Martha Flanders - US grants

The generation of motor patterns is an important function of the central nervous system. Motor cortical areas, the basal ganglia, the cerebellum, and the brainstem all contribute, in a complex way, to the formation of the "descending motor commands" that ultimately innervate motoneurons and excite muscle. The long-term objective of this project is to shed light on the process of motor pattern generation by describing the patterns of muscle activity that subserve natural arm movements in humans. The specific aims for our renewed efforts on this project are: AIM 1. To compare tonic patterns of muscle activity to phasic patterns of muscle activity. AIM 2. To describe the overall pattern of muscle activity across many elbow and/or Shoulder muscles, and across many directions of movement in 3D space. Electromyograms (EMGs) will be recorded as human subjects either move to targets in three-dimensional space or hold the arm in static postures. Both single-unit and multi-unit records will be used to compare the tonic and phasic patterns associated with posture and movement, respectively. These data will be used to test the hypothesis that posture and movement are fundamentally distinct. A principal component analysis and a simulation will be used to further describe how the central nervous system controls the most robust features of the motor pattern. The experiments and analyses can potentially be repeated in clinical settings, since EMGs can easily be recorded from patients.

The long-term objective is to reveal neuromuscular mechanisms for controlling complex movements. The previous grant cycle focused on the processes of shaping the hand and making movement sequences. The results showed that motor units, not muscles, are the functional elements, and that the control of movement sequences is continuous, not segmented. This renewal application aims to study the sensorimotor control of object manipulation, i.e., the control of arm and hand movements that involve substantial contact forces. Specific Aim 1 will test for a separation of transport and grasp components of arm and hand muscle activation. Based on prior evidence, it is thought that the arm and hand have distinct control mechanisms, with the arm responsible for transport and the hand solely responsible for grasping. Electromyographic (EMG) patterns from multiple muscles will be related to the direction of transport and the level of grasp stability. Preliminary studies suggest that the results will reveal an activation gradient from proximal to distal muscles, rather than distinct control mechanisms for the arm and hand. Specific Aim 2 will examine the efficacy of reflexive adjustments to the balance of hand muscle activation, during simple manipulation tasks. Since somatosensory feedback is generally thought to trigger new motor commands, its possible role in fine-tuning ongoing commands is largely unknown. The proposedexperiments are designed to test for a significant change in hand muscle directional tuning, due to the directional characteristics of a mild, unexpected frictional resistance or force field. Specific Aim 3 will characterize the influence ofjoint positions and contact forces on the perception of hand shape. Bimanual matching experiments will test 1) whether a discrete somatosensory distortion influences whole hand shape, and 2) whether the perception of hand shape is invariant to the level of contact force. The results will provide new information on neuromuscular control strategies for object manipulation and will determine the coordinate system in which multidimensional somatosensory information influences hand shape. These outcomes will be directly relevant to the design of prosthetic interfaces for restoring hand function and to the design of haptic interfaces (as in remote surgical operations). The results will also improve the scientific foundation for inventing treatments for focal hand dystonia, a disease with a complex and poorly understood somatosensory and motor basis.