Balancing consistency and flexibility: exploring trait and state optimism in rats using the cognitive judgement bias test.

Optimistic or pessimistic interpretation of ambiguous information has been widely used in animal welfare science as an indicator of emotional states. Yet, growing evidence suggests that besides this state dimension, animals differ consistently in their optimism levels, suggesting a trait dimension of optimism, too. The boundary as well as a potential interplay between these two dimensions, however, remains poorly understood. Here, we aimed to disentangle consistent trait from transient state optimism using a spatial cognitive judgement bias test in laboratory rats. Specifically, we repeatedly tested individuals for their optimism levels and assessed the repeatability to infer trait optimism. Then, we exposed the rats to either negatively valenced 22 kHz or positively valenced 50 kHz USV playbacks to manipulate state optimism. Lastly, we conducted tests for anxiety-like behavior to assess whether the manipulation was accompanied by broader changes consistent with altered emotional state. Building on previous work, we found repeatable optimism levels over 3 weeks, supporting the existence of a trait dimension of optimism. Surprisingly, however, we did not detect any statistically significant effects of the USV playbacks on optimism levels and anxiety-like behavior, hindering the systematic investigation of a state dimension. Overall, our study provides further evidence that optimism has a consistent trait dimension, while highlighting the ongoing need for future research to successfully disentangle trait and state components of behavior.