The representation of predictive force control and internal forward models: evidence from lesion studies and brain imaging

How the conception of control influences our understanding of actions

Wilful movement requires neural control. Commonly, neural computations are thought to generate motor commands that bring the musculoskeletal system - that is, the plant - from its current physical state into a desired physical state. The current state can be estimated from past motor commands and from sensory information. Modelling movement on the basis of this concept of plant control strives to explain behaviour by identifying the computational principles for control signals that can reproduce the observed features of movements. From an alternative perspective, movements emerge in a dynamically coupled agent-environment system from the pursuit of subjective perceptual goals. Modelling movement on the basis of this concept of perceptual control aims to identify the controlled percepts and their coupling rules that can give rise to the observed characteristics of behaviour. In this Perspective, we discuss a broad spectrum of approaches to modelling human motor control and their notions of control signals, internal models, handling of sensory feedback delays and learning. We focus on the influence that the plant control and the perceptual control perspective may have on decisions when modelling empirical data, which may in turn shape our understanding of actions.

Are We Real When We Fake? Attunement to Object Weight in Natural and Pantomimed Grasping Movements

Behavioral and neuropsychological studies suggest that real actions and pantomimed actions tap, at least in part, different neural systems. Inspired by studies showing weight-attunement in real grasps, here we asked whether (and to what extent) kinematics of pantomimed reach-to-grasp movement can reveal the weight of the pretended target. To address this question, we instructed participants (n = 15) either to grasp or pretend to grasp toward two differently weighted objects, i.e., a light object and heavy object. Using linear discriminant analysis, we then proceeded to classify the weight of the target - either real or pretended - on the basis of the recorded movement patterns. Classification analysis revealed that pantomimed reach-to-grasp movements retained information about object weight, although to a lesser extent than real grasp movements. These results are discussed in relation to the mechanisms underlying the control of real and pantomimed grasping movements.

Clinical application of computerized evaluation and re-education biofeedback prototype for sensorimotor control of the hand in stroke patients

Background

Hemianaesthesia patients usually exhibit awkward and inefficient finger movements of the affected hands. Conventionally, most interventions emphasize the improvement of motor deficits, but rarely address sensory capability and sensorimotor control following stroke. Thus it is critical for stroke patients with sensory problems to incorporate appropriate strategies for dealing with sensory impairment, into traditional hand function rehabilitation programs. In this study, we used a custom-designed computerized evaluation and re-education biofeedback (CERB) prototype to analyze hand grasp performances, and monitor the training effects on hand coordination for stroke patients with sensory disturbance and without motor deficiency.

Methods

The CERB prototype was constructed to detect momentary pinch force modulation for 14 sub-acute and chronic stroke patients with sensory deficiency and 14 healthy controls. The other ten chronic stroke patients (ranges of stroke period: 6–60 months) were recruited to investigate the effects of 4-weeks computerized biofeedback treatments on the hand control ability. The biofeedback procedures provide visual and auditory cues to the participants when the interactive force of hand-to-object exceeded the target latitude in a pinch-up-holding task to trigger optimal motor strategy. Follow-up measurements were conducted one month after training. The hand sensibility, grip forces and results of hand functional tests were recorded and analyzed.

Results

The affected hands of the 14 predominant sensory stroke patients exhibited statistically significant elevation in the magnitude of peak pinch force (p = 0.033) in pinching and lifting-up tasks, and poor results for hand function tests (p = 0.005) than sound hands did. In addition, the sound hands of patients were less efficient in force modulation (p = 0.009) than the hands of healthy subjects were. Training with the biofeedback system produced significant improvements in grip force modulation (p = 0.020) and better performances in the subtests of pin insertion (p = 0.019), and lifting of lightweight objects (p = 0.005).

Conclusions

The CERB prototype can provide momentary and interactive information for quantitative assessing and re-educating force modulation appropriately for stroke patients with sensory deficits. Furthermore, the patients could transfer the learned strategy to improve hand function.

