Modulation of Motor Awareness: A Transcranial Magnetic Stimulation Study in the Healthy Brain

Abstract

Previous studies on the mechanisms underlying willed actions reported that the premotor cortex may be involved in the construction of motor awareness. However, its exact role is still under investigation. Here, we investigated the role of the dorsal premotor cortex (PMd) in motor awareness by modulating its activity applying inhibitory rTMS to PMd, before a specific motor awareness task (under three conditions: without stimulation, after rTMS and after Sham stimulation). During the task, subjects had to trace straight lines to a given target, receiving visual feedback of the line trajectories on a computer screen. Crucially, in most trials, the trajectories on the screen were deviated, and to produce straight lines, subjects had to correct their movements towards the opposite direction. After each trial, participants were asked to judge whether the line seen on the computer screen corresponded to the line actually drawn. Results show that participants in the No Stimulation condition did not recognize the perturbation until 14 degrees of deviation. Importantly, active, but not Sham, rTMS significantly modulated motor awareness, decreasing the amplitude of the angle at which participants became aware of the trajectory correction. These results suggest that PMd plays a crucial role in action self-monitoring.

Keywords: TMS; awareness; low frequency; motor awareness; premotor cortex; self-monitoring.

Figure 1

Experimental set up and procedure. The experimental set up consisted of a 30 × 40 cm graphic tablet placed in a wooden box on a desk and connected to the computer. An LCD screen was placed on top of the wooden box. A hole in the wooden box allowed the participants insertion of one of the hands inside the box, so that it could not be seen. The subjects were seated on a comfortable fixed chair in front of the desk (both the graphic tablet and the screen were aligned to the subjects’ trunk midline) and they could only see the screen below the chin (Panel A). The subjects were instructed to reach, with the pen tip, a yellow target (4 × 4 mm) located on the sagittal axis at 22 cm from the starting point, by drawing a continuous line as fast as possible. After the target was reached, they were asked to indicate with “yes” or “no” whether the trajectory they saw on the screen corresponded to the movement actually performed or not. Subjects performed 51 trials in each experimental condition (i.e., No Stimulation, rTMS and Sham). For each trial, the software randomly applied the trajectory deviation, which ranged from 25° to the left (LD, i.e., −25° from the 0, with negative values indicating a leftward perturbation) to 25° on the right (RD, i.e., +25° from the 0), with a trial for each degree of deviation. There was also one trial with a deviation of 0°, indicating a perfect coherence between the visual feedback and the actual movement (Panel B).

Figure 2

Mean degree at which subjects became aware of the artificial deviation in the three conditions. Mean degree at which subjects became aware of the artificial deviation in the BS (mean = 14.34, SE = 1.1), soon after the rTMS (mean = 11.09, SE = 0.9) and in the Sham conditions (mean = 13.3, SE = 1.1). Error bars represent standard error of means; *, significant. NS = No Stimulation, rTMS = repetitive Transcranial Magnetic Stimulation, Sham = Sham stimulation.

Figure 3

Mean degree at which subjects became aware of the artificial deviation in the three conditions and with the two deviations. Mean degree at which subjects became aware of the artificial deviation in the three conditions and with the two deviations when the task was performed with the left hand (Panel A) and with the right hand (Panel B). Error bars represent standard error of means; *, significant. NS = No Stimulation, rTMS = repetitive Transcranial Magnetic Stimulation, Sham = Sham stimulation. Taken together, these results indicate that rTMS of PMd significantly affected participants’ conscious self-monitoring, and that the deviation direction, as well as the hand used to perform the task, influenced subjects’ action self-monitoring.