Cortico-Cortical Connectivity between Right Parietal and Bilateral Primary Motor Cortices during Imagined and Observed Actions: A Combined TMS/tDCS Study

Abstract

Previous transcranial magnetic stimulation (TMS) studies showed functional connections between the parietal cortex (PC) and the primary motor cortex (M1) during tasks of different reaching-to-grasp movements. Here, we tested whether the same network is involved in cognitive processes such as imagined or observed actions. Single pulse TMS of the right and left M1 during rest and during a motor imagery and an action observation task (i.e., an index-thumb pinch grip in both cases) was used to measure corticospinal excitability changes before and after conditioning of the right PC by 10 min of cathodal, anodal, or sham transcranial direct current stimulation (tDCS). Corticospinal excitability was indexed by the size of motor-evoked potentials (MEPs) from the contralateral first dorsal interosseous (FDI; target) and abductor digiti minimi muscle (control) muscles. Results showed selective ipsilateral effects on the M1 excitability, exclusively for motor imagery processes: anodal tDCS enhanced the MEPs' size from the FDI muscle, whereas cathodal tDCS decreased it. Only cathodal tDCS impacted corticospinal facilitation induced by action observation. Sham stimulation was always uneffective. These results suggest that motor imagery, differently from action observation, is sustained by a strictly ipsilateral parieto-motor cortex circuits. Results might have implication for neuromodulatory rehabilitative purposes.

Keywords: M1; TMS; action observation; connectivity; motor imagery; right PC; tDCS.

Figure 1

Experimental task. Subjects were submitted to a resting, motor imagery and an action observation task previously and after each tDCS session (anodal, cathodal, sham) spaced 5–7 days apart. Corticospinal excitability was measured by TMS of left and right primary motor cortex. The order of tDCS and task and TMS target (left and right M1) conditions was fully randomized.

Figure 2

Percentage changes of mean MEPs’ amplitude values during the different experimental conditions. A value of 100% indicates, independently for each subject, the mean value during the baseline condition prior tDCS. The error bars correspond to 1 SE. An asterisk (*) indicates significant differences between the experimental conditions. tDCS was always delivered over the right parietal cortex. (A) The corticospinal facilitation induced by motor imagery, as indexed by the MEPs’ size. During the anodal session, after tDCS application, the MEPs’ size (post-tDCS) significantly increased with respect to the pre-tDCS, while during the cathodal session significantly decreased it with respect to the pre-tDCS: such a double dissociation effect emerged only when TMS was applied on the right M1, ipsilateral to the conditioned parietal cortex. (B) The corticospinal facilitation induced by the action observation task, as indexed by the MEPs’ size, was still significantly decreased by cathodal tDCS with respect to the pre-tDCS condition. Again, this effect was present when the TMS was applied on the ipsilateral right M1. (C) Experimental set-up summarizing the site of tDCS conditioning and the side of TMS.

Figure 3

The corticospinal facilitation, as indexed by the mean MEPs’ size, induced by motor imagery is generally higher than the one induced by action observation. This is evident for all conditions: this effect reaches significance when TMS is applied on the dominant hemisphere [left M1,(A)] before and after sham, and before and after anodal tDCS session. During the cathodal session the same trend was observed. On the other hand, when TMS is applied on the right hemisphere (B), such an effect is again significant before sham and cathodal tDCS, and only after anodal tDCS. A value of 100% indicates, independently for each subject, the mean value during the baseline condition (before right PC tDCS). The error bars correspond to 1 SE. An asterisk (*) indicates significant differences between the experimental conditions.

Figure A1

Percentage changes of mean MEPs’ amplitude values during the different experimental conditions. A value of 100% indicates, independently for each subject, the mean value during the baseline condition (before tDCS). The error bars correspond to 1 SE. An asterisk (*) indicates significant differences between the experimental conditions. tDCS was always delivered over the right parietal cortex. Cathodal tDCS significantly decreased the corticospinal facilitation induced by motor imagery, as indexed by the mean MEPs’ size, when TMS was applied on the right primary motor cortex with respect to the left one.