Contribution of the Fronto-parietal Cortical Dynamics to Grip Force Control

While holding a coffee mug filled to the brim, we strive to avoid spilling the coffee. This ability relies on the interaction between the control of finger forces on a moment-to-moment basis and the visual information about the object. Such sensorimotor interaction is affected in patients with stroke, Parkinson’s disease, and cerebral palsy. Studies investigating force control have shown that fluctuations in the exerted force are not mere noise but arise from systematic physiological processes. Most recent evidence points toward a link between neural activity within the fronto-parietal brain regions including primary motor cortex (M1) and the fluctuations in grip force. However, specific contribution of the cortical activity to regulation of grip force remains unclear. This is a significant research gap because it limits our understanding about how the brain enables efficient control of grip forces during grasping. The current dissertation focused on bridging this research gap using noninvasive neuromodulation and neuroimaging approaches via two specific aims. In Aim-1, we determined the causal involvement of M1 in regulating grip force variability using transcranial magnetic stimulation (TMS) among healthy young individuals. Consistent with our hypothesis, temporary disruption of M1 resulted in upregulation of the grip force variability when compared to that post sham (placebo) stimulation. Interestingly, this upregulation was observed when visual feedback of the exerted grip force was available, but not when the visual feedback of the exerted grip force was removed, indicating the critical role of M1 in integrating visuomotor information for regulating grip force variability. In Aim-2, we examined the dependence of lateralized fronto-parietal neural activity on grip force magnitude during a grip force control task using noninvasive electroencephalography (EEG). Accumulating evidence suggests mechanistic role of neural variability in cognitive processes that scale with task demands. Consequently, we hypothesized laterally specific modulation in EEG variability with increasing magnitude of the grip force exerted during an isometric grip force control task in healthy young individuals. Consistent with our hypothesis, the neural variability was found to be lateralized, topographically constrained, and functionally dependent on the grip force magnitude thereby, showcasing the influence of force-dependent behavioral processes on neural variability. Taken together, this dissertation underscores the integral role of M1 and associated fronto-parietal cortical activity during grip force control. We highlight the relevance of these findings to the rehabilitation of upper extremity motor functions among patients with sensorimotor deficits and propose directions for future studies investigating neural correlates of digit force control.