Wong and Brown describe work on writing gratitude letters suggesting that this improves mental health, and in the usual 'preliminary' fMRI studies.
"...when we compared those who wrote the gratitude letters with those who didn’t, the gratitude letter writers showed greater activation in the medial prefrontal cortex when they experienced gratitude in the fMRI scanner. This is striking as this effect was found three months after the letter writing began. This indicates that simply expressing gratitude may have lasting effects on the brain."And, Fox discusses her work:
...our data suggest that because gratitude relies on the brain networks associated with social bonding and stress relief, this may explain in part how grateful feelings lead to health benefits over time. Feeling grateful and recognizing help from others creates a more relaxed body state and allows the subsequent benefits of lowered stress to wash over us.