Neuromuscular modularity and behavioral correlates of motor control

Abstract

I studied organizational principles that may subserve the control and learning of forelimb movements. Among these principles, I focused on muscular coordination patterns, motor cortical excitability, and sensorimotor interactions. I found that muscle activity in grasping and reaching behaviors could be reconstructed by linear combinations of a small number of time-varying muscle synergies, each fit with coefficients unique to the behavior. However, the generalization of these synergies between behavioral conditions was limited, in part by the sensitivity of the extraction algorithm to stereotyped muscular relations within contrasted conditions. In reaching studies designed to assist or resist different movement directions, I found a gradual change in the structure, as well as recruitment, of synergies. When a perturbation was targeted to the activity within a single muscle, I found a transient, relative suppression of this muscle in response to descending motor commands. In other motor cortical microstimulation experiments, I confirmed that long-train microstimulation is able to evoke complex, convergent movements. Even during highly-trained reaching movements, I found that there was relatively little invariance of the muscular patterns in relation to kinematic variables coding for the hand's displacement and velocity.

(cont.) In two studies examining the kinematic consequences of modulating cortical excitability, I either infused tissue plasminogen activator into monkey cortex or applied transcranial magnetic stimulation to human cortex, either while or before each adapted to a clockwise force field. In both cases basal motor performance was spared, but each manipulation appeared to be associated with disruptions of subjects' ability to retain, improve, or recall recent adaptations. Among other human studies, I investigated the interaction of dynamic adaptation and sequence learning, and found that simultaneous acquisition of a force field and a sequence does not impair performance on either but may have enabled subjects to tune in to, and chunk, their movements. I found that motor consolidation may be dependent on the more effortful learning enabled by catch-trial interruptions of practice on a novel condition. Finally, I used functional imaging and manual cutaneous stimulation to show that the hemodynamic response was biased according to receptor density but generally non-somatotopic and distributed throughout sensorimotor cortex.