Anticipatory scaling of grip forces when lifting objects of everyday life

The ability to predict and anticipate the mechanical demands of the environment promotes smooth and skillful motor actions. Thus, the finger forces produced to grasp and lift an object are scaled to the physical properties such as weight. While grip force scaling is well established for neutral objects, only few studies analyzed objects known from daily routine and none studied grip forces. In the present study, eleven healthy subjects each lifted twelve objects of everyday life that encompassed a wide range of weights. The finger pads were covered with force sensors that enabled the measurement of grip force. A scale registered load forces. In a control experiment, the objects were wrapped into paper to prevent recognition by the subjects. Data from the first lift of each object confirmed that object weight was anticipated by adequately scaled forces. The maximum grip force rate during the force increase phase emerged as the most reliable measure to verify that weight was actually predicted and to characterize the precision of this prediction, while other force measures were scaled to object weight also when object identity was not known. Variability and linearity of the grip force-weight relationship improved for time points reached after liftoff, suggesting that sensory information refined the force adjustment. The same mechanism seemed to be involved with unrecognizable objects, though a lower precision was reached. Repeated lifting of the same object within a second and third presentation block did not improve the precision of the grip force scaling. Either practice was too variable or the motor system does not prioritize the optimization of the internal representation when objects are highly familiar.

Impaired prehension is associated with lesions of the superior and inferior hand representation within the human cerebellum

Impairment of patients with cerebellar disease in prehension is well recognized. So far specific localizations within the human cerebellum associated with the impairment have rarely been assessed. To address this question we performed voxel-based lesion symptom mapping (VLSM) in patients with chronic focal cerebellar lesions in relation to specific deficits in prehensile movements. Patients with stroke within the posterior inferior cerebellar artery territory (n = 13) or the superior cerebellar artery (SCA) territory (n = 7) and corresponding control subjects were included in the study. Participants reached out, grasped, and lifted an object with either the left or right hand and with fast or normal movement speed. Both kinematic and grip-force parameters were recorded. Magnetic resonance imaging anatomical scans of the cerebellum were acquired, and lesions were marked as regions of interest. For VLSM analysis, a nonparametric test (Brunner-Munzel) was applied. Cerebellar patients showed clear abnormalities in hand transport (impaired movement speed and straightness) and, to a lesser degree, in hand shaping (increased finger touch latencies) while grip function was preserved. Deficits were most prominent in patients with SCA lesions and for ipsilesional, fast movements. Disorders in hand transport may be more difficult to compensate than deficits in hand shaping and grip-force control in chronic focal lesions of the cerebellum because of higher demands on predictive control of interaction torques. Lesions of the superior cerebellar cortex (lobules IV, V, VI) were associated with slower hand transport, whereas lesions of both superior (lobules VI, V, VI) and inferior cerebellar cortex (lobules VII, VIII) were associated with impaired movement straightness. These findings show that both the superior and inferior hand representations within the cerebellum contribute to hand transport during prehensile movements; however, they may have a different functional role.

Size-weight illusion, anticipation, and adaptation of fingertip forces in patients with cerebellar degeneration

The smaller of two equally weighted objects is judged to be heavier when lifted (size-weight illusion [SWI]). In contrast, fingertip forces show an initial size effect but adapt to the true object weights within a few trials. The aim of this study was to investigate possible contributions of the cerebellum to SWI, force anticipation, and adaptation based on object size and weight. Eighteen participants with isolated cerebellar degeneration and 18 age- and gender-matched controls alternately lifted objects of equal weight but different size in 40 trials. All participants perceived the small object to be heavier after lifting (perceptive SWI). Fingertip forces were significantly higher during the first lift of the large object compared with the small object in the control and cerebellar groups. For the load-force rate and lifting acceleration, effects of anticipation were significantly less in the cerebellar compared with the control group. Grip and load forces were adapted to object weight during repeated lifts in both groups. Preserved perceptive SWI in cerebellar patients supports the hypothesis that perceptive SWI depends on the function of the ventral visual path that receives no or few efferents from the cerebellum. The findings of preserved anticipation and adaptation of grip forces in cerebellar patients, however, were unexpected. Reduced anticipation of load forces suggests that the neural presentation of predictive grip- and load-force control may be different. Findings show that representation and adaptation of internal models of object characteristics are not exclusively located in the cerebellum.

