Corticospinal and vestibulospinal control of motor actions

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The controversy between 2 major theories of motor control creates uncertainty in the understanding of basic principles and interferes with progress in behavioral neuroscience. One theory suggests that the brain pre-programs movement trajectories and motor commands pre-computed by internal models of the system. The other theory suggests that motor control is accomplished parametrically, without computations, by changing threshold positions of body segments at which motoneuronal recruitment begins. According to the latter, the brain shifts the balance (attractor point) of the system and converts posture-stabilizing reflexes and intrinsic muscle properties into movement-producing mechanisms such that motor commands emerge without computations. Objective 1 of the program is to help resolve the controversy through experiments directly designed to test hypotheses of each approach. Objective 2 is to determine the role of different descending systems in the control of whole-body postures and movements (i.e., corticospinal (CS) and vestibulospinal (VS) systems). Hypothesis 1 is that the CS system mainly controls the body configuration in a task-specific way in the limits pre-determined by the VS system. The latter is responsible for motor equivalency – reaching the motor goal regardless of the number of body segments involved (Hypothesis 2) and for the appropriate orientation of body posture and movement in the gravitational field (Hypothesis 3). VS and CS influences will be evaluated using transcranial magnetic stimulation of the motor cortex and galvanic vestibular stimulation during arm motion with or without trunk in sitting subjects and during forward body leaning in standing subjects (4 tasks). Objective 3 is to elaborate a new diagnostic tool − a portable ultrasound-based device for evaluation of VS deficits in patients with neurological movement problems. The device will be evaluated in arm reaching movements with and without trunk involvement (Task 5). The resolution of the controversy between two major theories of motor control (Objective 1) and progress in the understanding of functional differences between the VS and CS systems (Objective 2) will advance basic science of motor control, affect the way we teach students in behavioral neuroscience, help clinicians assess and treat neurological motor deficits (Objective 3), and provide an example of applying results of basic science to engineering and health research in Canada.