Research on the mechanism of acupuncture began as early as 1976 with the introduction of the endorphin hypothesis and the gate control theory of pain, which accounts for the inhibition of nociceptive information via interneurons [REF].
In recent years, research has been prompted by the introduction of neuroimaging techniques such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET), both of which have further elucidated the relation between acupuncture stimulation and activation of neural structures [REF].