Individual Premotor Drive Pulses, Not Time-Varying Synergies, Are the Units of Adjustment for Limb Trajectories Constructed in Spinal Cord

Abstract

Complex actions may arise by combining simple motor primitives. Our studies support individual premotor drive pulses or bursts as execution primitives in spinal cord. Alternatively, the fundamental execution primitives at the segmental level could be time-varying synergies. To distinguish these hypotheses, we examined sensory feedback effects during targeted wiping organized in spinal cord. This behavior comprises three bursts. We tested (1) whether feedback altered the structure of individual premotor drive bursts or primitives, and (2) whether feedback differentially modulated different drive bursts or pulses in the three burst sequence. At least two of the three bursts would need to always be comodulated to support a time-varying synergy. We used selective muscle vibration to control spindle feedback from a single muscle (biceps/iliofibularis). The structures of premotor drive bursts were conserved. However, biceps vibration (1) scaled the amplitudes of two bursts coactivated during the initial phase of wiping independently of one another without altering their phase, and (2) independently phase regulated the third burst but preserved its amplitude. Thus, all three bursts were regulated separately. Durations were unaffected. The independent effects depended on (1) time of vibration during wiping, (2) frequency of vibration, and (3) limb configuration. Because each of the three bursts was independently modulated, these data strongly support execution using individual premotor bursts rather than time-varying synergies at the spinal level of motor organization. Our data show that both sensory feedback and central systems of the spinal cord act in concert to adjust the individual premotor bursts in support of the straight and unimodal wiping trajectory.

Space-by-time modular decomposition effectively describes whole-body muscle activity during upright reaching in various directions

“…In addition, similar temporal modules have been described in different 6 studies (different motor tasks and different animal models) using different extraction the task constraints but are physiological. This supports the robustness of the identified 10 temporal structure of motor modules irrespective of the study-specific signal preprocess-11 ing procedure (Hart and Giszter, 2010;Kargo and Giszter, 2008). We also note that 12 when using an extension of the model that includes temporal shifts/delays in the tem- …”

A Neural Basis for Motor Primitives in the Spinal Cord

“…That is, most of their significant STA effect peaks matched with the corresponding significant projection weights of the independent component onto EMG. Furthermore, 86% (40 of 46) of cells had better than Components were related to primitives identified previously by both statistical and physiological means (Hart and Giszter, 2004;Kargo and Giszter, 2008;Kargo et al, 2009). Interneuronal firing was phasically related to primitive/component group activity.…”

Stability of muscle synergies for voluntary actions after cortical stroke in humans

“…Descending signals from the motor cortical areas activate these networks organizing each synergy, which in turn activate the motoneurons of a set of muscles with a particular muscle activation balance profile; different movements then emerge as the synergies are recruited to varying degrees. Evidence supporting the notion of muscle synergies as modules for movement construction has come from several studies examining the muscle patterns in a variety of behaviors and species, ranging from reflexes and wipes in spinalized frogs (18,19), frog natural behaviors (12,16,17,20), postural maintenance tasks and locomotion in cats (21,22), grasping in monkeys (23), to motor tasks in humans (24)(25)(26). These studies all use different factorization algorithms to identify a small number of basis vectors whose linear combination describes the EMGs well.…”