Procedure

The study was conducted at the patient bedside. Before the beginning of the experiment, the procedure was explained to the participant, who was exposed to a small number of test stimuli for familiarization. The experiment consisted of two sessions of four blocks each, one session per side of stimulation. In each block, the subject received stimuli belonging to one of four sensory modalities: nociceptive, vibrotactile, auditory, and visual. Each stimulation block consisted of 40 stimuli. The order of the stimulation blocks was randomized across participants. A blocked design was chosen to ensure that expecting the possible occurrence of a nociceptive stimulus would not affect the responses elicited by non-nociceptive stimuli [74]. The interstimulus interval (ISI) was large, variable, and self-paced by the experimenter (5–10 s). Participants were instructed to keep their gaze fixed on a black cross (3 x 3 cm) placed in front of them on the edge of the bed, at a distance of ~2 m, 30° below eye level, for the whole duration of each block. To ensure that each stimulus was perceived and to maintain vigilance across time, participants were asked to press a button as soon as they felt the stimulation. Furthermore, participants provided a subjective intensity rating for each stimulus on a scale ranging from 0 to 10 (0 was defined as “undetected” and 10 was defined as “maximum intensity”). At the end of each block, the patients were asked to report whether they had perceived any of the stimuli as painful.