Daily physical complaints and hippocampal function: an fMRI study of pain modulation by anxiety

Pain is a popular physical complaint in human. It is known that experimental anxiety modulates pain processing through hippocampal amplification, whereas it is not known whether a similar experimental reaction is related to daily physical complaints known as 'somatization'. The purpose of this study is to investigate the neural correlates of pain modulation induced by anxiety, particularly in the hippocampus, and how individual differences in this neural reaction relate to somatization. We measured neural response to noxious electrical stimulations, as well as the response to the preceding visual anticipatory cues (which induced low anxiety or high anxiety), by functional magnetic resonance imaging (fMRI). Individual daily physical symptoms were assessed by using the somatization subscale of the Symptom Checklist 90 revised (SCL-90-R). Correlation coefficients between the neural activations and the somatization scores were calculated. We found that manifestation of daily physical symptoms was related to smaller differences in hippocampus activation between high and low anxiety states, suggesting that the ability of the hippocampus to distinguish anxiety states was weakened by the chronic condition that caused the daily physical symptoms. The proper inhibition of neural activation in low anxiety states in the hippocampus and the anterior insula was observed to occur in companionship with lower daily physical complaints. These findings indicate that anxiety's alteration of the network that includes the hippocampus and that is associated with pain modulation underlies the manifestation of somatization.