Figure 1.
Behavioral ratings of pain stimuli.
Ratings for unpleasantness and intensity of pain and pressure were compared in self-induced and externally induced conditions. (A) Both unpleasantness and intensity of self-induced pain were rated significantly lower than externally induced pain. (B) While unpleasantness ratings of pressure induced either way were not significantly different, the self-induced ones were rated significantly lower on intensity. VAS, visual analog scale. ***p<0.001, n = 25.
Figure 2.
Regional brain activation in the conjunction analysis.
A) Activation maps in the four conditions. Panels shown from left to right: active pain (apn); passive pain (ppn); active pressure (apr); passive pressure (ppr). The first three rows display the activation of SI, and the fourth to sixth rows show the activation of ACC, MI, insula in sequential order. B) Differences between pairs of conditions. The left three panels: active minus passive pain. The most right panel: active minus passive pressure. Nos, number of subjects. +: activation; −: deactivation. All data were corrected, p<0.05. R stands for right. SI, primary somatosensory cortex; MI, primary motor cortex; SMA: supplementary motor area; mFG, middle frontal gyrus; sTC: superior temporal cortex; Thl: thalamus; ACC, anterior cingulate cortex; AG: angular gyrus; LG, lingual gyrus; mTC, middle temporal cortex; iFG: inferior frontal gyrus; IPL: inferior parietal lobule; sPL: superior parietal lobule; PCC, posterior cingulate cortex.
Table 1.
Brain areas affected by each condition.
Figure 3.
Correlation analysis between BOLD signal changes and pain ratings.
Linear correlation analyses were performed for the BOLD signal changes in the ROIs with the unpleasantness ratings (left two columns) or pain intensity ratings (right two columns). Panels in the six rows (top to bottom) are scatter plots for the anterior cingulate cortex (ACC), the primary somatosensory cortex (SI), the primary motor cortex (MI), the insula (Ins), the vermis of cerebellum (Ver), and the thalamus (Tha). All ROIs except for the insula were negatively correlated with the ratings in active pain and positively correlated with ratings in passive pain. The Insula, however, were consistently correlated with all ratings in both conditions. All of the ROIs are based on functional and structural masks.
Figure 4.
Statistics on functional brain activation.
Mean intensities of BOLD signals and activated voxel numbers in the ROIs in active and passive pain conditions were compared. (A) In all ROIs except the insula, BOLD signal intensities were higher in passive pain than that in active pain condition. (B) No such difference could be found between active and passive pressure. (C) Passive pain activated significantly more voxels in the primary somatosensory and motor cortices. (D) No significant difference was observed between the two types of pressure.
Figure 5.
Schematic diagram of the pressure and pain stimuli and experimental conditions.
(A) Pressure ring with spherical beads. (B) Pain ring with polyhedral beads. (C) Self-induced movement, in which the subject squeezes his own left hand with his right hand; the pain ring is pictured. (D) Externally induced movement, in which the experimenter squeezes the subject's left hand; the pain ring is pictured. Within the rubber ring was the cross holder, consisted of the yellow short arms and the grey long arms.
Figure 6.
Procedural timeline for behavioral tests and fMRI scanning.
(A) Subjects underwent self-induced and externally induced squeezes in the pressure and pain conditions in a within-subjects design. Squeeze for 2 seconds/trial was repeated three times in 60 seconds. The interval between conditions was 30 seconds. All conditions were repeated three times, and condition order was randomly determined and counterbalanced. (B) During fMRI scanning, squeeze was repeated thirteen times in each condition with 10–18 seconds between trials. Each condition lasted for 180 seconds, and the interval between conditions was 60 seconds. Subjects gave off-line VAS ratings after scanning was completed.