Figure 1.
A, Schematic representation of the experimental setup. The subject sat comfortably at a table in front of a PC monitor displaying a starting line (black line), a target zone (colored lines), and a cursor (green dot). The lowest line of the target zone (green) represented 80% of the subject's MVF, whereas the highest line (light blue) represented 140% of MVF, as measured for each participant in the calibration phase at the beginning of the experiment. The task was to press a piston with the right index finger as strongly as possible to reach the higher lines of the target zone. The pressures against the piston were recorded by a force transducer and converted in vertical displacements of the cursor by a DAQ board. Right, Magnified view of the hand constraint. A holder was used to keep the hand static during the performance of the motor task and to allow only movements with the index finger. B, Illustration of a trial of the motor task. When the cue START appeared on the PC monitor, the subjects could start the trial by pressing the mouse key with the left hand. The trial started with the appearance of the black line at the bottom of the monitor, the yellow cursor, and the lines of the target zone. As soon as the subject pressed the piston, the cursor moved toward the target zone; when entering the target zone, it changed from yellow to green. Subjects had to maintain the cursor in the highest position they could achieve until the end of the trial. Each trial lasted 1100 ms. C, Timeline of the experimental protocol with a representation of the sessions sequence (baseline, manipulation, and final) and TENS application. The dashed arrows indicate the moment at which the subjective scales were administered. As schematically shown in the picture, the controlTENS group and the two experimental groups (expverbal+conditioning and expverbal) received different verbal information about the effects of TENS. Moreover, in the manipulation session, the groups underwent different procedures: the expverbal+conditioning group performed the motor task with a surreptitious amplification of the cursor's excursion range (conditioning procedure), whereas the expverbal group and the controlTENS group performed the motor task without any amplification. Crucial for our interest was the comparison of performance before and after manipulation (i.e., in the baseline and final sessions). An additional control group performed the motor task in the same three sessions, but without TENS application (controlnoTENS). D, Illustration of a trial of the TMS task. When the cue START appeared on the PC monitor, the subjects could start the trial by pressing the mouse key with the left hand. The trial started with the appearance of the black line at the bottom of the monitor, the yellow cursor, the lines of the target zone, and a red line. The red line represented the 30% of MVF as measured in the calibration phase. Subjects had to maintain the cursor on the red line until the end of the trial; because the task was not to achieve the highest lines, the lines of the target zone were dimmed compared with the motor task. Each trial lasted 5000 ms. Right, Magnified schematic view of the TMS pulse delivery is presented. If the cursor remained stable on the red line, for at least 500 ms (i.e., when the subject exerted a stable pressure of 30% MVF), the TMS pulse was triggered. The pulse was delivered randomly in a time window ranging from 500 to 1300 ms from the onset of the 30% MVF stable pressure. The TMS task was performed after the motor task, in the baseline, manipulation and final sessions. During TENS application, the electrodes for EMG recording were removed from the FDI muscle to apply the TENS electrodes. Afterward, the EMG electrodes were positioned again over the FDI muscle.