One theoretical model of the prefrontal cortex posits that we can achieve goal-directed behavior via "biased competition" - that is, representations of our current goals and context are maintained in the prefrontal cortex and exert an influence on downstream areas, ultimately biasing our behavior in a goal-directed and context-appropriate way. By theory, this relatively simple function is enhanced by the anterior cingulate, functioning as a "conflict monitor" and upregulating the PFC when this so-called biased competition is a little too competitive, and not quite biased enough. However, new anatomical data reported in a recent issue of Neuron reveals some complexities to this simple theoretical story.
Medalla & Barbas report that in addition to excitatory projections from the anterior cingulate to the dorsolateral prefrontal cortex (which perhaps accomplish the "upregulation" of biased competition in the presence of conflict, as outlined above), the anterior cingulate is distinct from other areas projecting to the dorsolateral prefrontal cortex in the higher number of synapses onto inhibitory neurons. In other words, the anterior cingulate can not only upregulate, but also downregulate prefrontal activity. Instead, a different area of the prefrontal cortex (area 46) might be in a better position to upregulate activity, based on its anatomical connectivity to calretinin neurons - ultimately yielding a disinhibitory effect in the PFC.
This evidence touches on a larger debate in the cognitive neuroscience community surrounding the neural basis of cognitive control. Intuitively speaking, it seems reasonable that the brain might implement control over thought and behavior by suppressing unwanted representations. While neural network models indicate that the same function can be achieved by way of biased competition, the evidence reported by Medalla & Barbas suggests that biased competition may only be part of the story. Given this preferentially-inhibitory connectivity from the anterior cingulate to an area of the prefrontal cortex, one might expect that both biased competition and long-range inhibition play a role in cognitive control.