Neuroimaging techniques such as functional magnetic resonance imaging (fMRI), electroencephalography (EEG), and magnetoencephalography (MEG) allow us to noninvasively monitor the effects of acupuncture in the human brain. Recent fMRI data demonstrate that acupuncture modulates a distributed network of cortical, subcortical/limbic and brainstem regions.1
However, fMRI only measures hemodynamic changes that are slow (>1 second) and cannot directly track neuronal electrical activity following an acupuncture stimulus on a millisecond timescale. Fortunately, both EEG and MEG may be used to reveal the time course of these rapid somatosensory responses. Although there are no previously published data utilizing MEG to evaluate the effects of acupuncture on brain activity, EEG has been used extensively to study the effects of manual acupuncture (MA) and electroacupuncture (EA) on somatosensory evoked potentials*
(SEPs) to both nonpainful and painful stimuli.
EEG studies investigating the effects of MA on nonpainful SEPs evoked by stimulation of leg acupoints found that 20 minutes of MA decreased amplitude of early latency SEPs, suggesting that acupuncture may modulate activity within spinal nerves and/or primary somatosensory cortex.2
Yet, similar studies did not find early modulation with arm/hand3,4
or facial acupoint stimulation.5
Studies utilizing EA have less methodological variability associated with needling than MA and have demonstrated that the time course of SEPs generated by EA given at hand acupoints are similar to median nerve SEPs.6,7
Furthermore, to help determine whether acupuncture modulates early sensory/discriminative or late cognitive/affective components of SEPs, previous studies have combined administration of acupuncture and anesthetics. One study argued that acupuncture modulates attentional mechanisms as it decreased amplitude of the P250 pain component.8
However, modulation of pain SEPs may occur even when subjects are unconscious under anesthesia,9
and although confounding effects may occur when acupuncture is combined with some anesthetics,10
other data found no effect of acupuncture on pain regardless of whether anesthesia is given prior to or following EA,11
but again the results varied. Finally, the time courses of EA SEPs and their effects on both nonpainful and painful sensory stimuli have been found to be highly dependent on the interstimulus interval used, with short intervals (<2 seconds), commonly used in clinical settings, resulting in an overlap of long-latency components,6,12,13
thus making it difficult to interpret many of these studies.
Collectively, studies of acupuncture effects on non-painful and painful SEPs have produced mixed results and are confounded by the need to use long (>2 second, which is uncommon for clinical EA) interstimulus intervals when multiple stimuli are used. It is also unclear whether acupuncture acts similarly on experimental pain as on chronic pain, and difficulties interpreting the effects of anesthetics combined with EA demonstrate that their concurrent use provides little additional information regarding the neural mechanisms of acupuncture. Finally, although previous EEG studies provide some useful information regarding the timing of acupuncture effects, all of them lack information regarding the anatomical location of the underlying brain activity. In the present study, we used anatomically constrained MEG to spatiotemporally map somatosensory evoked brain response to EA and sham acupuncture (SA) given at a clinically relevant frequency (2
Hz) without confounding measurements with other somatosensory/pain stimuli or the use of anesthetics. To further mimic clinical intervention procedures, acupuncture stimulation was given continuously for 15 minutes while MEG was recorded. To our knowledge, this is the first MEG investigation of acupuncture, thus providing novel insight into the spatiotemporal dynamics of neural responses underlying this healing modality.