Role of The Cortex in Visuomotor Control of Arm Stability

Whereas numerous motor control theories describe the control of arm trajectory during reach, the control of stabilization in a constant arm position (i.e., visuomotor control of arm posture) is less clear. Three potential mechanisms have been proposed for visuomotor control of arm posture: 1) increased impedance of the arm through co-contraction of antagonistic muscles, 2) corrective muscle activity via spinal/supraspinal reflex circuits, and/or 3) intermittent voluntary corrections to errors in position. We examined the cortical mechanisms of visuomotor control of arm posture and tested the hypothesis that cortical error networks contribute to arm stabilization. We collected electroencephalography (EEG) data from 10 young healthy participants across four experimental planar movement tasks. We examined brain activity associated with intermittent voluntary corrections of position error and antagonist co-contraction during stabilization. EEG beta-band (13–26 Hz) power fluctuations were used as indicators of brain activity, and coherence between EEG electrodes was used as a measure of functional connectivity between brain regions. Cortical activity in the sensory, motor, and visual areas during arm stabilization was similar to activity during volitional arm movements and was larger than activity during co-contraction of the arm. However, cortical connectivity between the sensorimotor and visual regions was higher during arm stabilization compared with volitional arm movements and co-contraction of the arm. The difference in cortical activity and connectivity between tasks might be attributed to an underlying visuomotor error network used to update motor commands for visuomotor control of arm posture

Disruption of State Estimation in the Human Lateral Cerebellum

The cerebellum has been proposed to be a crucial component in the state estimation process that combines information from motor efferent and sensory afferent signals to produce a representation of the current state of the motor system. Such a state estimate of the moving human arm would be expected to be used when the arm is rapidly and skillfully reaching to a target. We now report the effects of transcranial magnetic stimulation (TMS) over the ipsilateral cerebellum as healthy humans were made to interrupt a slow voluntary movement to rapidly reach towards a visually defined target. Errors in the initial direction and in the final finger position of this reach-to-target movement were significantly higher for cerebellar stimulation than they were in control conditions. The average directional errors in the cerebellar TMS condition were consistent with the reaching movements being planned and initiated from an estimated hand position that was 138 ms out of date. We suggest that these results demonstrate that the cerebellum is responsible for estimating the hand position over this time interval and that TMS disrupts this state estimate

Unravelling cossed wires : dysfunction in obstetric brachial plexus lesions in the light of intertwined effects of the peripheral and central nervous system

Sensory function is impaired in adults with conservatively treated OBPL. There is widespread motor misrouting together with motor functional impairment in conservatively treated OBPL, not explained by muscle weakness. There were no differences in the degree of cocontraction between OBPL patients and healthy subjects for either the triceps or deltoid muscles during supramaximal biceps stimulation. However, elbow stiffness was approximately 1.7 times higher in OBPL patients than in control subjects during voluntary levels of contraction, suggesting a significant effect of misrouting in the patients. In children with OBPL the deficit during automatic arm abduction was not observed during voluntary movements and therefore cannot be explained by a peripheral deficit, suggesting a central component. In adults OBPL affected imagined but not actual elbow flexion suggested an impairment of motor planning. LUMC / Geneeskund

Cortical Models for Movement Control

Defense Advanced Research Projects Agency and Office of Naval Research (N0014-95-l-0409)

Principles of involuntary vs. voluntary control of human action: investigations using the Kohnstamm phenomenon

