Somatosensory Stimulation

Somatosensory stimuli consisted of constant-current, 200-µs long square pulses generated by an electrical stimulator (DS7A, Digitimer). Stimuli were delivered using a surface bipolar electrode placed on the median nerve at the wrist. In Experiment, 1 stimuli were delivered to the right hand; In Experiment 2, stimuli were delivered to either hand. Stimulus intensity was adjusted in each participant at the beginning of each recording block, to elicit a clear HBR in three consecutive trials^8,10. Participants who refused an increase in stimulus intensity before three clear HBR responses were observed, or who had a clear HBR in less than 50% of trials were considered non-responders and did not take part in the experiment¹². In HBR responders the mean stimulus intensity across participants (±SD) was 39.4 ± 15.0 mA [experiment 1], and 46.8 ± 21.3 mA [experiment 2]. In Experiment 1, participants were recruited from a group of previously screened HBR responders^8,21. In Experiment 2, out of 64 participants tested, 20 (i.e. 31%) were classified as HBR-responders and took part in the rest of the experiment. This 31% response rate is lower than commonly reported (~60%^8,10,12,13), which may have been due to the head mounted display acting as a protective screen for certain participants¹².