed ( experiments 3a and 3b). Overestimation of the matched torques decreased significantly when subjects self-selected the reference torque that they were matching. Collectively, our data suggest that perceived effort between arms can be influenced by signals relating to the relative geometry of the limbs and the personal control of motor output during action. NEW & NOTEWORTHY This work highlights how limb geometry influences our sense of effort during voluntary motor actions. It also suggests that loss of personal control during motor actions leads to an increase in perceived effort.