Balancing the excitability of M1 circuitry during movement observation without overt replication.

Pablo Arias,Kenneth Grieve,Yoanna Corral-Bergantiã±Os,Javier Cudeiro,Antonio Oliviero,Laura Mordillo-Mateos,Nelson Espinosa,Verã³Nica Robles-Garcã­A

doi:10.3389/fnbeh.2014.00316

Abstract

Although observation of a movement increases the excitability of the motor system of the observer, it does not induce a motor replica. What is the mechanism for replica suppression? We performed a series of experiments, involving a total of 66 healthy humans, to explore the excitability of different M1 circuits and the spinal cord during observation of simple movements. Several strategies were used. In the first and second experimental blocks, we used several delay times from movement onset to evaluate the time-course modulation of the cortico-spinal excitability (CSE), and its potential dependency on the duration of the movement observed; in order to do this single pulse transcranial magnetic stimulation (TMS) over M1 was used. In subsequent experiments, at selected delay times from movement-onset, we probed the excitability of the cortico-spinal circuits using three different approaches: (i) electric cervicomedullary stimulation (CMS), to test spinal excitability, (ii) paired-pulse TMS over M1, to evaluate the cortical inhibitory-excitatory balance (short intracortical inhibition (SICI) and intracortical facilitation (ICF)], and (iii) continuous theta-burst stimulation (cTBS), to modulate the excitability of M1 cortical circuits. We observed a stereotyped response in the modulation of CSE. At 500 ms after movement-onset the ICF was increased; although the most clear-cut effect was a decrease of CSE. The compensatory mechanism was not explained by changes in SICI, but by M1-intracortical circuits targeted by cTBS. Meanwhile, the spinal cord maintained the elevated level of excitability induced when expecting to observe movements, potentially useful to facilitate any required response to the movement observed.

Highlights

It is well-established that the observation of movements executed by others induces changes in the observer’s motor system (Fadiga et al, 1995)
It may be suggested that during movement observation (MO) there is a balance in the excitability of cortical circuits allowing mirror facilitating activity within M1 while at the same time retaining the descending drive to the spinal cord; this study is aimed at evaluating if such a balancing mechanism is present in humans
The TMS pulses were delivered at specific delays from movement-onset, and the Control delay was delivered before MO but when it was expected to be observed

Summary

Introduction

It is well-established that the observation of movements executed by others induces changes in the observer’s motor system (Fadiga et al, 1995). The MNS was first described in monkey (Di Pellegrino et al, 1992) and at its heart contains neurons whose responses to an observed action are similar to the responses when the animal undertakes the same task (Gallese et al, 1996). It permits action understanding (Rizzolatti et al, 1996; Umilta et al, 2001) and has powerful influences on development (Rizzolatti and Craighero, 2004). This has extended the potential for the human MNS to be involved in a much wider range of activities (Rizzolatti et al, 2009)

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Conclusion