Abstract
Current theories suggest that decision-making arises from multiple, competing action-selection systems. Rodent studies dissociate deliberation and procedural behavior, and find a transition from procedural to deliberative behavior with experience. However, it remains unknown how this transition from deliberative to procedural control evolves within single trials, or within blocks of repeated choices. We adapted for rats a two-step task which has been used to dissociate model-based from model-free decisions in humans. We found that amixture ofmodel-based andmodel-free algorithms was more likely to explain rat choice strategies on the task than either model-based or model-free algorithms alone. This task contained two choices per trial, which provides a more complex and non-discrete per-trial choice structure. This task structure enabled us to evaluate how deliberative and procedural behavior evolved within-trial and within blocks of repeated choice sequences. We found that vicarious trial and error (VTE), a behavioral correlate of deliberation in rodents, was correlated between the two choice points on a given lap. We also found that behavioral stereotypy, a correlate of procedural automation, increased with the number of repeated choices. While VTE at the first choice point decreased with the number of repeated choices, VTE at the second choice point did not, and only increased after unexpected transitions within the task. This suggests that deliberation at the beginning of trialsmay correspond to changes in choice patterns, while mid-trial deliberation may correspond to an interruption of a procedural process.
Highlights
It has long been known that multiple systems within the brain contribute to making decisions (O’Keefe and Nadel, 1978; Adams and Dickinson, 1981; Sloman, 1996; Dayan and Balleine, 2002; Lieberman, 2003; Loewenstein and O’Donoghue, 2004; Balleine et al, 2008; van der Meer et al, 2012; Dolan and Dayan, 2013; Redish, 2013)
We found that rat choices on the two-step task were better explained by a mixture of the model-based and model-free systems than by either system alone
We were able to use the fact that each lap on the two-step task contained multiple decisions to evaluate how vicarious trial and error (VTE) changed over the course of a trial, and found that VTE at the two choice points were correlated within lap, suggesting rats likely entered deliberative modes for entire laps
Summary
It has long been known that multiple systems within the brain contribute to making decisions (O’Keefe and Nadel, 1978; Adams and Dickinson, 1981; Sloman, 1996; Dayan and Balleine, 2002; Lieberman, 2003; Loewenstein and O’Donoghue, 2004; Balleine et al, 2008; van der Meer et al, 2012; Dolan and Dayan, 2013; Redish, 2013). The probabilities change slowly over time, so the subject is constantly trying to find the best option and should not settle on one option, but can use observations of reward as a signal that the option is a good one to return to (at least for a while) This two-step task is able to dissociate model-based from model-free decisions because it creates conditions where the two decision-making algorithms make different choices, mostly on laps following a rare transition (e.g., choosing A at C1 leads to C3, a choice between E and F). There were return corridors from the reward offer sites to the start of the maze, and rats ran laps freely for 45 min per session
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