The protocol included three sessions: baseline, manipulation and final (Fig 1B ). The baseline and final sessions were identical in both groups and consisted in the execution of the motor task described above. These two sessions allowed comparing subjects’ performance before and after the nocebo manipulation.
Nocebo effects were obtained by applying an inert treatment (10 Hz transcutaneous electrical nerve stimulation, TENS) for 5 minutes over the region of the FDI belly. The intensity of TENS was adjusted until the subject reported a slight sensation without muscle contraction. Subjects were also asked to report whether TENS was painful or uncomfortable. None of the participants reported these sensations. Participants of the experimental group were told that TENS could reduce the recruitment of muscle fibers, thereby decreasing force production. Because of the cutaneous sensation perceived by the subjects over the region of the hand muscle involved in the task, TENS can be expected to manipulate the subject’s belief of bad motor performance. In order to reinforce the subjects’ belief about the effects of TENS, the experimental group underwent a conditioning phase. A pre-determined, surreptitious reduction of the cursor’s excursion range was introduced stepwise. More precisely, after TENS, the motor task was executed again, but this time unbeknown to the subjects an attenuation coefficient was introduced and the excursion of the cursor was gradually decreased in steps of 0.0029 from trial 1 to trial 35 and remained stable until the end of the session (from trial 36 to trial 50). Consequently, by applying the same amount of force as in the baseline, the participants of the experimental group could see the cursor achieving lower lines of the target zone than before, and therefore believed to be weaker because of TENS.
Before starting the final session, TENS was applied again together with verbal suggestion of worse motor performance. Subjects then repeated the motor task (50 trials), but this time without any manipulation, that is without the reduction of the cursor’s excursion range.
The same motor task was performed by the subjects of the control group, who also underwent the TENS application as described above, but with different verbal information. In particular, these subjects were clearly told that they have been assigned to a control group in which TENS was completely inert in affecting force. They executed the motor task three times, like the experimental group, but without reduction of the cursor’s displacements in the manipulation session.
For each subject, the whole experiment took about 1.5 hours to be completed. Participants were tested at different times during the day, starting from 9.00am to 5.00pm. In the experimental group, 9 subjects were tested in the morning (9.00am-12.00am) and 8 subjects were tested in the afternoon (1.00pm-5.00pm). In the control group, 9 subjects were tested in the morning (9.00am-12.00am) and 6 subjects were tested in the afternoon (1.00pm-5.00pm). By analyzing the distribution of the subjects tested in the morning and in the afternoon, we found no differences between the two groups (Chi-square test, χ2 = 0.161, df = 1, p = 0.688).
Nocebo effects were obtained by applying an inert treatment (10 Hz transcutaneous electrical nerve stimulation, TENS) for 5 minutes over the region of the FDI belly. The intensity of TENS was adjusted until the subject reported a slight sensation without muscle contraction. Subjects were also asked to report whether TENS was painful or uncomfortable. None of the participants reported these sensations. Participants of the experimental group were told that TENS could reduce the recruitment of muscle fibers, thereby decreasing force production. Because of the cutaneous sensation perceived by the subjects over the region of the hand muscle involved in the task, TENS can be expected to manipulate the subject’s belief of bad motor performance. In order to reinforce the subjects’ belief about the effects of TENS, the experimental group underwent a conditioning phase. A pre-determined, surreptitious reduction of the cursor’s excursion range was introduced stepwise. More precisely, after TENS, the motor task was executed again, but this time unbeknown to the subjects an attenuation coefficient was introduced and the excursion of the cursor was gradually decreased in steps of 0.0029 from trial 1 to trial 35 and remained stable until the end of the session (from trial 36 to trial 50). Consequently, by applying the same amount of force as in the baseline, the participants of the experimental group could see the cursor achieving lower lines of the target zone than before, and therefore believed to be weaker because of TENS.
Before starting the final session, TENS was applied again together with verbal suggestion of worse motor performance. Subjects then repeated the motor task (50 trials), but this time without any manipulation, that is without the reduction of the cursor’s excursion range.
The same motor task was performed by the subjects of the control group, who also underwent the TENS application as described above, but with different verbal information. In particular, these subjects were clearly told that they have been assigned to a control group in which TENS was completely inert in affecting force. They executed the motor task three times, like the experimental group, but without reduction of the cursor’s displacements in the manipulation session.
For each subject, the whole experiment took about 1.5 hours to be completed. Participants were tested at different times during the day, starting from 9.00am to 5.00pm. In the experimental group, 9 subjects were tested in the morning (9.00am-12.00am) and 8 subjects were tested in the afternoon (1.00pm-5.00pm). In the control group, 9 subjects were tested in the morning (9.00am-12.00am) and 6 subjects were tested in the afternoon (1.00pm-5.00pm). By analyzing the distribution of the subjects tested in the morning and in the afternoon, we found no differences between the two groups (Chi-square test, χ2 = 0.161, df = 1, p = 0.688).
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