Visuo-motor adaptation during inactivation of the cerebellar nuclei: A preliminary report

The role of the cerebellum in adaptation: ALE meta-analyses on sensory feedback error

It is widely accepted that unexpected sensory consequences of self‐action engage the cerebellum. However, we currently lack consensus on where in the cerebellum, we find fine‐grained differentiation to unexpected sensory feedback. This may result from methodological diversity in task‐based human neuroimaging studies that experimentally alter the quality of self‐generated sensory feedback. We gathered existing studies that manipulated sensory feedback using a variety of methodological approaches and performed activation likelihood estimation (ALE) meta‐analyses. Only half of these studies reported cerebellar activation with considerable variation in spatial location. Consequently, ALE analyses did not reveal significantly increased likelihood of activation in the cerebellum despite the broad scientific consensus of the cerebellum's involvement. In light of the high degree of methodological variability in published studies, we tested for statistical dependence between methodological factors that varied across the published studies. Experiments that elicited an adaptive response to continuously altered sensory feedback more frequently reported activation in the cerebellum than those experiments that did not induce adaptation. These findings may explain the surprisingly low rate of significant cerebellar activation across brain imaging studies investigating unexpected sensory feedback. Furthermore, limitations of functional magnetic resonance imaging to probe the cerebellum could play a role as climbing fiber activity associated with feedback error processing may not be captured by it. We provide methodological recommendations that may guide future studies.

“Abnormal” movements: What are they reflections of?

We agree with Latash & Anson that therapeutic approaches should be directed toward solving the underlying problem, not toward adapting the abnormal to normal behaviour. The fundamental obstacle, however, is that doing so requires a solution of the “equivalence problem” in movement control.

Defective preprogramming does not account for the clinical deficits of Parkinson's disease

Latash & Anson argue that changed motor patterns should not be viewed as pathological. Instead, they should be viewed as adaptations to a primary deficit. We argue that the evidence shows: (1) bradykinesia is not an adaptation to a different primary deficit, and (2) bradykinetic movements are not “normal” slowed movements but, to the contrary, bradykinesia is part of the pathophysiology of Parkinson's disease.

Generic mechanisms of coordination in special populations

We show that left-handers can be considered as a “special” population. We indicate that the asymmetries in performance exhibited by left-handers are due to a basic asymmetry in the underlying coordination dynamics that constrains bimanual coordination. In contrast to the claims of Latash & Anson, we argue that considerable knowledge has been gained regarding the essential equations of motion that govern biological coordination.

“Normal” is not the issue: It is “effective” goal attainment that counts

Adaptive motor patterns that emerge after a neuromusculoskeletal lesion reflect not only the primary lesion, but also the dynamic characteristics of the musculoskeletal linkage and environment in which the action is performed. Although these patterns may be optimal, they may also be ineffective as goal-directed actions; effectiveness may only be regained if training addresses primary deficits and ensures practice without gross biomechanical adaptations.

The goal of treatment for motor impairment is not to “normalize” but to “functionalize” through facilitative modulation and enabling context

Adaptations occurring in the central nervous system (CNS) in the presence of pathology are not uniformly “good” for the organism when viewed in a functional context. A functional reordering of CNS priorities can be produced by allowing restructuring of the external context or through modification of neuromuscular physiology or anatomy designed to reduce the inherent restriction of functional movement in upper motor neuron syndrome. In fact, volitional control can often be “unmasked” through such interventions. Therapeutic interventions should not be directed toward “normalization” of motor patterns but should permit a functional reordering of CNS priorities that would otherwise not be possible.

What are “normal movements” in atypical populations?

Redundancy of the motor control system is an important feature that gives the central control structures options for solving everyday motor problems. The choice of particular control patterns is based on priorities (coordinative rules) that are presently unknown. Motor patterns observed in unimpaired young adults reflect these priorities. We hypothesize that under certain atypical conditions, which may include disorders in perception of the environment and in decision making, structural or biochemical changes within the central nervous system (CNS), and/or structural changes of the effectors, the central nervous system may reconsider its priorities. A new set of priorities will reflect the current state of the system and may lead to different patterns of voluntary movement. Under such conditions, changed motor patterns should be considered not pathological but rather adaptive to a primary disorder and may even be viewed as optimal for a given state of the system of movement production. Therapeutic approaches should not be directed toward restoring the motor patterns to as close to “normal” as possible but rather toward resolving the original underlying problem. We illustrate this approach using, as examples, movements in amputees, in patients with Parkinson's disease, in patients with dystonia, and in persons with Down syndrome.

