Visual control of isometric force in Parkinson's disease

Two classes of action-stabilizing synergies reflecting spinal and supraspinal circuitry

We explored force-stabilizing synergies during accurate four-finger constant force production tasks in spaces of finger modes (commands to fingers computed to account for the finger interdependence) and of motor unit (MU) firing frequencies. The main specific hypothesis was that the multifinger synergies would disappear during unintentional force drifts without visual feedback on the force magnitude, whereas MU-based synergies would be robust to such drifts. Healthy participants performed four-finger accurate cyclical force production trials followed by trials of constant force production. Individual MUs were identified in the flexor digitorum superficialis (FDS) and extensor digitorum communis (EDC). Principal component analysis was applied to motor unit frequencies to identify robust MU groups (MU-modes) with parallel scaling of the firing frequencies in FDS, in EDC, and the combined MUs of FDS + EDC. The framework of the uncontrolled manifold hypothesis was used to quantify force-stabilizing synergies when visual feedback on the force magnitude was available and 15 s after turning the visual feedback off. Removing visual feedback led to a force drift toward lower magnitudes, accompanied by the disappearance of multifinger synergies. In contrast, MU-mode synergies were minimally affected by removing visual feedback off and continued to be robust for the FDS and for the EDC, while being absent for the (FDS + EDC) analysis. We interpret the findings within the theory of hierarchical control of action with spatial referent coordinates. The qualitatively different behavior of the multifinger and MU-mode-based synergies likely reflects the difference in the involved neural circuitry, supraspinal for the former and spinal for the latter.NEW & NOTEWORTHY Two types of synergies, in the space of commands to individual fingers and in the space of motor unit groups, show qualitatively different behaviors during accurate multifinger force-production tasks. After removing visual feedback, finger force synergies disappear, whereas motor unit-based synergies persist. These results point at different neural circuitry involved in these two basic classes of synergies: supraspinal for multieffector synergies, and spinal for motor unit-based synergies.

Advanced Force Coordination of Lower Extremities During Visuomotor Control Task in Soccer Players

Purpose: This study is aimed to determine specific bilateral lower extremity motor performances and coordination patterns in soccer players with healthy controls using the bilateral force control paradigm. We hypothesized that soccer players would show more advanced bilateral force control performances than untrained controls. Methods: Participants were 13 university soccer players and 13 healthy controls. Each group performed bilateral ankle dorsiflexion force control tasks across two vision conditions (i.e., vision and no-vision) and two targeted force levels (i.e., 10% and 40% maximum voluntary contraction). We calculated force accuracy, variability, and symmetry to assess force control performances. To estimate bilateral force coordination, we calculated Pearson's correlation coefficients between feet-within a single trial and considered uncontrolled manifold variables across multiple trials. In the no-vision condition, we performed secondary analyses for initial force control patters after removal of visual feedback. Results: There were no significant group differences in bilateral force accuracy and variability but, compared to the control group, soccer players showed higher force symmetry between left and right ankle dorsiflexion forces. For force coordination between feet, soccer players revealed more negative values of the correlation coefficient and greater good variability from the uncontrolled manifold analysis than those for the control group. The secondary analysis revealed no significant group differences in the time until force drift and amount of force adjustments. Conclusions: Soccer players have more compensatory and flexible interlimb force coordination strategies between feet.

Exploring the ability of strength and dexterity tests to detect hand function impairment in individuals with Parkinson's disease

BACKGROUND

Parkinson's disease (PD) can affect hand function since the beginning of the motor symptoms.

OBJECTIVE

To compare the ability of different hand function tests to: 1) distinguish individuals with PD from healthy controls; 2) differentiate stages of the disease; and 3) indicate changes over time due to disease progression.

METHODS

Twenty-four individuals with PD (Hoehn and Yahr: I-III) and 24 age- and sex-matched controls performed the Jebsen-Taylor Hand Function Test (JTHFT), the Nine-Hole Peg Test (9HPT), and the maximum grip and the maximum pinch strength tests using their right and left hands. Eight individuals with PD (six females and two males) were reassessed after 18 months. The ROC analyses and Mann-Whitney U tests (for disease progression) using the average performance of the hands were done.

RESULTS

Individuals with PD presented worse test performances than controls, except for the writing subtest of the JTHFT and the grip strength test. The JTHFT without the writing subtest (JTHFT_noW) was the most accurate to discriminate PD from controls (AUC = 0.899; sensitivity 75% and specificity 95.8%). The 9HPT and the simulated feeding and moving large, light objects JTHFT subtests were sensitive to distinguish stages, while the 9HPT, the moving small, common objects JTHFT subtest, and the grip strength were sensitive to changes with disease progression.

CONCLUSION

The JTHFT_noW was highly discriminative of the hand function impairments in PD. TwoJTHFT subtests were the most sensitives to distinguish PD stages (i.e. simulated feeding JTHFT subtest) and disease progression (i.e. moving small, common objects JTHFT subtest).

Sleep restriction impairs visually and memory-guided force control

Sleep loss is a common phenomenon with consequences to physical and mental health. While the effects of sleep restriction on working memory are well documented, it is unknown how sleep restriction affects continuous force control. The purpose of this study was to determine the effects of sleep restriction on visually and memory-guided force production magnitude and variability. We hypothesized that both visually and memory-guided force production would be impaired after sleep restriction. Fourteen men participated in an eleven-day inpatient sleep study and completed a grip force task after two nights of ten hours’ time in bed (baseline); four nights of five hours’ time in bed (sleep restriction); and one night of ten hours’ time in bed (recovery). The force task entailed four 20-second trials of isometric force production with the thumb and index finger targeting 25% of the participant’s maximum voluntary contraction. During visually guided trials, participants had continuous visual feedback of their force production. During memory-guided trials, visual feedback was removed for the last 12 seconds of each trial. During both conditions, participants were told to maintain the target force production. After sleep restriction, participants decreased the magnitude of visually guided, but not memory-guided, force production, suggesting that visual attention tasks are more affected by sleep loss than memory-guided tasks. Participants who reported feeling more alert after sleep restriction and recovery sleep produced higher force during memory-guided, but not visually guided, force production, suggesting that the perception of decreased alertness may lead to more attention to the task during memory-guided visual tasks.

The Nature of Finger Enslaving: New Results and Their Implications

We present a review on the phenomenon of unintentional finger action seen when other fingers of the hand act intentionally. This phenomenon (enslaving) has been viewed as a consequence of both peripheral (e.g., connective tissue links and multifinger muscles) and neural (e.g., projections of corticospinal pathways) factors. Recent studies have shown relatively large and fast drifts in enslaving toward higher magnitudes, which are not perceived by subjects. These and other results emphasize the defining role of neural factors in enslaving. We analyze enslaving within the framework of the theory of motor control with spatial referent coordinates. This analysis suggests that unintentional finger force changes result from drifts of referent coordinates, possibly reflecting the spread of cortical excitation.

