Abstract
Apathy, a disabling and poorly understood neuropsychiatric symptom, is characterised by impaired self-initiated behaviour. It has been hypothesised that the opportunity cost of time (OCT) may be a key computational variable linking self-initiated behaviour with motivational status. OCT represents the amount of reward which is foregone per second if no action is taken. Using a novel behavioural task and computational modelling, we investigated the relationship between OCT, self-initiation and apathy. We predicted that higher OCT would engender shorter action latencies, and that individuals with greater sensitivity to OCT would have higher behavioural apathy. 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. We show that the latency of self-initiation is driven by changes in the OCT. Furthermore, we demonstrate, for the first time, that participants with higher apathy 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. Our results suggest that OCT is an important variable for determining free-operant action initiation and understanding apathy.
Highlights
Apathy is a common, disabling and hard-to-treat neuropsychiatric symptom found in a range of neuropsychiatric disorders such as schizophrenia, depression, Parkinson’ disease (PD), Alzheimer’ disease (AD) and Huntington’ disease (HD) (Husain & Roiser, 2018; Krishnamoorthy & Craufurd, 2011; Le Heron, Apps, & Husain, 2018)
Using mixed linear models to summarise the group level effect of both OCT manipulations on action latency, we found that participants in both Exp. (1) and Exp. (2) adapted their action latencies with respect to OCT for both price and environment
Reward sensitivity and OCT correlate with apathy scores in young adults Using our average reward RL model, we found that reward sensitivity in young adults was correlated with independently assessed apathy scores in both Exp. (1) (ρ = 0.62, p = 0.008) and Exp. (2)
Summary
Apathy is a common, disabling and hard-to-treat neuropsychiatric symptom found in a range of neuropsychiatric disorders such as schizophrenia, depression, Parkinson’ disease (PD), Alzheimer’ disease (AD) and Huntington’ disease (HD) (Husain & Roiser, 2018; Krishnamoorthy & Craufurd, 2011; Le Heron, Apps, & Husain, 2018) In these populations, clinical apathy is commonly associated with reduced self-care, functional decline and the need for external support (Konstantakopoulos et al, 2011; Pagonabarraga, Kulisevsky, Strafella, & Krack, 2015; Starkstein, Jorge, Mizrahi, & Robinson, 2006; Van Duijn, Reedeker, Giltay, Roos, & Van Der Mast, 2010). Using a novel behavioural task and computational modelling, we investigated the relationship between OCT, self-initiation and apathy. Our results suggest that OCT is an important variable for determining freeoperant action initiation and understanding apathy
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