Figure 2. Area of secondary hyperalgesia with stroking stimuli (n = 23). Medians (quartiles) are shown. White bars = naloxone infusion followed by ketamine infusion; hatched bars = placebo infusion followed by ketamine infusion; black bars = placebo infusion followed by placebo infusion; asterisk = significantly larger area of secondary hyperalgesia in the group not receiving ketamine (P < 0.05). There was no significant difference between the two groups receiving ketamine.