Force variability is mostly not motor noise: Theoretical implications for motor control

Variability in muscle force is a hallmark of healthy and pathological human behavior. Predominant theories of sensorimotor control assume 'motor noise' leads to force variability and its 'signal dependence' (variability in muscle force whose amplitude increases with intensity of neural drive). Here, we demonstrate that the two proposed mechanisms for motor noise (i.e. the stochastic nature of motor unit discharge and unfused tetanic contraction) cannot account for the majority of force variability nor for its signal dependence. We do so by considering three previously underappreciated but physiologically important features of a population of motor units: 1) fusion of motor unit twitches, 2) coupling among motoneuron discharge rate, cross-bridge dynamics, and muscle mechanics, and 3) a series-elastic element to account for the aponeurosis and tendon. These results argue strongly against the idea that force variability and the resulting kinematic variability are generated primarily by 'motor noise.' Rather, they underscore the importance of variability arising from properties of control strategies embodied through distributed sensorimotor systems. As such, our study provides a critical path toward developing theories and models of sensorimotor control that provide a physiologically valid and clinically useful understanding of healthy and pathologic force variability.

Production and Perception of Intentional and Unintentional Actions

Physical approach to biological movement is based on the idea of control with referent spatial coordinates for effectors, from the whole body to single muscles. Within this framework, neural control signals induce changes in parameters of corresponding biology-specific laws of nature, and motor performance emerges as a result of interaction with the external force field. This approach is naturally compatible with the principle of abundance and the uncontrolled manifold hypothesis, which offer the framework for analysis of movement stability. The presence of two basic commands, reciprocal and co-activation, makes even single-effector tasks abundant and allows stabilizing their performance at the control level. Kinesthetic perception can be viewed as the process of estimating afferent signals within a reference system provided by the efferent process. Percepts are reflections of stable iso-perceptual manifolds in the combined afferent-efferent multi-dimensional space. This approach offers new, logical and based on laws of nature, interpretations for such phenomena as muscle co-activation, unintentional drifts in performance, and vibration-induced kinesthetic illusions. It also allows predicting new phenomena such as counter-intuitive effects of muscle co-activation of force production and perception, vibration-induced force illusions, performance drifts at two different speeds, and high variability in matching the contribution of individual elements in multi-element tasks. This approach can be developed for various subfields of movement studies including studies of athletics, movement disorders, and movement rehabilitation.

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 multielement task compared with the same elements in single-element tasks, dissociation between motor and perceptual effects of muscle coactivation, 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.

On the origin of finger enslaving: control with referent coordinates and effects of visual feedback

When a person tries to press with a finger, other fingers of the hand produce force unintentionally. We explored this phenomenon of enslaving during unintentional force drifts in the course of continuous force production by pairs of fingers of a hand. Healthy subjects performed accurate force production tasks by finger pairs Index-Middle, Middle-Ring, and Ring-Little with continuous visual feedback on the combined force of the instructed (master) fingers or of the noninstructed (enslaved) fingers. The feedback scale was adjusted to ensure that the subjects did not know the difference between these two, randomly presented, conditions. Across all finger pairs, enslaved force showed a drift upward under feedback on the master finger force, and master force showed a drift downward under feedback on the enslaved finger force. The subjects were unaware of the force drifts, which could reach over 50% of the initial force magnitude over 15 s. Across all conditions, the index of enslaving increased by ∼50% over the trial duration. The initial moment of force magnitude in pronation-supination was not a consistent predictor of the force drift magnitude. These results falsify the hypothesis that the counter-directional force drifts reflected drifts in the moment of force. They suggest that during continuous force production, enslaving increases with time, possibly due to the spread of excitation over cortical finger representations or other mechanisms, such as increased synchronization of firing of α-motoneurons innervating different compartments of extrinsic flexors. These changes in enslaving, interpreted at the level of control with referent coordinates for the fingers, can contribute to a variety of phenomena, including unintentional force drifts.NEW & NOTEWORTHY We report a consistent slow increase in finger enslaving (force production by noninstructed fingers) when visual feedback was presented on the force produced by either two instructed fingers or two noninstructed fingers of the hand. In contrast, force drifts could be in opposite directions depending on the visual feedback. We interpret enslaving and its drifts at the level of control with referent coordinates for the involved muscles, possibly reflecting spread of cortical excitation.

Precision Sensorimotor Control in Aging FMR1 Gene Premutation Carriers

BACKGROUND

Individuals with premutation alleles of the FMR1 gene are at risk of developing fragile X-associated tremor/ataxia syndrome (FXTAS), a neurodegenerative condition affecting sensorimotor function. Information on quantitative symptom traits associated with aging in premutation carriers is needed to clarify neurodegenerative processes contributing to FXTAS.

MATERIALS AND METHODS

26 FMR1 premutation carriers ages 44-77 years and 31 age-matched healthy controls completed rapid (2 s) and sustained (8 s) visually guided precision gripping tasks. Individuals pressed at multiple force levels to determine the impact of increasing the difficulty of sensorimotor actions on precision behavior. During initial pressing, reaction time, the rate at which individuals increased their force, the duration of pressing, and force accuracy were measured. During sustained gripping, the complexity of the force time series, force variability, and mean force were examined. During relaxation, the rate at which individuals decreased their force was measured. We also examined the relationships between visuomotor behavior and cytosine-guanine-guanine (CGG) repeat length and clinically rated FXTAS symptoms.

RESULTS

Relative to controls, premutation carriers showed reduced rates of initial force generation during rapid motor actions and longer durations of their initial pressing with their dominant hand. During sustained force, premutation carriers demonstrated reduced force complexity, though this effect was specific to younger premutation carries during dominant hand pressing and was more severe for younger relative to older premutation carriers at low and medium force levels. Increased reaction time and lower sustained force complexity each were associated with greater CGG repeat length for premutation carriers. Increased reaction time and increased sustained force variability were associated with more severe clinically rated FXTAS symptoms.

CONCLUSION

Overall our findings suggest multiple sensorimotor processes are disrupted in aging premutation carriers, including initial force control guided by feedforward mechanisms and sustained sensorimotor behaviors guided by sensory feedback control processes. Results indicating that sensorimotor issues in aging premutation carriers relate to both greater CGG repeat length and clinically rated FXTAS symptoms suggest that quantitative tests of precision sensorimotor ability may serve as key targets for monitoring FXTAS risk and progression.

Motor Memory Deficits Contribute to Motor Impairments in Autism Spectrum Disorder

Sensorimotor abnormalities are common in individuals with autism spectrum disorder (ASD); however, the processes underlying these deficits remain unclear. This study examined force production with and without visual feedback to determine if individuals with ASD can utilize internal representations to guide sustained force. Individuals with ASD showed a faster rate of force decay in the absence of visual feedback. Comparison of force output and tests of social and verbal abilities demonstrated a link between motor memory impairment and social and verbal deficits in individuals with ASD. This finding suggests that deficits in storage or retrieval of motor memories contribute to sensorimotor deficits and implicates frontoparietal networks involved in short-term consolidation of action dynamics used to optimize ongoing motor output.

Effects of practice on visual finger-force control in children at risk of developmental coordination disorder

BACKGROUND

The production of finger force control is essential for a large number of daily activities. There is evidence that deficits in the mechanisms of accuracy and control of finger force tasks are associated with children's motor difficulties.