What are “normal movements” inanypopulation?

It is the contention of Latash & Anson (L&A) that in atypical populations, such as those with cognitive, central neurological, or peripheral disorders, the central nervous system (CNS) is capable of producing more effective, though often less “normal,” movement patterns ifleft to its own devices. It is the aim of this commentary to extend their argument to other populations by pointing out the many parallels with development of movement patterns in sport.

Should stereotypic movement synergies in hemiparetic patients be considered adaptive?

The motor deficits observed in patients following some lesions of the central nervous system may be viewed as falling at one extreme of the continuum of possible motor behaviours. They are usually associated with an impaired ability to select and control specific movements from the available repertoire. Such movements may not be viewed as abnormal. However, it is unlikely that the primary motor deficits can all be considered adaptive.

“Constraint” versus “choice” in preferred movement patterns

Movement patterns in impaired gait are not selected by a smart central nervous system (CNS), but arise by virtue of mutual constraints of task, limitations in personal dynamics available for the task, and optimality criteria. An oscillatory model of gait cycle is presented that exemplifies this control and coordination scheme. Preferred gait patterns may be based on physical principles rather than CNS “coordinative rules.”

Developmental “movement disorders” and problem solving

Normal infants show a wide range of “atypical” movements. These, like the movements of atypical and normal adults, are best characterized as solutions to motor problems. Motor patterns alone may not be precise indicators of neurological status.

Is motor pathology associated with setting new CNS priorities or with increased difficulty in overcoming or suppressing preexisting CNS priorities?

Whereas Latash & Anson (L&A) have underscored the rearrangement or setting of new priorities, our primary focus is on preexisting central nervous system (CNS) priorities that become even more prevalent and intrusive under pathological conditions. The adaptations observed in the disordered motor system can often be understood against the backdrop of these primary CNS constraints. Even though this perspective has not been specifically addressed in the target article, we consider it complementary and not necessarily in opposition to L&A's primary thesis.

Feedback-dependent modulation of isometric force control: an EEG study in visuomotor integration

The primary purpose of this investigation was to examine the cortical mechanisms underlying visuomotor integration in an experiment directly manipulating visual feedback (control-signal gain) as participants executed a grasping task. This was accomplished by assessing human electroencephalograms in both time and frequency domains and relating these measures to the performance accuracy of isometric force control. The basic experimental manipulation consisted of subjects controlling a grip dynamometer and the subsequent force trace displayed on a computer monitor at various magnitudes of force output and control-signal gain. Several findings from this study were of interest. First, the effects of control-signal gain and its interplay with the magnitude of force were most evident across the parietal and frontocentral electrode locations--areas specifically related to multi-modal sensory evaluation (parietal lobe) and higher-order movement control (supplementary and mesial premotor areas). Second, electroencephalography (EEG) measures in the time domain, i.e., slow-wave potentials, were sensitive to control-signal gain only during the ramp phase of force production (period of reaching the target force), not the static phase (period of maintaining the target force level). Third, EEG measures within the frequency domain (event-related desynchronization), unlike the slow-wave potential measures, were sensitive to control-signal gain during the static phase of force production--a sensitivity that was directly related to improvements in the accuracy of isometric force control. The findings of this investigation are described in relation to the existent literature on human visuomotor integration with special attention paid to the distinct spatial and temporal electrocortical patterns exhibited under varying degrees of visual feedback and magnitudes of force output during grasping.

A study of motor performance and motor learning in episodic ataxia

Episodic ataxias are rare disorders in which periodic episodes of ataxia are separated by normal or near normal motor behaviour. They probably arise from dysfunctional membrane ion channels in the cerebellum. A patient with episodic ataxia EA-2 performed three motor tasks, before, during and after an ataxic episode. In all three tasks there were significant performance deficits during the ataxic episode. Two of the tasks also assessed motor adaptation (prism adaptation) or motor learning (ideogram drawing). In neither task was there significant disruption of motor adaptation or learning. These results suggest that the cerebellum may have separate roles in learning and in performance of visually guided movements, and that the dysfunction in this patient affected only his motor performance.

