Dopaminergic signatures of state and trait-like pessimism in mice

The midbrain dopaminergic system is critical for learning reward expectations, but how it contributes to pessimistic bias in reward expectation remains unclear. Using a Pavlovian ambiguous-cue task while recording mesolimbic dopaminergic activity, we find that trial-to-trial, "state" pessimism is associated with decreased dopaminergic activity during ambiguous cues, followed by a self-correcting positive shift in the balance of responses to reward and reward omission. Mice with pessimistically skewed cue interpretation, or "trait-like" pessimism, also had decreased activity during ambiguous cues. However, opposite of state pessimism, trait-like pessimism was associated with a negative shift in outcome signaling driven by larger reward-omission responses. These data indicate that state and trait-like pessimism arise from distinct neural mechanisms and support a model in which trait-like pessimism is maintained by asymmetrically weighted dopaminergic outcome signals.