The longing and wistfulness of nostalgia can make a person feel many different things. Amidst the memories, both happy and sad, you might even find a unique form of pain relief.
Yet exactly where such analgesic effects of nostalgia originate in the brain has remained a mystery, however.
In a new study, researchers from the Chinese Academy of Sciences tried to solve the puzzle in a truly bittersweet experiment.
Using fMRI scanners to monitor the brain activity of 34 volunteers, the researchers presented participants with images designed to induce nostalgic feelings, depicting objects or scenes from childhood.
Non-nostalgic images were also shown to the group, acting as control images, showing objects and scenes from modern life, but not evoking childhood memories.
While being shown these images during the experiment, the volunteers were also subjected to a thermosensory device giving them a sensation of painful heat on their forearm.
As the team expected based on previous research into nostalgia-induced analgesia, the nostalgic-themed images did appear to bring some measure of pain relief to the participants, with the volunteers rating the painfulness of the heat stimulator device lower when they were looking at the nostalgic pictures.
This effect was most apparent when the heat device was set to its lowest level of pain. Nostalgia seemed to lose its powers of pain relief when the thermal stimulator was set to its highest pain intensity level in the experiment.
The team suggests that could be because nostalgic cues last longer when pain is low, or because severe pain occupies more cognitive resources, diminishing the analgesic effects of nostalgia.
When participants' ratings of the pictures (in terms of their respective nostalgia levels) were taken into account, the most nostalgic images were linked with a greater analgesic effect.
"The more nostalgic the participants felt, the less pain they perceived," the team writes in its paper, led by first author and psychology researcher Ming Zhang.
"These regions are all involved in retro scene processing, the sensation of the self, and emotional appraisal," the researchers write, noting that when nostalgia images were shown at the same time as the pain stimulus, activity levels changed in other parts of the brain.
Most importantly, the team says, the thalamocortical system can be seen playing a vital role in how analgesia functions in the brain.
It's possible, the researchers suggest, that nostalgia might attenuate pain by strengthening the activity of the thalamus during painful sensations.
"Based on our findings, we propose a possible model of thalamus-centered pathways to explain the analgesic effect of nostalgia," the researchers write.
"These findings offer implications and perspectives for the further development and improvement of non-drug, psychological analgesia."
The findings are reported in JNeurosci.