Are friends really the family we choose? Local variations of hypothalamus activity when viewing personally known faces.

Sibling and friend relationships have significant impact on individuals' socio-emotional development. Hypothalamic supraoptic nucleus (SON) and paraventricular nucleus (PVN) synthesize and secrete neuropeptides, including oxytocin, associated with attachment behaviors. Here, using fMRI, we investigate the implication of these two hypothalamic nuclei in the visual processing of personally known faces.  Faces of same-sex sibling, best friend, celebrity, and unknown person appear in the middle of the screen while participants perform a task requiring a button click each time a central white dot turns red. Ratings of familiarity (time spent together) and emotionality (feelings toward individual) toward the four individuals are recorded. Local activation within the hypothalamus is assessed via two complementary methods: (1) voxel-based analyses within inclusive mask of the hypothalamus; (2) region-of-interest (ROI) analysis of partial hypothalamic volumes using SON and PVN as center of mass coordinates, with percent signal change extracted and analyzed within these ROIs. Results suggest that the SON responds to all familiar individuals while the PVN has increased response to sibling compared to friend faces and is correlated to familiarity but not emotionality. These findings support differential involvement of local hypothalamic substructures SON and PVN in response to faces of individuals with different social relationships.