In their already-classic 2001 article, Miller & Cohen use a “train track” metaphor to illustrate the function of prefrontal cortex. The idea is that myriad learned associations interconnect sensory representations with motor commands (metaphorically, these are the “train tracks”). The important associations will change depending on the animal’s current task (these are the “switching stations” which allow crossover between the train tracks). Thus, the role of prefrontal cortex is to bias activity in the brain (the “train”) to undergo the sensory-motor transformations which are task- and goal-appropriate. Metaphorically speaking, the prefrontal cortex is the conductor manages the brain’s “switching yard.”
As the metaphorical “conductor,” prefrontal cortex has naturally been the focus of “higher-level” cognitive neuroscience. Adding to prefrontal cortex’s appeal, it’s also one of the brain regions most expanded in humans relative to the apes. But another brain region to have also undergone rapid evolutionary expansion (in the form of gyrification) – in fact, the only other area, by some estimates – is an area near the temporoparietal junction. Like prefrontal cortex, this area showed disproportionate increases in gyrification specific to homonids, relative to the increased gyrification that would be expected based on that occurring across earlier phases of mammalian evolution. This is also compatible with Broddmann’s early suggestions that the temporoparietal junction (and surrounding inferior parietal lobule) is unique to humans, based on his examination of cortical layer organization, cell type distributions and interconnectivity (reviewed here.) While some anatomists have subsequently disagreed, other work supports the idea that novel parietal regions exist in humans (for example this, which suggests something closer to the mid-intraparietal sulcus).
This anatomical data might be taken to suggest that a fronto-parietal circuit is really the substrate of homonid evolution.
This indicates the need for close examination of the role of parietal cortex in higher-level cognition. Consistent with this idea, metanalysis of the human cognitive neuroscience data sometimes show common activation only in parietal cortex across a variety of high-level cognitive tasks. For example, Wager & Smith‘s metanalysis revealed that Broddman’s area 7 was “active in all types of executive function.” Of course, this is a relatively large area, occupying all of the inferior parietal lobe.
While Miller & Cohen implied that the prefrontal “conductor” could bias activity in any number of posterior regions representing information that was important for the current task, the above results suggest that parietal areas may be a particularly important locus of this “cortical railyard.” Indeed, classic views of the parietal cortex focused almost exclusively on its role as “association cortex,” involved in transforming sensory & motor representations to produce a unified representation of space, as reviewed by Anderson, Snyder, Bradley & Xing. At the conclusion of this review, the authors argue that this archaic division of cortical areas into motor- and sensory-specific regions must be revamped in order to cast parietal cortex as responsible for “highly cognitive functions” related to “attention, intention, and decisions.”
The parietal lobe refers to some 30% of the cortical surface, so finer parcellation of these areas is important for any intelligent discussion of what parietal cortex does: how it interacts with frontal cortex, how sensory-motor transformations are implemented computationally, and what kinds of nonspatial processing these computations enable.
There’s a ton of research relevant to this topic. I’ll be covering more in subsequent posts, but here are some relevant oldies:
Neglected Facets of Unilateral Neglect
Binding in Parietal Cortex (also, When TMS Helps)
Two maps, One Territory: Parietal vs. Hippocampal Representations
Working Memory Arrays in Parietal Cortex
Symbols, Numbers, Tools and Attention: Does Parietal Cortex Do Everything?
Task-Switching: A Role for Inferior Parietal Cortex