Probabilistic modeling reveals coordinated social interaction states and their multisensory bases.

Social behavior across animal species ranges from simple pairwise interactions to thousands of individuals coordinating goal-directed movements. Regardless of the scale, these interactions are governed by the interplay between multimodal sensory information and the internal state of each animal. Here, we investigate how animals use multiple sensory modalities to guide social behavior in the highly social zebrafish (Danio rerio) and uncover the complex features of pairwise interactions early in development. To identify distinct behaviors and understand how they vary over time, we developed a new hidden Markov model with constrained linear-model emissions to automatically classify states of coordinated interaction, using the movements of one animal to predict those of another. We discovered that social behaviors alternate between two interaction states within a single experimental session, distinguished by unique movements and timescales. Long-range interactions, akin to shoaling, rely on vision, while mechanosensation underlies rapid synchronized movements and parallel swimming, precursors of schooling. Altogether, we observe spontaneous interactions in pairs of fish, develop novel hidden Markov modeling to reveal two fundamental interaction modes, and identify the sensory systems involved in each. Our modeling approach to pairwise social interactions has broad applicability to a wide variety of naturalistic behaviors and species and solves the challenge of detecting transient couplings between quasi-periodic time series.

Physiological stress reactivity and empathy following social exclusion: a test of the defensive emotional analgesia hypothesis.

Experiences of social exclusion elicit social pain responses. The current study examined the ability of social exclusion to activate physiological stress responses and adaptively modulate affect and empathy consistent with "defensive emotional analgesia." Measures of affect and empathy, and saliva samples for cortisol and alpha-amylase (sAA) analysis, were collected before and after subjects participated in a computer game ("Cyberball") designed to manipulate feelings of social exclusion. Contrary to our hypotheses, social exclusion was associated with a reduction in cortisol, and social inclusion with an increase in cortisol. Both Cyberball groups showed increases in sAA and decreases in both positive and negative affect, with the greatest drop in affect occurring after social exclusion. Empathy did not differ between the social exclusion and inclusion groups and was not correlated with cortisol or sAA levels. These results support the presence of a defensive response to social exclusion in which central stress pathways controlling cortisol release are inhibited. Cortisol and sAA were shown to have distinct patterns of responses to psychological stress, with sAA responding more rapidly. Related methodological concerns for the use of these physiological stress markers and of Cyberball in social neuroscience research are discussed.