Kurzban et al.'s model aligns well with emerging metacognitive proposals of fatigue and effort, and provides a useful account of why task switching occurs. However, under many circumstances individuals continue to persist in a primary task where the immediate costs are considerable and where alternative options present significant benefits. As an example, in academia the tangible rewards of funding, publication, and tenure are far removed from the months of executive-function–demanding writing, research, and teaching required to reach these goals. The level of persistence needed in such scenarios is not well accounted for in the proposed model, which predicts that the large opportunity costs associated with long-term goals should prompt a high mental-effort signal and subsequent re-prioritization of behavior. So how do we sometimes “stay the course” in the face of subjective effort?

Several potential mechanisms could be involved in persistence. While the fatigue signal could be directly attenuated, say, by reward receipt, other mechanisms could operate upstream of this point. For example, the discounting of the primary goal that normally occurs under conditions of temporal distance, uncertainty, or exertion, could be attenuated during the cost/benefit evaluation. Alternatively, the degree of discounting of competing tasks could be increased. Kurzban et al. suggest that the key to task persistence involves attenuating the effort signal through reward, although they are not specific about the underlying brain mechanisms. Here we propose that activity in the anterior cingulate cortex (ACC) is critical, functioning to integrate cost/benefit ratios to provide a relative utility signal that may work directly to suppress subjective effort.

As the target article notes, as ACC activity decreases, so does task performance. One interpretation of this effect is that as long as activity in this region of the prefrontal cortex (PFC) remains high, vigilance and persistence are maintained. We and others have examined single-unit ACC activity during decision tasks and found that heightened firing appears to indicate a worthwhile course of action; however, a sufficiently strong signal may be required to drive pursuit and persistence (Amiez et al. Reference Amiez, Joseph and Procyk2005; Hillman & Bilkey Reference Hillman and Bilkey2010; Quilodran et al. Reference Quilodran, Rothe and Procyk2008; Sallet et al. Reference Sallet, Quilodran, Rothe, Vezoli, Joseph and Procyk2007; Shidara & Richmond Reference Shidara and Richmond2002). Importantly, this ACC signal appears only when significant cost/benefit analysis is required; furthermore, heightened firing does not always correspond to the most costly action, but rather seems to indicate the most worthwhile choice in terms of relative cost/benefit computed utility (Hillman & Bilkey Reference Hillman and Bilkey2010; Kennerley et al. Reference Kennerley, Walton, Behrens, Buckley and Rushworth2006; Rudebeck et al. Reference Rudebeck, Behrens, Kennerley, Baxter, Buckley, Walton and Rushworth2008). Moreover, the ACC is recruited regardless of the actual type of effort involved – be it physical exertion, competitive fighting, or mental taxation – suggesting that the region may be responding to generalized opportunity cost calculations inherent in cost/benefit decision tasks.

These encoding characteristics of ACC match the descriptions of several of the opportunity cost model components illustrated in Figure 1 of the target article: The ACC's experience-based encoding of cost/benefit computations provides an output signal that drives allocation of cognitive processes towards completion of tasks with optimal utility. Viewing the ACC in this way – as a dynamic utility encoder versus a cost encoder – represents a minor but important shift in thinking, one that could account for the persistence signal missing from the current model. Strong ACC signals could drive task persistence; however, as the ACC output signal wavers (“utility decreasing”), the phenomenology of effort begins, leading to reductions in persistence. Hence, the subjective experience of effort is, we propose, neither the result of the initial ACC recruitment nor the result of low levels of ACC activity, but rather, it results from a decrement in ACC activity from some prior, higher level. When tasks require cost/benefit computations, the ACC is recruited but subjective effort is usually not immediately reported. As a course of action is pursued, the expectation of high or imminent reward – which behaviorally serves to promote motivation and task engagement – also keeps ACC activity high (Croxson et al. Reference Croxson, Walton, O'Reilly, Behrens and Rushworth2009; Shidara & Richmond Reference Shidara and Richmond2002). However, if rewards are not anticipated (e.g., in randomized tasks or repetitive practice conditions), then ACC activity decreases (Raichle et al. Reference Raichle, Fiez, Videen, MacLeod, Pardo, Fox and Petersen1994; Shidara & Richmond Reference Shidara and Richmond2002), cingulate motor area activity increases (Shima & Tanji Reference Shima and Tanji1998) and task switching is likely to occur in response to the subjective effort experienced.

In our adaptation of the model, maintaining ACC activity at a high level is the key to persisting. We know that extrinsic incentives help ameliorate decrements in ACC activity over time; however, frequent tangible rewards are not always present in day-to-day life. What other mechanisms might work to maintain heightened ACC activity? We propose that experience also modifies ACC functionality over time such that in future tasks, higher ACC activity (and subsequent lateral PFC engagement) can be sustained in the absence of immediate rewards. This might be due to one of the mechanisms mentioned earlier – for example, reduced discounting of potential rewards available through the primary task or increased discounting of rewards associated with alternative actions – and ties into the learning process highlighted in the target article (sect. 3.3, para. 9). As one example of how this might occur, recent work has shown that individuals who are better able to visualize the future rewarded outcome of a choice tend to pursue long-term, over short-term, goals (Berns et al. Reference Berns, Laibson and Loewenstein2007; Boyer Reference Boyer2008). Thus, the discounting of reward value that normally occurs for temporally distant rewards is attenuated. Interestingly, scanning data indicate that that when this effect occurs, the hippocampus and PFC, including the ACC, co-activate in a coordinated manner (Benoit et al. Reference Benoit, Gilbert and Burgess2011; Peters & Buchel Reference Peters and Büchel2010). The exact nature of any communication that occurs between these regions is unclear, but it is possible that “future thinking” mediated through the hippocampus has a critical role. Future studies will help determine exactly how such systems enable us to persist in the face of subjective effort, further clarifying the neurophysiological framework that underlies the opportunity cost model.