The role of the cerebellum for predictive control of grasping

Predictive control of grasping forces when manipulating objects in the environment is suggested to reflect internal models that capture the causal relationship between actions and their consequences. The anatomical correlate of predictive control of grasping within the central nervous system is not completely understood. One structure which has been related to the neural representation of internal models is the cerebellum. Given its stereotyped cytoarchitecture, the widespread connections with cortical and subcortical sensory-motor structures and the neural activity of cerebellar Purkinje cells during sensory-motor tasks, the cerebellum has long been considered to play a major role in the establishment and maintenance of sensory-motor representations related to voluntary movement. Such representations are necessary to predict the consequences of our own movements. Here we review theoretical concepts, electrophysiological, imaging and behavioural data suggesting the cerebellum to be the anatomical and functional correlate of internal models relevant for predictive control of grasping.

Objective evaluation of manual performance deficits in neurological movement disorders

Impaired hand function is a frequent finding in movement disorders. The skilled control of prehensile finger forces is an essential feature of tool use in daily life. In healthy subjects, grip force is precisely adjusted to the mechanical object properties, such as weight and surface friction. Grip force is accurately scaled to be only a small amount higher than the minimum necessary to prevent a hand-held object from slipping. When an object is lifted and moved around in space, grip force is modulated in parallel with the movement-induced fluctuations in load. The absence of a temporal delay between grip and load force profiles implies that the central nervous system is able to predict the load variations before the intended manipulation. Sensory information is used to adjust the level of applied finger forces efficiently to the requirements of the mechanical object properties and the task at hand. The characteristics of impaired finger force control include inefficient grip force scaling and imprecision of the temporal coupling between grip and load force profiles. Here, we review the characteristics of deficient grip force behavior in movement disorders, e.g. Parkinson's disease, task-specific dystonia, Gille de la Tourette's syndrome and cerebellar disease. Grip force analysis is a highly sensitive method to document even subtle impairments of finger force control and may be used both as a diagnostic tool and for the objective evaluation of treatment in neurological movement disorders.

The role of the feedforward paradigm in cognitive psychology

Feedforward control is a process adjusting behaviour in a continuative way. Feedforward takes place when an equilibrium state is disrupted and the system has to automatically retrieve the homeostatic stable state. It also occurs when a perturbation is previewed and must be eliminated in order to achieve a desired goal. According to the most general definition, a feedforward process operates by fixing the future representation of the desired state, the achieving of which stops the process. Then, feedforward works by means of the refinement determined by successive comparisons between the actual and target products. In its applications, a feedforward process is thought to be modulated by the subject's purpose and the environmental state. Over the years, the feedforward process has assumed different connotations in several contests of cognitive psychology. An overview of the research fields in psychology that significantly progressed with the introduction of a feedforward paradigm is provided by: (a) reviewing models in which the feedforward concept plays a fundamental role in the system control; (b) examining critical experiments related to the interaction of feedforward and feedback processes; (c) evidencing practical applications for some of the presented feedforward-based architectures.

Predictive and reactive control of grasping forces: on the role of the basal ganglia and sensory feedback

We comparatively investigated predictive and reactive grip force behaviour in 12 subjects with basal ganglia dysfunction (six subjects with Parkinson's disease, six subjects with writer's cramp), two subjects chronically lacking all tactile and proprioceptive sensory feedback and 16 sex- and age-matched control subjects. Subjects held an instrumented receptacle between the index finger and thumb. A weight was dropped into the receptacle either unexpectedly from the experimenter's hand with the subject being blindfolded or expectedly from the subject's opposite hand. This paradigm allowed us to study predictive and reactive modes of grip force control. All patients generated an overshoot in grip force, irrespective of whether the weight was dropped expectedly or unexpectedly. When the weight was dropped from the experimenter's hand, a reactive grip force response lagged behind the load perturbation at impact in patients with basal ganglia dysfunction and healthy controls. When the weight was dropped expectedly from the subject's opposite hand, patients with basal ganglia dysfunction and healthy subjects started to increase grip force prior to the release of the weight, indicating a predictive mode of control. We interpret these data to support the notion that the motor dysfunction in basal ganglia disorders is associated with deficits of sensorimotor integration. Both deafferented subjects did not show a reactive mode of force control when the weight was dropped unexpectedly, underlining the importance of sensory feedback to initiate reactive force responses. Also in the predictive mode, grip force processing was severely impaired in deafferented subjects. Thus, at least intermittent sensory information is necessary to establish and update predictive modes of grasping force control.