Gratitude is necessary for social behavior and the default network is largely involved in many aspects of this emotion and subsequent reciprocity. One section of the anterior cingulate cortex, according to this research, is where gratitude is experienced or felt, and actions resulting from this feeling are implemented through more accessible sections of ACC.
Fz activation should, among other results, increase gratitude and reciprocal behaviors in individuals.
Yu et al (2018). Decomposing Gratitude: Representation and Integration of Cognitive Antecedents of Gratitude in the Brain. J Neuroscience, 38, 4886-4898. https://www.ncbi.nlm.nih.gov/pubmed/29735557