Sex differences in task engagement and lapse rate during reward learning plateaus

Our understanding of sex differences in reward learning has been limited due to the predominant study of males, yet recent studies have uncovered significant differences in the use of adaptive strategies, sensitivity to negative feedback, and impulsivity. Here, we evaluated sex differences in flexible learning in two domains: the learning of stimulus- and action-based associations and their reversals. During action-based learning, rats selected between two identical visual stimuli presented on a touchscreen, where the spatial location predicted a higher probability of reward. For stimulus-based learning, rats chose between two distinct visual stimuli presented in pseudorandom spatial locations, one of which was associated with a higher probability of reward. Reversal phases involved switching reward contingency between the two actions or stimuli. To gain a detailed understanding of diffferences across conditions, we modeled animals trial-by-trial choices using reinforcement learning (RL) models and examined their steady-state behavior to capture transitions between distinct behavioral states. We found that female rats were more likely to omit trials and take longer to initiate trials in both domains. The omissions were more frequent in late-stage action-based reversal learning, once learning had plateaued. Moreover, although the estimated parameters of the best-fitting RL model revealed some sex differences, the model that incorporated transitions between different behavioral states provided a better overall fit to the data. This model also revealed that across all reversal phases, females exhibited a higher transition-specific lapse rate than males, indicating greater task disengagement once there was no need for further learning. Together, our fine-grained analysis of behavior adds to a growing literature on sex differences in flexible reward learning.