abstract: The development of advanced, anthropomorphic artificial hands aims to provide upper extremity amputees with improved functionality for activities of daily living. However, many state-of-the-art hands have a large number of degrees of freedom that can be challenging to control in an intuitive manner. Automated grip responses could be built into artificial hands in order to enhance grasp stability and reduce the cognitive burden on the user. To this end, three studies were conducted to understand how human hands respond, passively and actively, to unexpected perturbations of a grasped object along and about different axes relative to the hand. The first study investigated the effect of magnitude, direction, and axis of rotation on precision grip responses to unexpected rotational perturbations of a grasped object. A robust "catch-up response" (a rapid, pulse-like increase in grip force rate previously reported only for translational perturbations) was observed whose strength scaled with the axis of rotation. Using two haptic robots, we then investigated the effects of grip surface friction, axis, and direction of perturbation on precision grip responses for unexpected translational and rotational perturbations for three different hand-centric axes. A robust catch-up response was observed for all axes and directions for both translational and rotational perturbations. Grip surface friction had no effect on the stereotypical catch-up response. Finally, we characterized the passive properties of the precision grip-object system via robot-imposed impulse perturbations. The hand-centric axis associated with the greatest translational stiffness was different than that for rotational stiffness. This work expands our understanding of the passive and active features of precision grip, a hallmark of human dexterous manipulation. Biological insights such as these could be used to enhance the functionality of artificial hands and the quality of life for upper extremity amputees. / Dissertation/Thesis / Ph.D. Mechanical Engineering 2013

The main goal of the dissertation was to determine task-dependent modulation of corticospinal descending output. From this main goal, I conducted three different studies to determine how corticospinal output to muscles of the upper arm and hand changed as a function of the task demands. In study 1, I examined how a somatosensory-motor circuit changes when a muscle needs to be active in a task and found that this circuit may be dependent on the movement phase, type of afferent input, and the task demands. In study 2, I examined how this same somatosensory-motor circuit acts to both allow and prevent muscle activity before movement. I revealed that this somatosensory-motor circuit may function to prevent muscle activity when a muscle is not needed in a task and creates facilitation of corticospinal output when it needs to be active in a task. These effects, however, are dependent on the movement phase and the digit the muscle is controlling. Study 3 determined how corticospinal output is modulated to upper arm muscles when performing movements that required different combinations of segmental interactions to achieve the task successfully. Corticospinal output was increased when inertia and the BBC moment at a joint resisted the intended joint rotation and these effects were dependent on the muscle and movement phase. I propose a model of the connectivity between the primary motor and somatosensory cortices that would increase, modulate, or decrease corticospinal output to a muscle depending on its role in the task. The findings from this work provides information to guide future neural rehabilitative interventions for individuals who have movement disorders arising from altered somatosensory-motor processing such as Cerebellar Ataxia, Developmental Coordination Disorder, Focal Hand Dystonia, Parkinson’s disease, and stroke. / Dissertation / Doctor of Philosophy (PhD) / On a day to day basis, we perform a variety of movements without giving much thought to how complicated it is for our nervous system to perform said movements. There are many different areas of the brain that are responsible for controlling movement. This dissertation focuses on two key areas that are critical for movement performance, namely the primary motor and somatosensory cortices. The primary motor cortex is largely responsible for sending signals to the muscles to control movement, while the primary somatosensory cortex plays a crucial role in receiving and understanding sensory input from our body. The studies in this dissertation describe how these two areas of the brain communicate during finger and arm movements to produce or prevent muscle activity. This work has implications for individuals with disorders that impact their everyday movements.