Linking Individual Movements to a Skilled Repertoire: Fast Modulation of Motor Synergies by Repetition of Stereotyped Movements

Abstract

Motor skills emerge when practicing individual movements enables the motor system to extract building instructions that facilitate the generation of future diverse movements. Here we asked how practicing stereotyped movements for minutes affects motor synergies that encode human motor skills acquired over years of training. Participants trained a kinematically highly constrained combined index-finger and thumb movement. Before and after training, finger movements were evoked at rest by transcranial magnetic stimulation (TMS). Post-training, the angle between posture vectors describing TMS-evoked movements and the training movements temporarily decreased, suggesting the presence of a short-term memory for the trained movement. Principal component analysis was used to identify joint covariance patterns in TMS-evoked movements. The quality of reconstruction of training or grasping movements from linear combinations of a small subset of these TMS-derived synergies was used as an index of neural efficiency of movement generation. The reconstruction quality increased for the trained movement but remained constant for grasping movements. These findings suggest that the motor system rapidly reorganizes to enhance the coding efficiency of a difficult movement without compromising the coding efficiency of overlearned movements. Practice of individual movements may drive an unsupervised bottom-up process that ultimately shapes synergistic neuronal organization by constant competition of action memories.

Synergistic Organization of Neural Inputs from Spinal Motor Neurons to Extrinsic and Intrinsic Hand Muscles

“…Also, the function of motor cortex in this synergistic control should be clarified. It was indeed shown that a synergistic structure of motor control may be coded in the primary motor cortex (Schieber and Santello, 2004;Ejaz et al, 2015;Leo et al, 2016;Fricke et al, 2020). Invasive cortical stimulation of monkeys further confirmed this phenomenon (Overduin et al, 2015), which nevertheless needs to be associated with how descending commands are organized at the spinal level.…”

The Topography of Visually Guided Grasping in the Premotor Cortex: A Dense-Transcranial Magnetic Stimulation (TMS) Mapping Study

“…Kinematic information on finger movements was acquired by means of a glove in which flexion sensors were embedded. This technique is widely used to record finger dynamics (see, e.g., Gentner and Classen, 2006;Kumar et al, 2012;Cattaneo et al, 2015;Fricke et al, 2019), and instructions for its realization have been published by Gentner and Classen (2009). Three commercial righthand gloves of a stretchable grade of Lycra, of three different sizes (small, medium, and large hand size) were modified to accommodate four 114-mm-long flexion sensors (flexsensors 4.5 inch, Spectrasymbol) over the metacarpophalangeal and proximal interphalangeal joints of the thumb, index, and little fingers (1 sensor for each finger).…”

Gaze direction influences grasping actions towards unseen, haptically explored, objects

“…The literature on visually-guided grasp tells us that the peak aperture is already fully influenced by, and therefore informative of, the target size 30 . Similarly to previous investigation on fingers dynamics 37 – 40 , we measured MFE, by means of a custom-made glove made of a stretchable fabric of Lycra equipped with four flexible resistive bend sensors strategically placed to detect the voltage deriving from the extension of each finger involved in whole-hand grasps (Fig. 2 A).…”