Though model-based decision making is fundamental to flexible behavior, its implementation in the brain remains poorly understood. Mechanistically dissecting model-based control necessitates dissociating it from simpler model-free systems. This requires tasks in which each system recommends a different course of action. Historically, tasks that achieved this, such as outcome devaluation (Adams and Dickinson, 1981), were poorly suited to neurophysiology as they generated only a limited number of informative trials. More recently, sequential decision tasks for humans have been developed that disambiguate model-based and model-free control in a stable way over many trials. The most popular of these is the so-called two-step task (Daw et al., 2011), which has been used to probe mechanisms of model-based RL (Daw et al., 2011; Wunderlich et al., 2012; Smittenaar et al., 2013; Doll et al., 2015), arbitration between controllers (Keramati et al., 2011; Lee et al., 2014; Doll et al., 2016), and behavioral differences in psychiatric disorders (Sebold et al., 2014; Voon et al., 2015; Gillan et al., 2016). The original version of the task has also been adapted in work with rats and