Thought is action

It is difficult to chart “normal movements” in atypical populations, such as those with Parkinson's disease, because there is great variability in the pattern of motor changes, both within and between patients. However, the potential clinical implications of Latash & Anson's theme are positive and powerful.

When are adaptive motor patterns nonadaptive?

The assumption that adaptive changes in motor patterns are optimal is questioned. Instances are cited where alteration of the adaptive motor patterns may be warranted. Other issues discussed are: (1) the relationship between central nervous system (CNS) priorities and the individual's priorities, (2) the use of cognitive bypass strategies by impaired individuals, and (3) conceiving CNS priorities as coordinative rules.

Dynamic similarities in action systems

Latash & Anson's contention that movement patterns that are different from those typically observed in persons without impairments should not be considered abnormal and usually should not be corrected is consistent with Davis & Burton's Ecological Task Analysis (ETA). Extending from the ETA concept of performer-scaled performance measures, the use of Froude numbers may offer insight into the global dynamics of a person's action system.

On optimality and movement disorders: A dynamic systems perspective

The argument that disordered or changed motor patterns can be adaptive or optimal given the constraints imposed is supported. Two major points of criticism are made, concerning the perceived superstates of the primary disorder and the absence of a systematic approach to identifying this adaptivity and optimality. We have tried to give the general outlines of such an approach to disordered movements from a dynamic systems perspective.

Bradykinesia in Parkinson's disease and cocontraction activity in dystonia are unlikely to be due to adaptive changes in the CNS

Latash & Anson's explanation of bradykinesia in patients with Parkinson's disease and cocontraction in dystonic patients is intriguing. However, the proposed adaptive changes in the central nervous system do not fit well with both clinical and experimental evidence of motor impairment in these patients. In particular, we question the explanation of: (1) the role of postural reactions and spatial accuracy in bradykinesia, (2) certain abnormalities during the execution of sequential and simultaneous movements, (3) the sudden changes in mobility (ON and OFF) of Parkinsonian patients, and (4) the meaning of reflex circuitry changes in dystonia.

Evaluation of central commands: Toward a theoretical basis for rehabilitation

The commentary focuses on the need for motor control modeling as a rationale for rehabilitation. We give examples in a bimanual unloading task and examine the potential consequences for recovery in patients with cerebral lesions. Hierarchical models of motor control lead to a distinction between “task optimization” and “motor optimization” and to a qualifier on Latash & Anson's “hands-off” position.

Toward peaceful coexistence of adaptive central strategies and medical professionals

We start with a number of philosophical and theoretical issues related to motor control, proceed through a spectrum of problems related to the role of adaptive changes in the central nervous system (CNS) in response to a primary disorder, and end with tentative practical recommendations. We consider the hierarchical and dynamic approaches to motor control not as incompatible alternatives but as ways of approaching two equally important issues, those of control and coordination. Professionals working in the area of physical therapy and rehabilitation should make use of the adaptive abilities of the CNS, identify goals, provide tools, and allow the CNS to develop optimal strategies. Therapists should intervene if they suspect that the CNS settles down in a local rather than a global optimum because of such factors as pain or the lack of a long-term predictive ability. Adaptive changes within the CNS may be important not only in pathologies but also in cases of specialized training, normal growth, and normal ageing.

Rehabilitation promotes functional movement in atypical populations

The suggestion that movement disorders exhibited by people with central nervous system (CNS) dysfunction should be considered normal and therefore not rehabilitated is rejected from three standpoints: (1) the CNS does not always select the best movement patterns for optimal long term outcome, (2) there is literature demonstrating that rehabilitation enhances motor function and independence, and (3) there exists a capacity for motor recovery and motor skill learning following brain damage.

Posturo-kinetic capacity in the disabled

“Normal movements” in atypical populations address the question of postural control: Voluntary movement is a perturbation of body balance and cannot be executed without a convenient counterperturbation. Despite a change in the postural program in relation to the impairment (Parkinson's disease, paraplegia), the performance level is decreased. Movements are not “normal,” owing to a reduction in posturokinetic capacity.

