Dynamic population coding of social novelty in the insular cortex

The familiarity of socially interacting peers has a profound impact on behavior ^1–3 , but little is known about the neuronal representations distinguishing familiar from novel conspecifics. The insular cortex (IC) regulates social behavior ^4–9 , and our previous study revealed that neurons in the agranular IC (aIC) encode ongoing social interactions ¹⁰ . To elucidate how these neurons discriminate between interactions with familiar and novel conspecifics, we monitored neuronal activity in mice by microendoscopic calcium imaging during social recognition memory (SRM) and linear chamber social discrimination (LCSD) tasks. In the SRM task, repeated interactions with the same target activated largely nonoverlapping cells during each session. The fraction of cells associated with social investigation (social cells) decreased as the subject repeatedly interacted with the same target, whereas substitution of a second novel target and subsequent exchange with the first familiar target recruited more new social cells. In the LCSD task, the addition of a novel target to an area containing a familiar target transiently increased the number of cells responding to both targets, followed by an eventual increase in the number of cells responding to the novel target. These results support the view that the aIC dynamically encodes social novelty, rather than consistently encode social identity, by rapidly reorganizing the neural representations of conspecific information.