Anticipatory corticospinal control of movements in healthy subjects and in patients with neurological motor deficits

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When a person holds a heavy book in the palm of one hand and if the book is suddenly lifted off by someone else, the arm moves smoothly to another position (unloading reflex). If the person him/herself moves the book using the other hand, the arm stays almost at the same position (self-unloading). In anticipation of the self-unloading, the brain may modify motor control signals so that the arm position is preserved. To unveil the underlying control mechanisms, this project will analyze the corticospinal influences in the two unloading tasks. Previous studies suggested that the threshold arm position, at which muscles become activated, is a control variable influenced by the corticospinal system. This project hypothesizes that, in order to preserve the arm position during self-unloading, the subject will start changing corticospinal influences on threshold arm position before and during the unloading onset, and will maintain the final corticospinal influences after unloading. To test this hypothesis, single-pulse transcranial magnetic stimulation will be applied on primary motor cortex before and after the two unloading tasks to trigger electromyographic responses. The hypothesis will be confirmed if evoked electromyographic responses before and after self-unloading are different from those during unloading reflex.A computer model which predicts electromyographic patterns and movement kinematics will be developed to simulate the two unloading tasks. The hypothetical control input is a constant threshold position for unloading reflex and a ramp-shaped threshold position for self-unloading. Model prediction will be compared with experimental measurements.This project will also test whether or not post-stroke subjects have deficits in anticipatory control of self-unloading. By comparing responses elicited in the patients to healthy controls, new insights into the mechanisms underlying disordered motor control will be identified.