OBJECTIVE

To compare the effect of practice of an isometric finger force/torque task between children with significant movement difficulty and those with no difficulty movement.

METHODS

Twenty-four children aged between 9 and 10 years (12 at risk of developmental coordination disorder and 12 with no movement difficulty - typically developing children) were asked to produce finger force/torque control in a continuous and constant 25% of maximum voluntary torque with visual feedback during 15s. Practice was given during five consecutive days with 15 trials per day. After the practice with visual feedback, children were asked to perform five trials without visual feedback. In these trials, feedback was removed 5s after the start of the trial.

RESULTS

Typically developing children were consistently more accurate in maintaining finger force/torque control than those children at risk of developmental coordination disorder. Children from both groups improved the performance in the task according to practice sessions. Also, children at risk of developmental coordination disorder poorly performed the task without visual feedback as they did when visual feedback was available.

CONCLUSION

The present study give support to the idea that movement difficulty is associated with finger force/torque control and children at risk of developmental coordination disorder can improve finger force/torque control with practice when visual feedback is available.

Performance drifts in two-finger cyclical force production tasks performed by one and two actors

We explored changes in the cyclical two-finger force performance task caused by turning visual feedback off performed either by the index and middle fingers of the dominant hand or by two index fingers of two persons. Based on an earlier study, we expected drifts in finger force amplitude and midpoint without a drift in relative phase. The subjects performed two rhythmical tasks at 1 Hz while paced by an auditory metronome. One of the tasks required cyclical changes in total force magnitude without changes in the sharing of the force between the two fingers. The other task required cyclical changes in the force sharing without changing total force magnitude. Subjects were provided with visual feedback, which showed total force magnitude and force sharing via cursor motion along the vertical and horizontal axes, respectively. Further, visual feedback was turned off, first on the variable that was not required to change and then on both variables. Turning visual feedback off led to a mean force drift toward lower magnitudes while force amplitude increased. There was a consistent drift in the relative phase in the one-hand task with the index finger leading the middle finger. No consistent relative phase drift was seen in the two-person tasks. The shape of the force cycle changed without visual feedback reflected in the lower similarity to a perfect cosine shape and in the higher time spent at lower force magnitudes. The data confirm findings of earlier studies regarding force amplitude and midpoint changes, but falsify predictions of an earlier proposed model with respect to the relative phase changes. We discuss factors that could contribute to the observed relative phase drift in the one-hand tasks including the leader-follower pattern generalized for two-effector tasks performed by one person.

Force illusions and drifts observed during muscle vibration

We explored predictions of a scheme that views position and force perception as a result of measuring proprioceptive signals within a reference frame set by ongoing efferent process. In particular, this hypothesis predicts force illusions caused by muscle vibration and mediated via changes in both afferent and efferent components of kinesthesia. Healthy subjects performed accurate steady force production tasks by pressing with the four fingers of one hand (the task hand) on individual force sensors with and without visual feedback. At various times during the trials, subjects matched the perceived force using the other hand. High-frequency vibration was applied to one or both of the forearms (over the hand and finger extensors). Without visual feedback, subjects showed a drop in the task hand force, which was significantly smaller under the vibration of that forearm. Force production by the matching hand was consistently higher than that of the task hand. Vibrating one of the forearms affected the matching hand in a manner consistent with the perception of higher magnitude of force produced by the vibrated hand. The findings were consistent between the dominant and nondominant hands. The effects of vibration on both force drift and force mismatching suggest that vibration led to shifts in both signals from proprioceptors and the efferent component of perception, the referent coordinate and/or coactivation command. The observations fit the hypothesis on combined perception of kinematic-kinetic variables with little specificity of different groups of peripheral receptors that all contribute to perception of forces and coordinates. NEW & NOTEWORTHY We show that vibration of hand/finger extensors produces consistent errors in finger force perception. Without visual feedback, finger force drifted to lower values without a drift in the matching force produced by the other hand; hand extensor vibration led to smaller finger force drift. The findings fit the scheme with combined perception of kinematic-kinetic variables and suggest that vibration leads to consistent shifts of the referent coordinate and, possibly, of coactivation command to the effector.

Improvements in force variability and structure from vision- to memory-guided submaximal isometric knee extension in subacute stroke

We examined changes in variability, accuracy, frequency composition, and temporal regularity of force signal from vision-guided to memory-guided force-matching tasks in 17 subacute stroke and 17 age-matched healthy subjects. Subjects performed a unilateral isometric knee extension at 10, 30, and 50% of peak torque [maximum voluntary contraction (MVC)] for 10 s (3 trials each). Visual feedback was removed at the 5-s mark in the first two trials (feedback withdrawal), and 30 s after the second trial the subjects were asked to produce the target force without visual feedback (force recall). The coefficient of variation and constant error were used to quantify force variability and accuracy. Force structure was assessed by the median frequency, relative spectral power in the 0-3-Hz band, and sample entropy of the force signal. At 10% MVC, the force signal in subacute stroke subjects became steadier, more broadband, and temporally more irregular after the withdrawal of visual feedback, with progressively larger error at higher contraction levels. Also, the lack of modulation in the spectral frequency at higher force levels with visual feedback persisted in both the withdrawal and recall conditions. In terms of changes from the visual feedback condition, the feedback withdrawal produced a greater difference between the paretic, nonparetic, and control legs than the force recall. The overall results suggest improvements in force variability and structure from vision- to memory-guided force control in subacute stroke despite decreased accuracy. Different sensory-motor memory retrieval mechanisms seem to be involved in the feedback withdrawal and force recall conditions, which deserves further study. NEW & NOTEWORTHY We demonstrate that in the subacute phase of stroke, force signals during a low-level isometric knee extension become steadier, more broadband in spectral power, and more complex after removal of visual feedback. Larger force errors are produced when recalling target forces than immediately after withdrawing visual feedback. Although visual feedback offers better accuracy, it worsens force variability and structure in subacute stroke. The feedback withdrawal and force recall conditions seem to involve different memory retrieval mechanisms.

Game-Based Augmented Visual Feedback for Enlarging Speech Movements in Parkinson's Disease

PURPOSE

The purpose of this pilot study was to demonstrate the effect of augmented visual feedback on acquisition and short-term retention of a relatively simple instruction to increase movement amplitude during speaking tasks in patients with dysarthria due to Parkinson's disease (PD).

METHOD

Nine patients diagnosed with PD, hypokinetic dysarthria, and impaired speech intelligibility participated in a training program aimed at increasing the size of their articulatory (tongue) movements during sentences. Two sessions were conducted: a baseline and training session, followed by a retention session 48 hr later. At baseline, sentences were produced at normal, loud, and clear speaking conditions. Game-based visual feedback regarding the size of the articulatory working space (AWS) was presented during training.

RESULTS

Eight of nine participants benefited from training, increasing their sentence AWS to a greater degree following feedback as compared with the baseline loud and clear conditions. The majority of participants were able to demonstrate the learned skill at the retention session.

CONCLUSIONS

This study demonstrated the feasibility of augmented visual feedback via articulatory kinematics for training movement enlargement in patients with hypokinesia due to PD.

