We use our experiences to form and update beliefs about the hidden states of the world1–3. When possible, we also gather evidence by observing others. However, how the brain integrates experiential and observational evidence is not understood. We studied the dynamics of evidence integration in a two-player game with volatile hidden states. Both humans and monkeys successfully updated their beliefs while playing the game and observing their partner, although less effectively when observing. Electrophysiological recordings in animals revealed that the anterior cingulate cortex integrates independent sources of experiential and observational evidence into a coherent neural representation of dynamic belief about the environment’s state. The geometry of population activity revealed the computational architecture of this integration and provided a neural account of the behavioural asymmetry between experiential and observational evidence accumulation. This work lays the groundwork for understanding the neural mechanisms underlying evidence accumulation in social contexts in the primate brain.
