Control of mechanically stable movement by spinalized frogs

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2003.Includes bibliographical references (leaves 54-58).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Evidence suggests that the isolated vertebrate spinal motor system might use only a few muscle synergies for the production of a range of movements. The evolution of such synergies encoded in the spinal cord could be dictated by mechanical stability requirements for interacting with the environment or by particular performance advantages. Previous work in frogs and cats has shown that the isometric forces measured during movements evoked by intraspinal stimulation converge to a stable equilibrium. In non-isometric conditions, however, there is no guarantee that a similar property of convergence will be observed. We therefore characterized the stability properties of trajectories produced by spinalized frogs. Hindlimb movements in frogs were measured and phasic force perturbations were applied by a Phantom robot (Sensable Tech., Inc) attached at the ankle. EMGs were recorded from 12 hindlimb muscles and used to trigger the perturbations in both hindlimb-to-hindlimb wipes and withdrawals. In both behaviors, we found that the final position of the movements was stable in that the ankle trajectory after perturbation moved to the final position of the unperturbed trajectory. Following deafferentation, wiping movements showed a similar, although weaker, recovery after perturbation. Thus the stability properties found during isometric conditions also hold in dynamic conditions. These results show that spinal neural systems are able to stabilize goal-directed movements.by Andrew Garmory Richardson.S.M

Role of the precentral cortex in adapting behavior to different mechanical environments

Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 155-171).We routinely produce movements under different mechanical contexts. All interactions with the physical environment, such as swinging a hammer or lifting a carton of milk, alter the forces experienced during movement. With repeated experience, sensorimotor maps are adapted to maintain a high level of movement performance regardless of the mechanical environment. This dissertation explored the contribution of the precentral cortex to this process of motor adaptation. In the first experiment, we recorded precentral neural activity in rhesus monkeys that were trained to perform visually-cued reaching movements while holding on to a robotic manipulandum capable of changing the forces experienced during the task. Preparation and control of the reaching movements were correlated with single cell activity throughout the precentral cortex, including the primary motor cortex and five different premotor areas. Precentral field potential activity was also modulated during the reaching behavior, particularly in the beta and high gamma frequency bands. When novel forces were introduced, single cell activity changed in a manner that specifically compensated for the applied forces and mirrored the time course of behavioral adaptation.(cont.) Force-related changes were present in the field potential activity as well. Some of these changes were maintained following removal of the forces. Control data and simulations revealed that these residual changes were well described by a model of noisy adaptation in a redundant cortical network. In the second experiment, human subjects performed the same reaching paradigm after receiving transcranial magnetic stimulation to transiently inhibit cortical activity. Initial learning of the novel force environment was normal but recall of the field 24 hours later was impaired relative to controls. Taken together, the results suggest that distributed areas within the precentral cortex are involved in recalibrating sensorimotor maps to fit the present mechanical context and in initiating a memory trace of newly-experienced environments.by Andrew Garmory Richardson.Ph.D