Affiliation:
1. Department of Neuroscience and Rehabilitation Section of Physiology University of Ferrara Ferrara Italy
2. Center for Translational Neurophysiology of Speech and Communication Istituto Italiano di Tecnologia Ferrara Italy
3. Department of Computer Science Western University London Ontario Canada
4. IRCSS Santa Lucia Roma Italy
Abstract
AbstractIn the absence of disease, humans produce smooth and accurate movement trajectories. Despite such ‘macroscopic’ aspect, the ‘microscopic’ structure of movements reveals recurrent (quasi‐rhythmic) discontinuities. To date, it is unclear how the sensorimotor system contributes to the macroscopic and microscopic architecture of movement. Here, we investigated how corticospinal excitability changes in relation to microscopic fluctuations that are naturally embedded within larger macroscopic variations in motor output. Participants performed a visuomotor tracking task. In addition to the 0.25 Hz modulation that is required for task fulfilment (macroscopic scale), the motor output shows tiny but systematic fluctuations at ∼2 and 8 Hz (microscopic scales). We show that motor‐evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) during task performance are consistently modulated at all (time) scales. Surprisingly, MEP modulation covers a similar range at both micro‐ and macroscopic scales, even though the motor output differs by several orders of magnitude. Thus, corticospinal excitability finely maps the multiscale temporal patterning of the motor output, but it does so according to a principle of scale invariance. These results suggest that corticospinal excitability indexes a relatively abstract level of movement encoding that may reflect the hierarchical organisation of sensorimotor processes.
imageKey points
Motor behaviour is organised on multiple (time)scales.
Small but systematic (‘microscopic’) fluctuations are engrained in larger and slower (‘macroscopic’) variations in motor output, which are instrumental in deploying the desired motor plan.
Corticospinal excitability is modulated in relation to motor fluctuations on both macroscopic and microscopic (time)scales.
Corticospinal excitability obeys a principle of scale invariance, that is, it is modulated similarly at all (time)scales, possibly reflecting hierarchical mechanisms that optimise motor encoding.