Training the motor cortex by observing the actions of others during immobilization

Abstract

Limb immobilization and nonuse are well-known causes of corticomotor depression. While physical training can drive the recovery from nonuse-dependent corticomotor effects, it remains unclear if it is possible to gain access to motor cortex in alternative ways, such as through motor imagery (MI) or action observation (AO). Transcranial magnetic stimulation was used to study the excitability of the hand left motor cortex in normal subjects immediately before and after 10 h of right arm immobilization. During immobilization, subjects were requested either to imagine to act with their constrained limb or to observe hand actions performed by other individuals. A third group of control subjects watched a nature documentary presented on a computer screen. Hand corticomotor maps and recruitment curves reliably showed that AO, but not MI, prevented the corticomotor depression induced by immobilization. Our results demonstrate the existence of a visuomotor mechanism in humans that links AO and execution which is able to effect cortical plasticity in a beneficial way. This facilitation was not related to the action simulation, because it was not induced by explicit MI.

Keywords: action observation; direct-matching hypothesis; immobilization; internal simulation; motor imagery.

© The Author 2013. Published by Oxford University Press.

Figures

Figure 1.

Mean FDI corticomotor maps recorded before (Pre) and after (Post) immobilization in the 3 groups. (a) In ND (on the left), AO (in the middle), and MI (on the right) groups, participants performed the requested task for 10 hourly sessions during the immobilization period (from around 8 AM to 6 PM). (b) Each map was centered on the maximal response obtained for each participant in every condition, regardless of antero-posterior (here corresponding to above–below directions) and medio-lateral (here right-left) coordinates. Colors indicate the amplitude of MEP, normalized with respect to the maximal response obtained in the Pre-condition in every subject, from “blue” (the lowest values) to “red” (the highest responses).

Figure 2.

Effect of AO in preventing corticomotor modifications induced by immobilization. (a) MEP RCs of right FDI in ND, AO, and MI before (dashed light line, Pre) and after (solid dark line, Post) immobilization. On the abscissa, TMS stimulus intensity above RMT (% of MSO); on the ordinate, mean values of the normalized MEP ± standard error. A significant Pre–Post difference is present in ND and MI, but not in AO. (b) A typical MEP from one scalp site of one exemplificative subject from each group averaged among the repeated recordings before (light gray) and after (dark gray) immobilization. Two-dimensional image time series were filled with these data from all scalp sites. (c) Scalp sites significantly responding to TMS before immobilization (orange, P < 0.05) and significantly deactivated after nonuse (blue, P < 0.05, Pre - Post) are shown (overlaid onto a 3D rendering of the Montreal Neurological Institute, MNI, template brain by coregistering FDI cortical coordinates onto the MNI space, Niyazov et al. 2005). In ND and MI was evident a significant deactivated area (blue) that was completely absent in AO.