Human cerebral cortex consists of approximately 10^10 neurons that are organized into a complex network of local circuits and long-range fiber pathways. This complex network forms the basis of distributed interactions among specialized brain systems – for example visual processing in the posterior (back) of the brain, and language processing in the temporal (side) and frontal lobes of the brain. This brain circuitry forms a complex connection network. Brain scientists have been interested in determining both the structural and functional core of the human brain.
- The structural core of the brain exists where there are most connections and where the connection strengths are strongest.
- The functional core are brain areas that are more highly active when at rest than when doing cognitive tasks, and has a central role in integrating information across functionally segregated brain regions.
In a study published in 2008 in PLoS Biology [link], Martijn van den Heuvel, a neuroscientist at Utrecht University Medical Center, and his team used a cutting edge brain imaging technique that allows high resolution mapping of fiber pathways. From this they could build up connection maps covering the entire cortical surface. Computational analysis of this network revealed regions of cortex that are highly connected and highly central, forming a structural core of the human brain. ‘Core hubs’ in the network include the precuneus (medial posterior parietal cortex), and the posterior cingulate cortex. ‘Provincial hubs’ are members of the frontal (e.g., medioorbitofrontal cortex), temporoparietal (e.g., bank of the superior temporal sulcus, superior temporal cortex) or occipital modules (e.g., pericalcarine cortex). This network is shown in the diagram below.
Precuneus and cingulate cortex
In the same study, Martijn van den Heuvel and his team also found that the core precuneus and posterior cingulate cortex brain areas were more active than other brain areas in an alert, resting state. The functional core revealed in their brain scans shows highly active precuneus and cingulate cortex activity.
They propose that the structural core of the brain may have a central role in integrating information across functionally segregated brain regions.
The precuneus and conscious awareness
More highly developed (i.e. a larger proportion of the brain volume) in human beings than in non-human primates or other animals.
- Most complex columnar cortical organization of all cortical areas.
- Among the last regions to myelinate in ontogeny.
- Highest resting metabolic rate among brain areas – consuming 35% more glucose than any other area of the cerebral cortex in humans, and other species.
Brain imaging studies have shown that the precuneus (often in association with the pre-frontal cortex) has heightened activity for:
- retrieval / consolidation of autobiographical memory
- explicit processing of information (mental images / spontaneous thoughts)
- manipulation of this information for problem-solving and planning
- enhanced consciousness state of yoga meditation
- high levels of reflective self-awareness
- when comparing subliminal vs supraliminal word presentation there is precuneus activation only when conscious awareness of stimuli
- The precuneus is significantly less active than the rest of the brain during both SWS and REM sleep.
- During hypnosis there is decreased activation in the precuneus.
- During general anaesthetic infusion > decrease in precuneus associated with slipping into unconsciousness.
- The precuneus is one of the first regions of the brain to resume activity when regaining consciousness from a vegetative state.
Summing up, the precuneus is part of a recently evolved neural network subserving self-awareness and conscious experience. In association with prefrontal cortices, this system underlies not just conscious awareness but awareness of agency – in action and in thought. It is involved in representing the first person perspective (the viewpoint of the observing self) and interpreting an action as being controlled by oneself versus another person (third person perspective). [link] The precuneus is also part of the structural and functional core of the brain, integrating information across functionally segregated brain regions – a function that has been ascribed to consciousness in ‘global workspace’ theories of consciousness.