Spatiotemporal mapping the neural correlates of acupuncture with MEG

Abstract

Acupuncture is an ancient Eastern healing modality with putative therapeutic applications. Unfortunately, little is known about the central mechanisms by which acupuncture may exert its effects. In this study, 16 [corrected] healthy subjects were evaluated with magnetoencephalography (MEG) to map the location and timing of brain activity during low-frequency electroacupuncture (EA) and mechanical, noninsertive, sham acupuncture (SA) given at acupoint PC-6. Both EA and SA evoked brain responses that localized to contralateral primary somatosensory (SI) cortex. However, initial responses for EA peaked slightly earlier than those for SA and were located inferiorly within SI. Average equivalent current dipole strength was stronger (particularly at latencies >60 ms) for SA. These spatiotemporal differences between activations elicited by EA and SA are likely attributable to stimulus modality (electrical versus mechanical) and differences in the underlying somatosensory fibers transmitting these signals. The present data confirm that acupuncture modulates activity within somatosensory cortex, providing support for previous studies that suggest that the therapeutic effects of acupuncture are linked to SI modulation. Thus, MEG provides excellent spatiotemporal characterization of the somatosensory component of acupuncture, and future studies can contrast derived brain response parameters in healthy controls with those found in a diseased state.

FIG. 1.

Experimental design and the arm brace. A. Each magnetoencephalography (MEG) scan consisted of three rest runs and two acupuncture runs (electroacupuncture [EA] and sham acupuncture [SA]). Acupuncture run order was randomized across subjects and consisted of 15 minutes of continuous low-frequency (2 Hz) stimulation. During each 10-minute rest run, MEG was recorded while subjects sat quietly. B. During EA and SA, subjects wore an MEG-compatible arm brace to reduce hand movement. The brace was equipped with a piezo-driven stimulator (dashed black circle). C. An enlarged image of the piezo-driven stimulator. The stimulator was anchored to the arm but adjustable so that the stimulation tip (black arrow) could be placed at the correct position on the skin surface. A rectangular opening in the brace allowed access to underlying acupoints.

FIG. 2.

Equivalent current dipole (ECD) and distributed source analysis of electroacupuncture (EA) and sham acupuncture (SA) conditions. A. ECD localization for 16 subjects demonstrated that sources for both EA (red) and SA (blue) map proximally to one another along the contralateral somatosensory (SI) cortex (∼BA 3b). Source locations were mapped to the closest points on the cortical surfaces reconstructed from each subject's magnetic resonance imaging scan, morphed to the average brain surface, and visualized using the inflated cortical representation. The inflated representations are used to reveal activity within sulci (dark gray) as well as on gyri (light gray). B. Average SI dipole time courses for EA (red) and SA (blue) demonstrate that activity peaks earlier in EA than SA, possibly due to temporal dispersion of afferent signals. Furthermore, SI ECD strength differs between conditions at long latencies, possibly due to the number and/or type of somatosensory fibers recruited. C. The image shows distributed source modeling results averaged across subjects and displayed on an average brain surface. These data confirm that the primary sources of magnetoencephalography (MEG) activity are within the contralateral central sulcus. The first significant peak for EA (∼20 ms) occurs earlier than that for SA (∼35–40 ms) and is located in a slightly more inferior position along the posterior bank of the central sulcus. Response peaks are also seen at ∼70 ms and ∼120 ms within SI cortex for both EA and SA. Activity returns to baseline by ∼250 ms. Thresholds for the activity shown were selected to control the familywise error rate to be at 5%.

FIG. 3.

Post-acupuncture ratings. A. The mean MGH Acupuncture Sensation Scale (MASS) Index was on average slightly larger for electroacupuncture (EA) than sham acupuncture (SA), suggesting that in general subjects might have felt stronger sensations during EA. B. When considering both EA and SA, the most commonly reported sensations were “pressure” and “tingling,” both being reported by >60% of subjects.