Summary
The prefrontal cortex (PFC) has long been thought to serve as an ‘executive’ that controls the selection of actions, and cognitive functions more generally. However, the mechanistic basis of this executive function has not been clearly specified, often amounting to a homunculus. This chapter reviews recent attempts to deconstruct this homunculus by elucidating the precise computational and neural mechanisms underlying the executive functions of the PFC. The overall approach builds upon existing mechanistic models of the basal ganglia (BG) and frontal systems known to play a critical role in motor control and action selection, where the BG provide a ‘Go’ versus ‘NoGo’ modulation of frontal action representations. In our model, the BG modulate working memory representations in prefrontal areas to support more abstract executive functions. We have developed a computational model of this system that is capable of developing human-like performance on working memory and executive control tasks through trial-and-error learning. This learning is based on reinforcement learning mechanisms associated with the midbrain dopaminergic system and its activation via the BG and amygdala. Finally, we briefly describe various empirical tests of this framework.
Introduction
There is widespread agreement that some regions of the brain play a larger role in controlling our overall behaviour than others, with a strong consensus that the prefrontal cortex (PFC) is a ‘central executive’ (e.g., Baddeley, 1986; Christoff et al., 2009; Conway et al., 2005; Duncan, 2001; Koechlin and Summerfield, 2007; Miller and Cohen, 2001; Shallice, 1988). However, this central executive label raises many more questions than it answers. How does the PFC know what actions or plans to select? How does experience influence the PFC? How do the specific neural properties of the PFC enable this kind of function, and how do these differ from those in other, non-executive areas? Without answers to these kinds of questions, the notion of a central executive is tantamount to positing a homunculus (small man) living inside the PFC and controlling our actions.