Stroke Rehabilitation Reaches a Threshold

PLoS Computational Biology, Aug 2008

Motor training with the upper limb affected by stroke partially reverses the loss of cortical representation after lesion and has been proposed to increase spontaneous arm use. Moreover, repeated attempts to use the affected hand in daily activities create a form of practice that can potentially lead to further improvement in motor performance. We thus hypothesized that if motor retraining after stroke increases spontaneous arm use sufficiently, then the patient will enter a virtuous circle in which spontaneous arm use and motor performance reinforce each other. In contrast, if the dose of therapy is not sufficient to bring spontaneous use above threshold, then performance will not increase and the patient will further develop compensatory strategies with the less affected hand. To refine this hypothesis, we developed a computational model of bilateral hand use in arm reaching to study the interactions between adaptive decision making and motor relearning after motor cortex lesion. The model contains a left and a right motor cortex, each controlling the opposite arm, and a single action choice module. The action choice module learns, via reinforcement learning, the value of using each arm for reaching in specific directions. Each motor cortex uses a neural population code to specify the initial direction along which the contralateral hand moves towards a target. The motor cortex learns to minimize directional errors and to maximize neuronal activity for each movement. The derived learning rule accounts for the reversal of the loss of cortical representation after rehabilitation and the increase of this loss after stroke with insufficient rehabilitation. Further, our model exhibits nonlinear and bistable behavior: if natural recovery, motor training, or both, brings performance above a certain threshold, then training can be stopped, as the repeated spontaneous arm use provides a form of motor learning that further bootstraps performance and spontaneous use. Below this threshold, motor training is “in vain”: there is little spontaneous arm use after training, the model exhibits learned nonuse, and compensatory movements with the less affected hand are reinforced. By exploring the nonlinear dynamics of stroke recovery using a biologically plausible neural model that accounts for reversal of the loss of motor cortex representation following rehabilitation or the lack thereof, respectively, we can explain previously hard to reconcile data on spontaneous arm use in stroke recovery. Further, our threshold prediction could be tested with an adaptive train–wait–train paradigm: if spontaneous arm use has increased in the “wait” period, then the threshold has been reached, and rehabilitation can be stopped. If spontaneous arm use is still low or has decreased, then another bout of rehabilitation is to be provided.

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Stroke Rehabilitation Reaches a Threshold

Citation: Han CE, Arbib MA, Schweighofer N ( Stroke Rehabilitation Reaches a Threshold Cheol E. Han 0 Michael A. Arbib 0 Nicolas Schweighofer 0 Karl J. Friston, University College London, United Kingdom 0 1 Department of Computer Science, University of Southern California, Los Angeles, California, United States of America, 2 USC Brain Project, University of Southern California, Los Angeles, California, United States of America, 3 Department of Computer Science, University of Southern California, Los Angeles, California, United States of America, 4 Department of Neuroscience, University of Southern California, Los Angeles, California, United States of America, 5 Department of Biokinesiology and Physical Therapy, University of Southern California , Los Angeles, California , United States of America Motor training with the upper limb affected by stroke partially reverses the loss of cortical representation after lesion and has been proposed to increase spontaneous arm use. Moreover, repeated attempts to use the affected hand in daily activities create a form of practice that can potentially lead to further improvement in motor performance. We thus hypothesized that if motor retraining after stroke increases spontaneous arm use sufficiently, then the patient will enter a virtuous circle in which spontaneous arm use and motor performance reinforce each other. In contrast, if the dose of therapy is not sufficient to bring spontaneous use above threshold, then performance will not increase and the patient will further develop compensatory strategies with the less affected hand. To refine this hypothesis, we developed a computational model of bilateral hand use in arm reaching to study the interactions between adaptive decision making and motor relearning after motor cortex lesion. The model contains a left and a right motor cortex, each controlling the opposite arm, and a single action choice module. The action choice module learns, via reinforcement learning, the value of using each arm for reaching in specific directions. Each motor cortex uses a neural population code to specify the initial direction along which the contralateral hand moves towards a target. The motor cortex learns to minimize directional errors and to maximize neuronal activity for each movement. The derived learning rule accounts for the reversal of the loss of cortical representation after rehabilitation and the increase of this loss after stroke with insufficient rehabilitation. Further, our model exhibits nonlinear and bistable behavior: if natural recovery, motor training, or both, brings performance above a certain threshold, then training can be stopped, as the repeated spontaneous arm use provides a form of motor learning that further bootstraps performance and spontaneous use. Below this threshold, motor training is ''in vain'': there is little spontaneous arm use after training, the model exhibits learned nonuse, and compensatory movements with the less affected hand are reinforced. By exploring the nonlinear dynamics of stroke recovery using a biologically plausible neural model that accounts for reversal of the loss of motor cortex representation following rehabilitation or the lack thereof, respectively, we can explain previously hard to reconcile data on spontaneous arm use in stroke recovery. Further, our threshold prediction could be tested with an adaptive train-wait-train paradigm: if spontaneous arm use has increased in the ''wait'' period, then the threshold has been reached, and rehabilitation can be stopped. If spontaneous arm use is still low or has decreased, then another bout of rehabilitation is to be provided. - Stroke is the leading cause of disability in the US, and about 65% of stroke survivors experience long-term upper extremity functional limitations [1]. Although patients may regain some motor functions in the months following stroke due to spontaneous recovery, stroke often leaves patients with predominantly unilateral motor impairments. Indeed, recovery of upper extremity function in more than half of patients after stroke with severe paresis is achieved solely by compensatory use of the less-affected limb [2]. Improving use of the more affected arm is important however, because difficulty to use this arm in daily tasks has been associated with reduced quality of life [3]. There is now definite evidence however that physical therapy interventions targeted at the more affected arm can improve both the amount of spontaneous arm use and arm and hand function after stroke [4]. Further, even after motor retraining is terminated, performance can further improve in patients with less severe strokes in the months following therapy [5,6]. A possible interpretation of this result is that the repeated attempts to use the affected arm in daily activities are a form of motor practice that can lead to further improvements in motor performance [5]. The neural correlates of motor training after stroke have been invest (...truncated)


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Cheol E. Han, Michael A. Arbib, Nicolas Schweighofer. Stroke Rehabilitation Reaches a Threshold, PLoS Computational Biology, 2008, Volume 4, Issue 8, DOI: 10.1371/journal.pcbi.1000133