What is the appropriate criterion for therapeutic intervention in the motor domain?

Observations are made on Latash & Anson's theme that changed motor patterns should not be viewed as pathological. Normality in motor control might be characterised in terms of qualitative differences in control mechanisms rather than differences in movement patterns. Understanding of the concepts implicit in instructions to subjects should be confirmed. Principles of energy expenditure in atypical movements are outlined. If restoration of normality is not adopted as a criterion for therapeutic intervention, what might be proposed as an alternative?

Altered bilateral muscle synergies after stroke

Using movement disorders to understand the central nervous system's goals for motor behaviour may be easier in neurological models with a focal lesion of sudden onset, because the distinction between primary and adaptive changes may be clearer than in slowly progressive and/or diffuse neurological disease.

What makes a population atypical–priorities or constraints?

Priorities for movement reflect constraints deriving from the motor system, task goals, and physical environment. Atypical and typical populations alike set and reset priorities in response to constraints, and they do so at many processing loci and time scales. Efforts to understand what is atypical about a population should focus first on the constraints it encounters.

Anticipatory postural mechanisms: Some evidence and methodological implications

To understand the basic priorities of the central nervous system in human motor control, neurophysiological parameters are important. Certain H-reflex methods related to anticipatory postural control are particularly useful and may have therapeutic implications.

The concept of “normal” movement and its consequences for therapy

The guideline for therapy should be the improvement of the individual's functionality, not the acquisition of the ill-defined “goldstandard” of normal movement. However, Latash & Anson's suggestion that only primary causes of dysfunction should be treated is problematic for two reasons: First, the distinction between genuine and adaptive changes in motor performance is not always possible, and second, adaptive changes do not necessarily improve motor function, but may actually be detrimental to the system's performance.

Towards functional movement: Implications for research and therapy

Researchers and clinicians have different goals and constraints in trying to understand and treat movement disorders. Research on primary and secondary aspects of disorders may lead to effective treatment aimed at restoring functional movements even if the primary disorder cannot be cured. Similarly, movement patterns may be maladaptive and hence need therapeutic intervention to restore functional movement skills.

Frames of reference and normal movement

Latash & Anson propose that so-called abnormal movements may be stable and adaptive coordination and control solutions to task goals in action. Their interpretation of this established viewpoint is confused throughout in the persistent crossing of frames of reference, both in the description of movement and action and the proposed theory for motor control.

Adaptive changes in postural reactions after unilateral leg amputation

Changes in anticipatory postural adjustment following below-the-knee amputation may be considered as adaptive for reacquiring balance control.

How functional are atypical motor patterns?

Latash & Anson are commended for pointing out that investigators of atypical motor performance have too often failed to recognize the adaptive nature of many atypical movement characteristics. However, the authors' assumption that atypical motor patterns can be considered “normal” and, by implication, that it is futile to attempt to teach different patterns to patients that may be more functional, is criticized.

Anthropomorphizing the CNS: Is it what or who you know?

The target article pays insufficient attention to (1) the difficulties that arise when the term “normal” is used to describe motor skill behavior, and (2) the need to resist ascribing motor control to an intelligent central nervous system. A complex systems perspective is proposed with attractor states and periodicity identified as markers of atypical coordination and control.

Optimal search strategies for optimal motor solutions: Self-determination or informed guidance?

Impoverished resources may direct the pathologic central nervous system toward local solutions during ontogeny. The resultant motor behavior can be systemically optimal without being functionally optimal. Therapeutic intervention should focus on facilitating function (rather than “normalcy”) by encouraging behavioral exploration through appropriate combinations of task and environmental variations with respect to individual capabilities. Some health professions have already adopted this philosophy.

Abnormal movements can be identified in “atypical” populations

Latash & Anson's discussion of movements of “atypical” populations is potentially topical but unhelpful. Abnormal movements can be distinguished from any compensatory adjustments in Parkinson's, Huntington's, and Alzheimer's diseases and schizophrenia. To suggest otherwise may do “atypical” populations and their caregivers a disservice. It would be more meaningful to discuss the relative modularity and separability of cognitive and motor processes.