Psychological accounts of human action control strongly distinguish between voluntary and involuntary movements. In the Kohnstamm phenomenon, a sustained voluntary contraction of a muscle is followed by sustained, involuntary aftercontraction of the same muscle. This offers a useful experimental model of the voluntary/involuntary distinction, because aftercontractions physically resemble voluntary movements, while feeling subjectively very different. Despite 100 years of study, many basic questions remain unanswered about the Kohnstamm phenomenon. This thesis presents several experiments addressing these questions, and using the phenomenon to shed light on the voluntary/involuntary distinction. First, the recruitment of the Kohnstamm generator was explored by systematically varying the muscle contractions and task goal during the initial voluntary activity that induces the Kohnstamm phenomenon. This revealed that the Kohnstamm generator is a low frequency integrator. Next, experiments on physical obstruction of the involuntarily rising arm showed that afferent input can temporarily gate output from the Kohnstamm generator. Subjective estimates of contact force against the obstacle were higher than for matched voluntary movements, suggesting that the generator does not produce efference copies. In a further experiment, resistive and assistive perturbations during a horizontal Kohnstamm aftercontraction produced EMG responses, consistent with principles of negative position feedback control operating during voluntary movements, but with lower gains. Experiments in which participants were instructed to inhibit the aftercontraction showed that, though involuntary, Kohnstamm movements could nevertheless be voluntarily controlled, suggesting the novel concept of a “negative motor command”. Such voluntary inhibition caused a strange subjective experience of upward force, again suggesting a lack of efference copy for the aftercontraction. A model is presented that shows how the Kohnstamm phenomenon is generated and controlled. This systematic study of the control principles of the Kohnstamm phenomenon sheds important new light on the classical distinction between involuntary and voluntary movement

Functional Electrical Stimulation mediated by Iterative Learning Control and 3D robotics reduces motor impairment in chronic stroke

Background: Novel stroke rehabilitation techniques that employ electrical stimulation (ES) and robotic technologies are effective in reducing upper limb impairments. ES is most effective when it is applied to support the patients’ voluntary effort; however, current systems fail to fully exploit this connection. This study builds on previous work using advanced ES controllers, and aims to investigate the feasibility of Stimulation Assistance through Iterative Learning (SAIL), a novel upper limb stroke rehabilitation system which utilises robotic support, ES, and voluntary effort. Methods: Five hemiparetic, chronic stroke participants with impaired upper limb function attended 18, 1 hour intervention sessions. Participants completed virtual reality tracking tasks whereby they moved their impaired arm to follow a slowly moving sphere along a specified trajectory. To do this, the participants’ arm was supported by a robot. ES, mediated by advanced iterative learning control (ILC) algorithms, was applied to the triceps and anterior deltoid muscles. Each movement was repeated 6 times and ILC adjusted the amount of stimulation applied on each trial to improve accuracy and maximise voluntary effort. Participants completed clinical assessments (Fugl-Meyer, Action Research Arm Test) at baseline and post-intervention, as well as unassisted tracking tasks at the beginning and end of each intervention session. Data were analysed using t-tests and linear regression. Results: From baseline to post-intervention, Fugl-Meyer scores improved, assisted and unassisted tracking performance improved, and the amount of ES required to assist tracking reduced. Conclusions: The concept of minimising support from ES using ILC algorithms was demonstrated. The positive results are promising with respect to reducing upper limb impairments following stroke, however, a larger study is required to confirm this

From Parallel Sequence Representations to Calligraphic Control: A Conspiracy of Neural Circuits

Calligraphic writing presents a rich set of challenges to the human movement control system. These challenges include: initial learning, and recall from memory, of prescribed stroke sequences; critical timing of stroke onsets and durations; fine control of grip and contact forces; and letter-form invariance under voluntary size scaling, which entails fine control of stroke direction and amplitude during recruitment and derecruitment of musculoskeletal degrees of freedom. Experimental and computational studies in behavioral neuroscience have made rapid progress toward explaining the learning, planning and contTOl exercised in tasks that share features with calligraphic writing and drawing. This article summarizes computational neuroscience models and related neurobiological data that reveal critical operations spanning from parallel sequence representations to fine force control. Part one addresses stroke sequencing. It treats competitive queuing (CQ) models of sequence representation, performance, learning, and recall. Part two addresses letter size scaling and motor equivalence. It treats cursive handwriting models together with models in which sensory-motor tmnsformations are performed by circuits that learn inverse differential kinematic mappings. Part three addresses fine-grained control of timing and transient forces, by treating circuit models that learn to solve inverse dynamics problems.National Institutes of Health (R01 DC02852