Patch-leaving decisions and pupil-linked arousal systems

Deciding when to abandon a depleting resource in favour of potentially richer alternatives is fundamental to adaptive behaviour. Such patch-leaving decisions require balancing the expected advantage of leaving against both the cost of moving and the reward foregone in the current environment. Previous research suggests that activity of noradrenergic (NE) neurons in the locus coeruleus (LC) underpins patch-leaving. In the current study, we used pupil dilation as a time-resolved readout of subcortical neuromodulation during patch-leaving. We hypothesised that leave decisions will be preceded by a transient pupil dilation. Participants harvested from exponentially depleting patches (blueberry bushes) with three initial reward values, in two environments, which differed in the variability of initial reward values. Behavioural results show that, as predicted by the mathematically optimal solution, participants adjusted their decisions based on the instantaneous reward rate, but also displayed a bias to overharvest (stay longer in compared to the optimum), which was more pronounced in the high variability environment. Pupil size was overall larger in the high variability environment, associated with increased uncertainty. Importantly, we observed an increased transient pupil dilation in response to reward outcomes immediately preceding leave compared to stay decisions, as well as an increase in pupil dilation (and RT) across successive stay trials, leading up to leave decisions, presumably indicating an increased LC-NE activity associated with abandoning current options to explore alternatives.