SUPPLEMENTAL MATERIALS

https://doi.org/10.23641/asha.5116840.

Memory-guided force control in healthy younger and older adults

Successful performance of a memory-guided motor task requires participants to store and then recall an accurate representation of the motor goal. Further, participants must monitor motor output to make adjustments in the absence of visual feedback. The goal of this study was to examine memory-guided grip force in healthy younger and older adults and compare it to performance on behavioral tasks of working memory. Previous work demonstrates that healthy adults decrease force output as a function of time when visual feedback is not available. We hypothesized that older adults would decrease force output at a faster rate than younger adults, due to age-related deficits in working memory. Two groups of participants, younger adults (YA: N = 32, mean age 21.5 years) and older adults (OA: N = 33, mean age 69.3 years), completed four 20-s trials of isometric force with their index finger and thumb, equal to 25% of their maximum voluntary contraction. In the full-vision condition, visual feedback was available for the duration of the trial. In the no vision condition, visual feedback was removed for the last 12 s of each trial. Participants were asked to maintain constant force output in the absence of visual feedback. Participants also completed tasks of word recall and recognition and visuospatial working memory. Counter to our predictions, when visual feedback was removed, younger adults decreased force at a faster rate compared to older adults and the rate of decay was not associated with behavioral performance on tests of working memory.

Unintentional force changes in cyclical tasks performed by an abundant system: Empirical observations and a dynamical model

The study explored unintentional force changes elicited by removing visual feedback during cyclical, two-finger isometric force production tasks. Subjects performed two types of tasks at 1Hz, paced by an auditory metronome. One - Force task - required cyclical changes in total force while maintaining the sharing, defined as relative contribution of a finger to total force. The other task - Share task - required cyclical changes in sharing while keeping total force unchanged. Each trial started under full visual feedback on both force and sharing; subsequently, feedback on the variable that was instructed to stay constant was frozen, and finally feedback on the other variable was also removed. In both tasks, turning off visual feedback on total force elicited a drop in the mid-point of the force cycle and an increase in the peak-to-peak force amplitude. Turning off visual feedback on sharing led to a drift of mean share toward 50:50 across both tasks. Without visual feedback there was consistent deviation of the two force time series from the in-phase pattern (typical of the Force task) and from the out-of-phase pattern (typical of the Share task). This finding is in contrast to most earlier studies that demonstrated only two stable patterns, in-phase and out-of-phase. We interpret the results as consequences of drifts of parameters in a dynamical system leading in particular to drifts in the referent finger coordinates toward their actual coordinates. The relative phase desynchronization is caused by the right-left differences in the hypothesized drift processes, consistent with the dynamic dominance hypothesis.

Effects of visual feedback and memory on unintentional drifts in performance during finger-pressing tasks

This study tested two hypotheses on the nature of unintentional force drifts elicited by removing visual feedback during accurate force production tasks. The role of working memory (memory hypothesis) was explored in tasks with continuous force production, intermittent force production, and rest intervals over the same time interval. The assumption of unintentional drifts in referent coordinate for the fingertips was tested using manipulations of visual feedback: young healthy subjects performed accurate steady-state force production tasks by pressing with the two index fingers on individual force sensors with visual feedback on the total force, sharing ratio, both, or none. Predictions based on the memory hypothesis have been falsified. In particular, we observed consistent force drifts to lower force values during continuous force production trials only. No force drift or drifts to higher forces were observed during intermittent force production trials and following rest intervals. The hypotheses based on the idea of drifts in referent finger coordinates have been confirmed. In particular, we observed superposition of two drift processes: a drift of total force to lower magnitudes and a drift of the sharing ratio to 50:50. When visual feedback on total force only was provided, the two-finger forces showed drifts in opposite directions. We interpret the findings as evidence for the control of motor actions with changes in referent coordinates for participating effectors. Unintentional drifts in performance are viewed as natural relaxation processes in the involved systems; their typical time reflects stability in the direction of the drift. The magnitude of the drift was higher in the right (dominant) hand, which is consistent with the dynamic dominance hypothesis.

Optimality and stability of intentional and unintentional actions: I. Origins of drifts in performance

We address the nature of unintentional changes in performance in two papers. This first paper tested a hypothesis that unintentional changes in performance variables during continuous tasks without visual feedback are due to two processes. First, there is a drift of the referent coordinate for the salient performance variable toward the actual coordinate of the effector. Second, there is a drift toward minimum of a cost function. We tested this hypothesis in four-finger isometric pressing tasks that required the accurate production of a combination of total moment and total force with natural and modified finger involvement. Subjects performed accurate force-moment production tasks under visual feedback, and then visual feedback was removed for some or all of the salient variables. Analytical inverse optimization was used to compute a cost function. Without visual feedback, both force and moment drifted slowly toward lower absolute magnitudes. Over 15 s, the force drop could reach 20% of its initial magnitude while moment drop could reach 30% of its initial magnitude. Individual finger forces could show drifts toward both higher and lower forces. The cost function estimated using the analytical inverse optimization reduced its value as a consequence of the drift. We interpret the results within the framework of hierarchical control with referent spatial coordinates for salient variables at each level of the hierarchy combined with synergic control of salient variables. The force drift is discussed as a natural relaxation process toward states with lower potential energy in the physical (physiological) system involved in the task.

Variability, frequency composition, and temporal regularity of submaximal isometric elbow flexion force in subacute stroke

We compared variability, frequency composition, and temporal regularity of submaximal isometric elbow flexion force at 10, 20, 35, and 50 % of peak torque between 34 stroke subjects (5-48 days post-onset, both arms) and 24 age-matched controls (dominant arm), and related the findings in the paretic arm to motor impairment. Force variability was quantified by the coefficient of variation (CV), frequency composition by the median frequency and relative power in 0-3-, 4-6-, and 8-12-Hz bands, and regularity by the sample entropy (SampEn). The paretic elbow flexors showed significantly increased CV and relative power in 0-3-Hz band, decreased power in 4-6- and 8-12-Hz bands, and decreased SampEn compared to both the non-paretic and control elbow flexors (P ≤ 0.0002), with no differences between the latter two (P ≥ 0.012). With increasing contraction intensity, the relative power in different frequency bands was insufficiently modulated and SampEn excessively decreased in the paretic elbow flexors. Also, CV in the paretic elbow flexors was non-linearly related to the relative power in different frequency bands and SampEn across contraction intensities (rectangular hyperbolic fit, 0.21 ≤ R ² ≤ 0.55, P ≤ 0.006), whereas no force parameter correlated with arm motor impairment. These results largely extend our previous findings in the paretic knee extensors to the elbow flexors in subacute stroke, except that here force variability was increased only in the paretic elbow flexors and modulation of force regularity with increasing contraction intensity showed the opposite, decreasing pattern, which was considerably exaggerated in the paretic muscles.

