Sequential Control Underlies Robust Ramping Dynamics in the Rostrolateral Prefrontal Cortex.

An essential human skill is our capacity to monitor and execute a sequence of tasks in the service of an overarching goal. Such a sequence can be as mundane as making a cup of coffee or as complex as flying a fighter plane. Previously, we showed that, during sequential control, the rostrolateral prefrontal cortex (RLPFC) exhibits activation that ramps steadily through the sequence and is necessary for sequential task execution using fMRI in humans (Desrochers et al., 2015). It remains unknown what computations may underlie this ramping dynamic. Across two independent fMRI experiments, we manipulated three features that were unique to the sequential control task to determine whether and how they modulated ramping activity in the RLPFC: (1) sequence position uncertainty, (2) sequential monitoring without external position cues (i.e., from memory), and (3) sequential monitoring without multilevel decision making (i.e., task execution). We replicated the ramping activation in RLPFC and found it to be remarkably robust regardless of the level of task abstraction or engagement of memory functions. Therefore, these results both replicate and extend previous findings regarding the function of the RLPFC. They suggest that sequential control processes are integral to the dynamics of RLPFC activity. Advancing knowledge of the neural bases of sequential control is crucial for our understanding of the sequential processes that are necessary for daily living.SIGNIFICANCE STATEMENT We perform sequences of tasks every day, but little is known about how they are controlled in the brain. Previously we found that ramping activity in the rostrolateral prefrontal cortex (RLPFC) was necessary to perform a sequence of tasks. We designed two independent fMRI experiments in human participants to determine which features of the previous sequential task potentially engaged ramping in the RLPFC. We found that any demand to monitor a sequence of state transitions consistently elicited ramping in the RLPFC, regardless of the level of the decisions made at each step in the sequence or engagement of memory functions. These results provide a framework for understanding RLPFC function during sequential control, and consequently, daily life.