Opportunity cost determines action initiation latency and predicts apathy

Background: Apathy, a disabling and poorly understood neuropsychiatric symptom, is characterised by impaired self-initiated behaviour. Although the computational mechanisms that determine self-initiation are poorly understood, it has been hypothesised that the opportunity cost of time (OCT) may be a key variable linking self-initiated behaviour with motivational status. Using a novel behavioural task and computational modelling, we investigated the relationship between OCT, self-initiation and apathy. OCT represents the amount of reward which is foregone per second if no action is taken. We predicted that higher OCT would engender shorter action latencies, and that individuals with greater sensitivity to OCT would have higher behavioural apathy.Methods: We modulated the OCT in a novel task called the ‘Fisherman Game’, Participants freely chose when to self-initiate actions to either collect rewards, or on occasion, to complete non-rewarding actions. We measured the relationship between action latencies, OCT and apathy for each participant across two independent non-clinical studies, one under laboratory conditions (n=21) and one online (n=90). ‘Average-reward’ reinforcement learning was used to model our data. We replicated our findings across both studies.Results: We show that the latency of self-initiation is driven by changes in the OCT. Furthermore, we demonstrate, for the first time, higher apathy was showed greater sensitivity to changes in OCT in younger adults. Our model shows that apathetic individuals experienced greatest change in subjective OCT during our task as a consequence of being more sensitive to rewards.Conclusions: Our results suggest that OCT is an important variable for determining free-operant action initiation and understanding apathy.