The synergic control of multi-finger force production: stability of explicit and implicit task components

Manipulating objects with the hands requires the accurate production of resultant forces including shear forces; effective control of these shear forces also requires the production of internal forces normal to the surface of the object(s) being manipulated. In the present study, we investigated multi-finger synergies stabilizing shear and normal components of force, as well as drifts in both components of force, during isometric pressing tasks requiring a specific magnitude of shear force production. We hypothesized that shear and normal forces would evolve similarly in time and also show similar stability properties as assessed by the decomposition of inter-trial variance within the uncontrolled manifold hypothesis. Healthy subjects were required to accurately produce total shear and total normal forces with four fingers of the hand during a steady-state force task (with and without visual feedback) and a self-paced force pulse task. The two force components showed similar time profiles during both shear force pulse production and unintentional drift induced by turning the visual feedback off. Only the explicitly instructed components of force, however, were stabilized with multi-finger synergies. No force-stabilizing synergies and no anticipatory synergy adjustments were seen for the normal force in shear force production trials. These unexpected qualitative differences in the control of the two force components-which are produced by some of the same muscles and show high degree of temporal coupling-are interpreted within the theory of control with referent coordinates for salient variables. These observations suggest the existence of two classes of neural variables: one that translates into shifts of referent coordinates and defines changes in magnitude of salient variables, and the other controlling gains in back-coupling loops that define stability of the salient variables. Only the former are shared between the explicit and implicit task components.

Memory-guided force output is associated with self-reported ADHD symptoms in young adults

Attention-deficit/hyperactivity disorder (ADHD) is the most commonly diagnosed mental health disorder in childhood and persists into adulthood in up to 65 % of cases. ADHD is associated with adverse outcomes such as the ability to gain and maintain employment and is associated with an increased risk for substance abuse obesity workplace injuries and traffic accidents A majority of diagnosed children have motor deficits; however, few studies have examined motor deficits in young adults. This study provides a novel examination of visuomotor control of grip force in young adults with and without ADHD. Participants were instructed to maintain force production over a 20-second trial with and without real-time visual feedback about their performance. The results demonstrated that when visual feedback was available, adults with ADHD produced slightly higher grip force than controls. However, when visual feedback was removed, adults with ADHD had a faster rate of decay of force, which was associated with ADHD symptom severity and trait impulsivity. These findings suggest that there may be important differences in the way that adults with ADHD integrate visual feedback during continuous motor tasks. These may account for some of the motor impairments reported in children with ADHD. These deficits could result from (1) dysfunctional sensory motor integration and/or (2) deficits in short-term visuomotor memory.

On the nature of unintentional action: a study of force/moment drifts during multifinger tasks

We explored the origins of unintentional changes in performance during accurate force production in isometric conditions seen after turning visual feedback off. The idea of control with referent spatial coordinates suggests that these phenomena could result from drifts of the referent coordinate for the effector. Subjects performed accurate force/moment production tasks by pressing with the fingers of a hand on force sensors. Turning the visual feedback off resulted in slow drifts of both total force and total moment to lower magnitudes of these variables; these drifts were more pronounced in the right hand of the right-handed subjects. Drifts in individual finger forces could be in different direction; in particular, fingers that produced moments of force against the required total moment showed an increase in their forces. The force/moment drift was associated with a drop in the index of synergy stabilizing performance under visual feedback. The drifts in directions that changed performance (non-motor equivalent) and in directions that did not (motor equivalent) were of about the same magnitude. The results suggest that control with referent coordinates is associated with drifts of those referent coordinates toward the corresponding actual coordinates of the hand, a reflection of the natural tendency of physical systems to move toward a minimum of potential energy. The interaction between drifts of the hand referent coordinate and referent orientation leads to counterdirectional drifts in individual finger forces. The results also demonstrate that the sensory information used to create multifinger synergies is necessary for their presence over the task duration.

The nature of constant and cyclic force production: unintentional force-drift characteristics

We explored unintentional changes in forces during performance of constant and cyclic force-production tasks (F-tasks) after visual feedback removal. Based on earlier studies, we expected all force parameters to drop exponentially with time. We also explored possible role of working memory in the force drop phenomena. Healthy subjects performed constant or cyclic isometric F-tasks with the index finger under visual feedback. The cyclic task was paced by a metronome. Removing visual feedback resulted in a consistent force drop in constant F-tasks and a qualitatively similar drift in the mean force in the cyclic F-task. Both were slow with characteristic times of about 10-20 s. In contrast, force amplitude in the cyclic F-task increased quickly (within 1-2 s). When the subjects were asked to stop producing force for 5 s after the visual feedback disappeared and then resume force production, no downward force drift was seen in constant F-tasks, while in cyclic F-tasks, the drift of the mean force was present and an exaggerated increase in force amplitude was also observed. We conclude that while working memory limitations may influence cyclic F-tasks, their role in determining the force drift in constant F-tasks is limited. The results of both experiments are interpreted within the referent configuration hypothesis supplemented with an idea of unintentional drift of referent coordinates (RC-back-coupling) induced by differences between the referent and actual body configurations.

Force-stabilizing synergies in motor tasks involving two actors

We investigated the ability of two persons to produce force-stabilizing synergies in accurate multi-finger force production tasks under visual feedback on the total force only. The subjects produced a time profile of total force (the sum of two hand forces in one-person tasks and the sum of two subject forces in two-person tasks) consisting of a ramp-up, steady-state, and ramp-down segments; the steady-state segment was interrupted in the middle by a quick force pulse. Analyses of the structure of inter-trial finger force variance, motor equivalence, anticipatory synergy adjustments (ASAs), and the unintentional drift of the sharing pattern were performed. The two-person performance was characterized by a dramatically higher amount of inter-trial variance that did not affect total force, higher finger force deviations that did not affect total force (motor equivalent deviations), shorter ASAs, and larger drift of the sharing pattern. The rate of sharing pattern drift correlated with the initial disparity between the forces produced by the two persons (or two hands). The drift accelerated following the quick force pulse. Our observations show that sensory information on the task-specific performance variable is sufficient for the organization of performance-stabilizing synergies. They suggest, however, that two actors are less likely to follow a single optimization criterion as compared to a single performer. The presence of ASAs in the two-person condition might reflect fidgeting by one or both of the subjects. We discuss the characteristics of the drift in the sharing pattern as reflections of different characteristic times of motion within the subspaces that affect and do not affect salient performance variables.

Finger force changes in the absence of visual feedback in patients with Parkinson's disease

OBJECTIVES

We investigated the unintentional drift in total force and in sharing of the force between fingers in two-finger accurate force production tasks performed without visual feedback by patients with Parkinson's disease (PD) and healthy controls. In particular, we were testing a hypothesis that adaptation to the documented loss of action stability could lead to faster force drop in PD.

METHODS

PD patients and healthy controls performed accurate constant force production tasks without visual feedback by different finger pairs, starting with different force levels and different sharing patterns of force between the two fingers.

RESULTS

Both groups showed an exponential force drop with time and a drift of the sharing pattern towards 50:50. The PD group showed a significantly faster force drop without a change in speed of the sharing drift. These results were consistent across initial force levels, sharing patterns, and finger pairs. A pilot test of four subjects, two PD and two controls, showed no consistent effects of memory on the force drop.

CONCLUSIONS

We interpret the force drop as a consequence of back-coupling between the actual and referent finger coordinates that draws the referent coordinate towards the actual one. The faster force drop in the PD group is interpreted as adaptive to the loss of action stability in PD. The lack of group differences in the sharing drift suggests two potentially independent physiological mechanisms contributing to the force and sharing drifts.

