Contribution of visual and proprioceptive sensory feedback to the online regulation of wrist posture in humans using fMRI
Abstract
Effective limb position regulation requires the use of sensory feedback from multiple sources (eg. vision and proprioception) to assess task performance and to correct motor output when performance fails to meet desired objectives. The neural mechanisms mediating limb position regulation in humans are incompletely understood, partly due to the lack of MR compatible robotic tools. This dissertation presents the development of a one degree-of-freedom, MR compatible, pneumatic manipulandum capable of providing computer-controlled perturbations about the wrist. Results of two functional magnetic resonance imaging (FMRI) studies exploring the contributions of sensory feedback to the regulation of wrist position are then described. In both studies, subjects stabilized the manipuladum's handle against constant and pseudo-random extensor torque perturbations while in a 1.5 Tesla MR Scanner. We first examined proprioceptive contributions to wrist position stabilization without ongoing visual feedback. Kinematic and electromyographic analyses revealed that subjects invoked three compensatory responses during stabilization: feedback regulation via long-loop reflexes; impedance modulation via antagonist muscle co-activation; and feedforward, discrete corrective movements. FMRI results revealed two distinct networks that were differentially excited by the task. One had significant BOLD activity when limb state estimation errors were robotically imposed, including the cerebello-thalamo-cortical pathways. The second had significant BOLD activity throughout active stabilization (independent of robotically-induced errors) and included regions in prefrontal and inferior parietal cortices. The second study examined how integrated visual and proprioceptive feedback contributed to wrist position. In separate trials, subjects were provided either with reliable or unreliable visual feedback during stabilization. Correlations between BOLD activation and performance errors revealed that posterior parietal cortex and the lateral cerebellum contribute importantly to the calculation of errors during stabilization. Furthermore, the magnitude of BOLD activation in the deep cerebellar nuclei depended on the temporal correlation between visual and proprioceptive inputs. These experiments revealed that wrist stabilization is controlled by a combination of feedback and feedforward mechanisms mediated by distinct neuronal networks, and that the cerebellum plays a role in integrating sensory feedback from multiple sources for the control of limb position. Future experiments designed to identify the temporal limits of visual/proprioceptive integration in cerebellar output pathways are proposed.
This paper has been withdrawn.