Review
Cognitive control, hierarchy, and the rostro–caudal organization of the frontal lobes

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Cognitive control supports flexible behavior by selecting actions that are consistent with our goals and appropriate for our environment. The prefrontal cortex (PFC) has an established role in cognitive control, and research on the functional organization of PFC promises to contribute to our understanding of the architecture of control. A recently popular hypothesis is that the rostro–caudal axis of PFC supports a control hierarchy whereby posterior-to-anterior PFC mediates progressively abstract, higher-order control. This review discusses evidence for a rostro–caudal gradient of function in PFC and the theories proposed to account for these results, including domain generality in working memory, relational complexity, the temporal organization of behavior and abstract representational hierarchy. Distinctions among these frameworks are considered as a basis for future research.

Introduction

Cognitive control permits selection of actions that are consistent with our goals and context. The prefrontal cortex (PFC) is a central component in the network of brain regions supporting cognitive control 1, 2, 3, 4, 5, 6, 7, 8, 9. Thus, a fruitful approach to understanding the architecture of control has been to investigate the functional organization of the PFC. In recent years, functionally selective PFC sub-regions have been associated with distinct forms of control 10, 11, 12, 13, 14, 15. However, it remains an important goal to understand these isolated control functions in context of broader functional and neuroanatomical organizing principles 2, 13, 16. This review considers one such organizing hypothesis: that the rostro–caudal axis of the frontal lobes is organized hierarchically, whereby posterior frontal regions support control involving temporally proximate, concrete action representations, and the anterior PFC supports control involving temporally extended, abstract representations 5, 16, 17, 18, 19, 20, 21, 22, 23, 24 (Figure 1). Of course, there are diverse ways of defining ‘abstraction’ and, likewise, many processing schemes by which these levels might interact, including non-hierarchical ones. Here, the evidence and associated theories of a frontal rostro–caudal gradient of function are reviewed.

Section snippets

Hierarchy in action and the action system

Hierarchical structures occur throughout artificial and natural systems. The defining properties of hierarchy depend on whether one is emphasizing processing or representation. Processing hierarchies require that superordinate levels, operating over longer time scales, asymmetrically modulate subordinate processing 25, 26. Representational hierarchies require that superordinate representations form abstractions over subordinate representations, favoring generality over detail and allowing

The rostro–caudal axis of PFC and working memory: domain generality

Working memory refers to active maintenance and manipulation of information over a brief interval in the service of a task. Initial evidence for rostro–caudal distinctions within the PFC comes from experiments designed to test regional differences based on working memory domain. In general, regional distinctions based on the content of maintained information, such as spatial or object, remain controversial 38, 39, 40. However, when content-based distinctions are evident, they are typically

The rostro–caudal axis of PFC and abstract thought: relational complexity

Christoff and colleagues 20, 56, 57 (also see Refs 2, 58, 59) have proposed a framework for the rostro–caudal gradient in the frontal lobe that is also based on the content of working memory representations. But, rather than emphasizing domain, abstraction derives from relational complexity [56] (Figure 2b). The number of variable dimensions that must be integrated to determine a response determines the order of relational complexity 60, 61.

First-order relational complexity, associated with

The rostro–caudal axis of PFC and cognitive control: representational hierarchy

Cognitive-control theories propose that the PFC maintains contextual information to bias selection of relevant representations over competitors 6, 66, 67, 68, 69 (Box 1). Within a uniform computational control structure, there is opportunity for hierarchy among both the representations that compete for processing and the representations that are maintained to provide a control signal.

Distinguishing theories of rostro-caudal function

Progressively anterior regions of the frontal cortex support increasingly complex, abstract representations and processes. On this point, the frameworks reviewed here agree. However, there are several points of divergence among the frameworks that are theoretically significant for our understanding of frontal-lobe function and cognitive control.

Concluding remarks

The fact that behavior can be organized hierarchically does not require that the system itself be structured hierarchically. Nevertheless, growing evidence supports spatially distinct regions of the frontal lobe that process differentially abstract components of action selection. Considerable controversy persists regarding the factors that distinguish this functional gradient and whether these processors interact hierarchically. Resolving these points of controversy will be fundamental to our

Acknowledgements

I thank M. D’Esposito, B. Buchsbaum and V. Van Veen for their valuable comments and discussions while writing this review. My work is supported by a National Research Service Award (F32 NS053337).

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