SIGNIFICANCE

The hypothesis on adaptive changes in PD with the purpose to ensure stability of steady states may have important implications for treatment of PD. The speed of force drop may turn into a useful tool to quantify such adaptive changes.

Hand function is impaired in healthy older adults at risk of Parkinson's disease

Abnormal substantia nigra morphology in healthy individuals, viewed with transcranial ultrasound, is a significant risk factor for Parkinson's disease. However, little is known about the functional consequences of this abnormality (termed 'hyperechogenicity') on movement. The aim of the current study was to investigate hand function in healthy older adults with (SN+) and without (SN-) substantia nigra hyperechogenicity during object manipulation. We hypothesised that SN+ subjects would exhibit increased grip force and a slower rate of force application compared to SN- subjects. Twenty-six healthy older adults (8 SN+ aged 58 ± 8 years, 18 SN- aged 57 ± 6 years) were asked to grip and lift a light-weight object with the dominant hand. Horizontal grip force, vertical lift force, acceleration, and first dorsal interosseus EMG were recorded during three trials. During the first trial, SN+ subjects exhibited a longer period between grip onset and lift onset (i.e. preload duration; 0.27 ± 0.25 s) than SN- subjects (0.13 ± 0.08 s; P = 0.046). They also exerted a greater downward force prior to lift off (-0.54 ± 0.42 N vs. -0.21 ± 0.12 N; P = 0.005) and used a greater grip force to lift the object (19.5 ± 7.0 N vs. 14.0 ± 4.3 N; P = 0.022) than SN- subjects. No between group differences were observed in subsequent trials. SN+ subjects exhibit impaired planning for manipulation of new objects. SN+ individuals over-estimate the grip force required, despite a longer contact period prior to lifting the object. The pattern of impairment observed in SN+ subjects shares similarities with de novo Parkinson's disease patients.

Cortical hypoexcitability persists beyond the symptomatic phase of a concussion

PRIMARY OBJECTIVE

The purpose of this research was to assess cortical excitability, voluntary activation of muscle and force sensation beyond the initial highly symptomatic period post-concussion (1-4 weeks post-injury). It was hypothesized that reduced excitability of the motor cortex may impair muscle activation and alter perceptions of force and effort.

RESEARCH DESIGN

Eight concussed varsity football players were age- and position-matched with eight healthy teammates to control for training and body size. Healthy controls had not suffered a concussion in the previous 12 months.

METHODS AND PROCEDURES

Paired-pulse transcranial magnetic stimulation was used to assess cortical excitability, voluntary activation was calculated using cortical twitch interpolation technique and sense of force was determined using constant-force sensation contractions.

MAIN OUTCOMES AND RESULTS

The concussed group had lower intra-cortical facilitation (p = 0.036), lower maximal voluntary muscle activation (p = 0.038) and greater perceptions of force (p < 0.05), likely due to compensatory increases in upstream drive, than their healthy matched teammates.

CONCLUSIONS

Taken together, these findings suggest a state of hypoexcitability that persists beyond the immediate acute phase of a concussion and may result in neuromuscular impairments that would call to question the athlete's readiness to return to sport.

The measurement of visual sampling during real-world activity in Parkinson's disease and healthy controls: a structured literature review

BACKGROUND

Visual sampling techniques are used to investigate the complex role of vision during real-world activities in Parkinson's disease. Earlier research is limited to static simple tasks or measurement of eye movements alone, but more recent investigations involve more real-world activities. The approach to the objective measurement of eye movements varies with respect to instrumentation, testing protocols, and mediating factors that may influence visual sampling.

OBJECTIVES

The aim of this review was to examine previous work measuring visual sampling during real-world activities in Parkinson's disease to inform the development of robust protocols. Within this review a real-world activity was considered to be a goal-orientated motor task involving more than one body segment such as reaching or walking.

METHODS

Medline, Embase, PsychInfo, Scopus, Web of Knowledge, PubMed and the Cochrane library databases were searched. Two independent reviewers and an adjudicator screened articles that described quantitative visual sampling in people with Parkinson's disease and healthy controls.

RESULTS

Twenty full-text articles were screened and 15 met inclusion/exclusion criteria. A wide range of instruments and outcome measures were reported which were generally used in a task-dependent manner. Instrument reliability and validity was insufficiently reported in all studies. Few studies considered mediators of visual sampling such as visual or cognitive deficits.

CONCLUSIONS

Future research is required to accurately characterise visual impairments in Parkinson's disease and during real-world activities. Composite use of instruments may be required to achieve reliability and validity of visual sampling outcomes which need to be standardised. Recommendations also include assessment of cognition and basic visual function.

Modulation of force below 1 Hz: age-associated differences and the effect of magnified visual feedback

Oscillations in force output change in specific frequency bins and have important implications for understanding aging and pathological motor control. Although previous studies have demonstrated that oscillations from 0–1 Hz can be influenced by aging and visuomotor processing, these studies have averaged power within this bandwidth and not examined power in specific frequencies below 1 Hz. The purpose was to determine whether a differential modulation of force below 1 Hz contributes to changes in force control related to manipulation of visual feedback and aging. Ten young adults (25±4 yrs, 5 men) and ten older adults (71±5 yrs, 4 men) were instructed to accurately match a target force at 2% of their maximal isometric force for 35 s with abduction of the index finger. Visual feedback was manipulated by changing the visual angle (0.05°, 0.5°, 1.5°) or removing it after 15 s. Modulation of force below 1 Hz was quantified by examining the absolute and normalized power in seven frequency bins. Removal of visual feedback increased normalized power from 0–0.33 Hz and decreased normalized power from 0.66–1.0 Hz. In contrast, magnification of visual feedback (visual angles of 0.5° and 1.5°) decreased normalized power from 0–0.16 Hz and increased normalized power from 0.66–1.0 Hz. Older adults demonstrated a greater increase in the variability of force with magnification of visual feedback compared with young adults (P = 0.05). Furthermore, older adults exhibited differential force modulation of frequencies below 1 Hz compared with young adults (P<0.05). Specifically, older adults exhibited greater normalized power from 0–0.16 Hz and lesser normalized power from 0.66–0.83 Hz. The changes in force modulation predicted the changes in the variability of force with magnification of visual feedback (R² = 0.80). Our findings indicate that force oscillations below 1 Hz are associated with force control and are modified by aging and visual feedback.

Low-frequency fluctuation in continuous real-time feedback of finger force: a new paradigm for sustained attention

OBJECTIVE

Behavioral studies have suggested a low-frequency (0.05 Hz) fluctuation of sustained attention on the basis of the intra-individual variability of reaction-time. Conventional task designs for functional magnetic resonance imaging (fMRI) studies are not appropriate for frequency analysis. The present study aimed to propose a new paradigm, real-time finger force feedback (RT-FFF), to study the brain mechanisms of sustained attention and neurofeedback.

