Box plot illustrating cold pain thresholds variations related to placebo and real tDCS.

<p>Statistically significant changes occurred in the cold pain thresholds of the left face (<i>p</i> = 0.012) throughout the experiment.</p

Changes in the μ-opioid receptor availability induced by placebo (A–C) and real (D–E) tDCS.

<p>A and D, Representation of precuneus MOR activation in the sagittal plane. B and E, PAG MOR activation in the axial plane. C, Left thalamus (Thal) MOR activation in the coronal plane. F, Left prefrontal cortex (PFC) MOR activation in the axial plane. All images are radiological in orientation, threshold T 3–8.</p

Experimental design used in the second PET scan.

<p>Placebo tDCS was applied during the early PET phase (15 to 35 min post-tracer administration) and real tDCS during the late PET phase (60 to 80 min). QST was performed before the PET, in the period between placebo and real tDCS and immediately after the PET.</p

Correlation between placebo and real tDCS-induced MOR activation.

<p>MOR BP_ND during placebo (x axis) and real (y axis) tDCS for each subject in the clusters of μ-opioid activation induced by placebo (A–C) and real (D–F) tDCS. The same clusters are illustrated in the <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0102350#pone-0102350-g002" target="_blank">figure 2</a>. Positive correlations can be observed in precuneus and PAG (red lines) but not in thalamus and PFC (blue lines). Statistically significant values at p<0.05 were found in the PAG cluster activated during placebo tDCS (r_p = 0.760, p = 0.013, 3B) and in the precuneus cluster activated during active tDCS (r_p = 0.788, p = 0.008, 3D).</p

Box plot representing tDCS (placebo and real) effects in the heat pain thresholds of both sides of the face.

<p>Statistically significant changes occurred in the heat pain thresholds of the left face (<i>p</i> = 0.032).</p