Figure 1.
A: Procedures for peripheral responses (Experiment 1). Two small paintbrushes stroked the rubber hand (strokes were approximately 2 cm in length) and the hidden real left hand of the participant at a frequency of 1 Hz for 180 seconds as synchronously as possible under one condition (synchronous condition: S) and asynchronously under the other condition (asynchronous condition: A); RHI: Rubber hand illusion induction. Participants received acupuncture needle stimulation at a frequency of 1 Hz for 10 seconds every 1 minute during the 3-minute acupuncture stimulation period on the hidden (real) left hand immediately after commencing the RHI under the synchronous or asynchronous condition (Acu: Acupuncture stimulation). B: Procedures for brain responses (Experiment 2). There were two randomized sessions: a synchronous condition (S) and an asynchronous condition (A). The acupuncture needle was stimulated at the LI4 acupoint in the left hand out of sight of the participant according to the beats of a 1-Hz metronome transmitted via earphones (Acu: Acupuncture stimulation). One session included a four-block tactile-stimulation condition (30 seconds) for the RHI and a four-block acupuncture stimulation (30 seconds); these were performed successively. Tactile stimulation was synchronously or asynchronously applied with short strokes from two brushes (approximately 2 cm in length) at a frequency of 1 Hz during the tactile-stimulation block; RHI: Rubber hand illusion induction.
Figure 2.
Peripheral responses to acupuncture stimulation during the RHI.
A representative example of the peripheral responses to acupuncture stimulation around the LI4 acupoint (2×2 cm) is shown in upper column A. The increase in peripheral responses to acupuncture stimulation was significantly different between the synchronous and asynchronous brush-stroke sessions, as seen in lower column A (18.6±3.3 vs. 9.3±2.8%, t = 2.544, P<0.05). Peripheral responses to acupuncture stimulation are presented as the mean peripheral blood-flow change over time. Values are means ± standard errors.
Figure 3.
Brain activations in response to acupuncture stimulation.
A: Brain activations in response to acupuncture stimulations were observed in the contralateral secondary somatosensory cortex (SII) and insula under both the asynchronous (right SII: 44, −26, 26; t = 11.52; Z = 5.9; right insula: 44, 2, 8; t = 9.78; Z = 5.5) and synchronous (right SII: 50, −20, 20; t = 9.33; Z = 5.39; right insula: 56, 20, 0; t = 8.9; Z = 5.27) conditions. All two peaks were p<0.05, corrected, and all coordinates are in MNI space. B: To plot the regions of brain activation involved in acupuncture stimulation, the averaged percent signal change in anatomical regions of interests (ROIs), including the right insula and SII, were extracted. When the acupuncture stimulation occurred, BOLD responses in the right insula (0.32±0.04 vs. 0.23±0.05%, t = 2.517, P<0.05; A) but not the SII (0.34±0.05 vs. 0.29±0.05%, t = 1.413, P>0.177; B) differed significantly under the asynchronous and synchronous sessions.
Table 1.
Activated brain regions to acupuncture stimulation in asynchronous session and synchronous session (n = 17).
Figure 4.
Individual differences in brain activations to acupuncture stimulation.
A: Correlation between rubber hand illusion score and brain activations to acupuncture stimulation during reduced body ownership. Normalized SPM T-maps overlaid on the corresponding axial T1-weighted images showing statistically significant (p<0.001, uncorrected with 10 continuous voxels) brain activation correlations between the rubber hand illusion (synchronous vs. asynchronous session) and brain responses to acupuncture stimulation in asynchronous condition compared to synchronous condition. Brain activation was observed in the right ventral premotor cortex (x = 52, y = 16, z = 26, Z = 3.34, r = 0.571, p<0.05). B: Correlation between DeQi score and brain activations to acupuncture stimulation during reduced body ownership. Brain activation related to DeQi score (asynchronous vs. synchronous session) was observed in the right posterior insula (x = 46, y = −8, z = −10, Z = 4.38, r = 0.752, p<0.001).