METHODS

We compared the low-frequency fluctuations in both behavioral and fMRI data from 38 healthy adults (19 males; mean age, 22.3 years). Two fMRI sessions, in RT-FFF and sham finger force feedback (S-FFF) states, were acquired (TR 2 s, Siemens Trio 3-Tesla scanner, 8 min each, counter-balanced). Behavioral data of finger force were obtained simultaneously at a sampling rate of 250 Hz.

RESULTS

Frequency analysis of the behavioral data showed lower amplitude in the low-frequency band (0.004-0.104 Hz) but higher amplitude in the high-frequency band (27.02-125 Hz) in the RT-FFF than the S-FFF states. The mean finger force was not significantly different between the two states. fMRI data analysis showed higher fractional amplitude of low-frequency fluctuation (fALFF) in the S-FFF than in the RT-FFF state in the visual cortex, but higher fALFF in RT-FFF than S-FFF in the middle frontal gyrus, the superior frontal gyrus, and the default mode network.

CONCLUSION

The behavioral results suggest that the proposed paradigm may provide a new approach to studies of sustained attention. The fMRI results suggest that a distributed network including visual, motor, attentional, and default mode networks may be involved in sustained attention and/or real-time feedback. This paradigm may be helpful for future studies on deficits of attention, such as attention deficit hyperactivity disorder and mild traumatic brain injury.

Feedback about isometric force production yields more random variations

We investigated the relation between visual feedback and the degree of structure versus randomness in the variability of single-digit, isometric force output. Participants were instructed to maintain a constant level of force during the presence or absence of visual feedback about force output. The structure of force output variability was quantified using spectral analysis and detrended fluctuation analysis. Both analyses revealed that force output was less structured (more random) when visual feedback was available than when it was not. More random performance variation seemed to reflect a corrective strategy in the control of action.

Proprioceptive deficits in Parkinson's disease patients with freezing of gait

Recent research has proposed that proprioceptive deficits may exist in Parkinson's disease (PD); however, proprioception has not been studied in those who experience freezing of gait (FOG). Proprioception was investigated through stimulation of proprioceptive receptors via patellar tendon vibration. In a force matching task to either 10% or 30% maximal voluntary contraction, response to vibration with and without vision of a force target was compared between 15 PD with FOG (PD-FOG), 16 PD without FOG (PD non-FOG), and 15 non-PD control participants (Controls). In a 15-s trial, vision of the target was provided for the first 10 s but in the last 5 s, four conditions were possible: (i) vision, no vibration; (ii) vision, vibration; (iii) no vision, no vibration; or (iv) no vision, vibration. The expected healthy response to vibration was an overshoot of the target. Controls and PD non-FOG did not perform significantly different with or without: vibration or vision. PD-FOG performed similarly to Controls and PD non-FOG in the baseline condition (i). Errors by PD-FOG on the other conditions (ii-iv) were significantly different from the baseline condition but were not significantly different from each other. The PD-FOG group significantly undershot the target when vibration was added [F((2,36))=4.8376, P<0.02] and when vision was removed [F((2,36))=4.8376, P<0.02]. It is suggested that any deviation from normal sensory availability contributes to severe deficits in PD-FOG.

The interaction of respiration and visual feedback on the control of force and neural activation of the agonist muscle

The purpose of this study was to compare force variability and the neural activation of the agonist muscle during constant isometric contractions at different force levels when the amplitude of respiration and visual feedback were varied. Twenty young adults (20-32 years, 10 men and 10 women) were instructed to accurately match a target force at 15% and 50% of their maximal voluntary contraction (MVC) with abduction of the index finger while controlling their respiration at different amplitudes (85%, 100% and 125% normal) in the presence and absence of visual feedback. Each trial lasted 22s and visual feedback was removed from 8-12 and 16-20s. Each subject performed three trials with each respiratory condition at each force level. Force variability was quantified as the standard deviation of the detrended force data. The neural activation of the first dorsal interosseus (FDI) was measured with bipolar surface electrodes placed distal to the innervation zone. Relative to normal respiration, force variability increased significantly only during high-amplitude respiration (∼63%). The increase in force variability from normal- to high-amplitude respiration was strongly associated with amplified force oscillations from 0 to 3 Hz (R(2) ranged from .68 to .84, p< .001). Furthermore, the increase in force variability was exacerbated in the presence of visual feedback at 50% MVC (vision vs. no-vision: .97 vs. .87N) and was strongly associated with amplified force oscillations from 0 to 1 Hz (R(2)= .82) and weakly associated with greater power from 12 to 30 Hz (R(2)= .24) in the EMG of the agonist muscle. Our findings demonstrate that high-amplitude respiration and visual feedback of force interact and amplify force variability in young adults during moderate levels of effort.

Parkinson's disease differentially affects adaptation to gradual as compared to sudden visuomotor distortions

Patients with Parkinson's disease (PD) have difficulties in movement adaptation to optimize performance in novel environmental contexts such as altered screen cursor-hand relationships. Prior studies have shown that the time course of the distortion differentially affects visuomotor adaptation to screen cursor rotations, suggesting separate mechanisms for gradual and sudden adaptation. Moreover, studies in human and non-human primates suggest that adaptation to sudden kinematic distortions may engage the basal ganglia, whereas adaptation to gradual kinematic distortions involves cerebellar structures. In the present studies, participants were patients with PD, who performed center-out pointing movements, using either a digitizer tablet and pen or a computer trackball, under normal or rotated screen cursor feedback conditions. The initial study tested patients with PD using a cross-over experimental design for adaptation to gradual as compared with sudden rotated hand-screen cursor relationships and revealed significant after-effects for the gradual adaptation task only. Consistent with these results, findings from a follow-up experiment using a trackball that required only small finger movements showed that patients with PD adapt better to gradual as against sudden perturbations, when compared to age-matched healthy controls. We conclude that Parkinson's disease affects adaptation to sudden visuomotor distortions but spares adaptation to gradual distortions.

Comparison of four evaluation approaches in transcutaneous electrical nerve stimulation treatment in two incomplete tetraplegic subjects

INTRODUCTION

  In the present investigation, we applied the whole-hand transcutaneous electrical nerve stimulation (TENS) therapy to two incomplete tetraplegic subjects and assessed their progress with four evaluation methods.

METHODS

  Two spinal cord injured subjects with spastic upper extremities participated in the study. The TENS therapy was added to their regular treatment. The TENS was delivered to the subject's hands by a conductive glove. The therapy consisted of 20-min sessions each working day during a period of four weeks. The used assessment methods were: maximal force test, force tracking task, Jebsen-Taylor hand function test, and modified Ashworth scale.

RESULTS

  The results show increased finger muscle strength, improved motor control and hand function in both patients. The reduction of muscle tone, as assessed by the modified Ashworth scale, was observed in one subject.

DISCUSSION

  There was no correlation found between the Jebsen-Taylor test and the maximal force test or force tracking task in this investigation. Assessment methods are complementary to each other as each one adds new or more detailed information about level of impairment.

CONCLUSION

  From the comparison of four evaluation methods, it is evident that different assessments and measurements should be used in order to get better picture of patient's upper extremity impairment.

