Brain Correlates of Phasic Autonomic Response to Acupuncture Stimulation: An Event-Related fMRI Study

This pioneering study used event-related functional magnetic resonance imaging (er-fMRI) to investigate how the brain controls autonomic nervous system responses during acupuncture. Eighteen healthy participants with no prior acupuncture experience underwent manual stimulation at two acupuncture points (ST-36 and SP-9) and one non-acupoint control site (SH1), while their brain and autonomic responses were simultaneously monitored. The experimental protocol used an event-related design, allowing detailed analysis of responses to each individual 2-second stimulus. During the study, electrocardiographic data were collected to measure heart rate, along with electrodes for skin conductance response, both important indicators of autonomic nervous system activity.

The results revealed distinct response patterns across the different points. Acupuncture at ST-36 produced the greatest heart rate decrease (-2.88 bpm), while SP-9 generated the greatest skin conductance response (1.99 µS/s), indicating different types of autonomic activation. Interestingly, even sham stimulation produced measurable responses, though of lesser magnitude. Brain analysis showed consistent activation in secondary somatosensory regions, insula, and middle cingulate cortex, areas known to process bodily sensations and pain.

Simultaneously, significant deactivation was observed in the default mode network, a set of brain regions active during rest. An important finding was the correlation between different autonomic response patterns and specific brain activity. Events that produced heart rate deceleration (orienting response) correlated with greater default mode network deactivation, while events with greater skin conductance response were associated with anterior insula activation. This differentiation suggests that acupuncture may activate two distinct types of psychophysiological reflexes: orienting response (characterized by heart rate deceleration) and defense/startle response (characterized by heart rate acceleration and increased skin conductance).

The intensity of sensations reported by participants correlated positively with autonomic responses, indicating that the magnitude of the physiological effect is related to the intensity of the sensory experience. The clinical implications are significant, suggesting that different acupuncture points may have distinct efficacies for modulating specific neural circuits, providing a scientific basis for point selection in therapeutic protocols. The study also indicates that autonomic responses to acupuncture are controlled by specific brain networks, providing insights into the mechanisms by which acupuncture may produce therapeutic effects through autonomic nervous system modulation. Limitations include the relatively small sample size and the focus on acute responses, with no investigation of long-term effects.

In addition, the correlational nature of the data does not allow definitive causal relationships between brain activity and autonomic response to be established.