The network of brain regions we associate here with semantic processing has also been linked with more specific functions. Nearly all parts of the network have been implicated in aspects of social cognition, including theory-of-mind (processing of knowledge pertaining to mental states of other people), emotion processing, and knowledge of social concepts [36
]. Much of the network has been implicated in retrieval of episodic and particularly autobiographical memories [68
], leading to the hypothesis that these regions function to retrieve event memories through a process of `scene construction' [70
]. The same scene construction processes have been proposed as the basis for `prospection,' i.e., imagining future scenarios for the purpose of planning and goal attainment [71
]. Finally, the association of these regions with autobiographical, `self-projection,' and self-referential processes has led to suggestions that they are specifically involved in processing self knowledge [73
]. Several recent reviews and meta-analyses attest to the high degree of neuroanatomical overlap between the networks supporting these purportedly distinct processes [67
Given this overlap, it is logical to ask whether there is a process common to all of these cognitive functions. A model based on self-referential processing cannot easily explain activation of the same regions by theory-of-mind tasks, which by definition emphasize knowledge pertaining to others. The general process of mental scene construction is common to episodic memory retrieval, prospection, and many theory-of-mind tasks, but this model cannot explain the consistent activation of these regions by single word comprehension tasks, as shown above in . Indeed, the contrasts analyzed in focused on general semantic knowledge (especially knowledge about object concepts) and did not emphasize episodic, autobiographical, social, emotional, self, or any other specific knowledge domain.
One process shared across semantic, social cognition, episodic memory, scene construction, and self-knowledge tasks is the retrieval of conceptual knowledge. The scene construction posited to underlie episodic memory retrieval and prospection refers to a partial, internal simulation of prior experience. But the construction of a scene requires content. The content of such a simulation is conceptual knowledge about particular entities, events, and relationships. The variety of this content is impressive, encompassing object, action, social, self, spatial, and other domains, yet these types of content all share a common basis in sensory-motor experience, learning through generalization across individual exemplars, and progressive abstraction from perceptual detail. We propose that the essential function of the high-level convergence zone network is to store and retrieve this conceptual content, which is employed over a variety of domain-specific tasks.
This network of high-level convergence zones also overlaps extensively with the `default mode network' of regions that show higher levels of activity during passive and `resting' states than during attentional tasks [47
]. The similarity between all of these networks lends strong support to proposals that `resting' is a cognitively complex condition characterized by episodic and autobiographical memory retrieval, manipulation of semantic and social knowledge, creativity, problem solving, prospection, and planning [75
]. Several authors have emphasized the profound adaptive value of these processes, which not only enable the attainment of personal goals but are also responsible for all of human cultural and technological development [78