Interference effects from observed movement in Parkinson's disease

Previous research has demonstrated that Parkinson's disease patients have an increased susceptibility to response conflict. In the present study, the authors investigate whether Parkinson's patients have a similar sensitivity to interference from observed movements. In all, 10 patients and 10 controls performed horizontal and vertical arm movements while watching a video of either a person performing similar movements or a moving dot. Movements were performed in the same plane (congruent) and orthogonal to the observed movement (incongruent). The off-axis variance of movements was our index of interference. Although patients tended to exhibit more off-axis variability than did controls, both groups demonstrated similar congruence effects, with greater variance in incongruent conditions. These results indicated that increased susceptibility to interference in Parkinson's disease does not extend to interference from observed movements.

Removal of visual feedback alters muscle activity and reduces force variability during constant isometric contractions

The purpose of this study was to compare force accuracy, force variability and muscle activity during constant isometric contractions at different force levels with and without visual feedback and at different feedback gains. In experiment 1, subjects were instructed to accurately match the target force at 2, 15, 30, 50, and 70% of their maximal isometric force with abduction of the index finger and maintain their force even in the absence of visual feedback. Each trial lasted 22 s and visual feedback was removed from 8-12 to 16-20 s. Each subject performed 6 trials at each target force, half with visual gain of 51.2 pixels/N and the rest with a visual gain of 12.8 pixels/N. Force error was calculated as the root mean square error of the force trace from the target line. Force variability was quantified as the standard deviation and coefficient of variation (CVF) of the force trace. The EMG activity of the agonist (first dorsal interosseus; FDI) was measured with bipolar surface electrodes placed distal to the innervation zone. Independent of visual gain and force level, subjects exhibited lower force error with the visual feedback condition (2.53 +/- 2.95 vs. 2.71 +/- 2.97 N; P < 0.01); whereas, force variability was lower when visual feedback was removed (CVF: 4.06 +/- 3.11 vs. 4.47 +/- 3.14, P < 0.01). The EMG activity of the FDI muscle was higher during the visual feedback condition and this difference increased especially at higher force levels (70%: 370 +/- 149 vs. 350 +/- 143 microV, P < 0.01). Experiment 2 examined whether the findings of experiment 1 were driven by the higher force levels and proximity in the gain of visual feedback. Subjects performed constant isometric contractions with the abduction of the index finger at an absolute force of 2 N, with two distinct feedback gains of 15 and 3,000 pixels/N. In agreement with the findings of experiment 1, subjects exhibited lower force error in the presence of visual feedback especially when the feedback gain was high (0.057 +/- 0.03 vs. 0.095 +/- 0.05 N). However, force variability was not affected by the vastly distinct feedback gains at this force, which supported and extended the findings from experiment 1. Our findings demonstrate that although removal of visual feedback amplifies force error, it can reduce force variability during constant isometric contractions due to an altered activation of the primary agonist muscle most likely at moderate force levels in young adults.

Error processing during online motor control depends on the response accuracy

We investigated which brain areas show error-related activity during online motor control while errors occur independently from decision making. During motor tasks, error is a deviation from accuracy or correctness. The effect of the accuracy level on error-related brain activity is unclear. Using functional Magnetic Resonance Imaging (fMRI), we investigated how error-related brain activity, especially in fronto-medial wall areas, depended on motor accuracy (MA). Subjects performed a force tracking task with the thumb-index grip: to continuously follow a moving target on a monitor with a cursor which position was controlled by the force amount produced by the fingers. Task difficulty varied with changes in the cursor size (the smaller the cursor, the more difficult the task). We measured the motor accuracy (mean distance between the cursor center and the target) and the error amount (cursor out of the target). Errors were produced when motor accuracy was low and also when motor accuracy was high. For fMRI data processing, we defined a model based on both the error amount and the motor accuracy. The results showed that supplementary motor area (SMA) and dorsal anterior cingulate cortex (ACC) activation increased with error and task difficulty independent of the accuracy of motor control. Interestingly, activity in the rostral part of left ACC only increased with error when the motor accuracy was low, independently from task difficulty. These results suggest a clear functional dissociation between dorsal and rostral ACC in error processing which depends on the amount of attentional resources allocated to motor accuracy.

The effect of age on the grip force control in lateral grip

In the paper we present the grip force tracking system for the evaluation of grip force control. We developed a grip measuring device which can be used for the computer assisted measurements of the grip force in real time. The device was used as an input to a force-tracking task where the subject applied the grip force according to the visual feedback from the computer screen. The performance of the task was evaluated by the tracking error between the target signal and the measured force. We assessed the grip force control in the groups of 10-year old children, 25- to 35-year old adults and 50- to 60-year old adults. The subjects performed a sinus tracking task which required periodic muscle activation to produce the desired output. The results of the average tracking errors show significant differences in grip force control among the three tested groups. The largest variability among subjects was observed in the group of children and older adults. No significant difference in force control was found between the dominant and non-dominant hand. The grip force tracking system presented is aimed to be used for the evaluation of grip force control in patients with different sensory-motor impairments.

Effects of STN DBS on memory guided force control in Parkinson's disease (June 2007)

This study examined the control of elbow force in nine patients with Parkinson's disease when visual feedback was available and when visual feedback was removed to determine how medication (Meds) and unilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) affect memory guided force control. Patients were examined in each of four treatment conditions: 1) off treatment; 2) Meds; 3) STN DBS; and 4) Meds plus STN DBS. With visual feedback available, there was no difference in force output across treatment conditions. When visual feedback was removed force output drifted under the target in both the off-treatment and the Meds conditions. However, when on STN DBS or Meds plus STN DBS force output drifted above the target. As such, only STN DBS had a significant effect on force output in the vision removed condition. Increased force output when on STN DBS may have occurred due to disruptions in the basal ganglia-thalamo-cortical circuitry. We suggest that modulation of output of the internal segment of the globus pallidus by STN DBS may drive the effect of STN DBS on memory guided force control.

Effects of focal hand dystonia on visually guided and internally guided force control

BACKGROUND

A fundamental feature underlying many movement disorders is increased variability in the motor response. Despite abnormalities of grip force control in people with dystonia, it is not clear whether dystonia is also associated with increased variability in force output and whether force variability in dystonia is affected by the presence or absence of visual feedback.

OBJECTIVE

To examine force variability in 16 patients with writer's cramp and 16 matched controls.

METHODS

The variability of force output at the wrist under conditions of both vision and no vision was examined. The underlying frequency structure of the force signal was also compared across groups. Participants produced isometric wrist flexion to targets at 25% and 50% of their maximum voluntary contraction strength under conditions of both vision and no vision.

RESULTS

Similar levels of force variability were observed in patients with dystonia and controls at the lower force levels, but patients with dystonia were less variable in their force output than controls at the higher force level. This reduction in variability in people with dystonia at 50% maximum voluntary contraction was not affected by vision. Although a similar dominant frequency in force output was observed in people with dystonia and controls, a reduced variability in the group with dystonia at the higher force level was due to reduced power in the 0-4-Hz frequency bin.

CONCLUSIONS

The first evidence of a movement disorder with reduced variability is provided. The findings are compatible with a model of dystonia, which includes reduced cortical activation in response to sensory input from the periphery and reduced flexibility in motor output.