A role for the insula in establishing social dominance: structural and functional MRI studies in nonhuman primates.

Awareness of one's position in the social hierarchy is essential for survival. Conversely, poor social cognition is associated with several neuropsychiatric diseases. Although brain regions that mediate understanding of the social hierarchy are poorly understood, recent evidence implicates the insula. Magnetic resonance imaging (MRI) scans were acquired in twelve individually housed male cynomolgus monkeys to determine whether structural and functional characteristics of the insular cortex predicted the social rank that monkeys would attain once they formed stable social hierarchies. Structural MRI revealed that left insular volume was significantly larger in monkeys that would become dominant vs. subordinate. No differences were observed in other areas including amygdala, caudate nucleus, or prefrontal cortex. Volumetric differences were localized to dorsal anterior regions of both left and right insulae. Functional MRI revealed that global correlation, a measure of connectedness to the rest of the brain, was significantly lower in the left insula of monkeys who would become dominant vs. subordinate. Moreover, the fractional amplitude of low-frequency fluctuations, a reflection of spontaneous brain activity, trended lower in bilateral insula in the future dominant monkeys. This prospective study provides evidence for a role of the insula in the establishment and maintenance of social dominance relationships.