The structure of consciousness

This is the title of an essay by Buzsaki in the March 15 issue of Nature. It has the following summary of three basic architectural schemes that are present in mammalian brains:

The simplest uses strictly local wiring. In this kind of circuit, typified by the cerebellum, a few neuronal types form individual 'modules' that may be repeated as necessary. Because interaction between modules is restricted to neighbours, it is massively parallel in nature. In different species, the size of locally organized brain structures — including the basal ganglia, thalamus and cerebellum — roughly scales with the number of modules they contain.

An entirely different type of network uses random connections, with a more or less equal probability of connecting local, intermediate or distant neurons. A unique example of such a random connectionist scheme is the recurrent excitatory circuit of the hippocampal CA3 region.

The third architectural scheme, exemplified by the neocortex, combines local modularity with more random, long-range connectivity. This complex wiring scheme shares many properties with 'small-world' or 'scale-free' networks. The advantage of this arrangement is that the number of intermediate steps between any two neurons — the synaptic path length — can remain relatively constant when network size is scaled up, because even a small fraction of long-range connections can dramatically reduce the average path length. Although intermediate and long-range interconnections demand resources and space, they are critical for globally distributing the results of local computations throughout the entire cerebral cortex.

He suggests a view of subjective consciousness (that has been proposed also by several other researchers):

.... that the local–global wiring of the cerebral cortex and the perpetual, self-organized complex dynamics it supports are necessary ingredients for subjective experiences. Environmental inputs can be seen as perturbations of the ongoing spontaneous activity. If they manage to perturb ongoing activity for a sufficiently long time in a big enough population of neurons, their effect will be noticed; that is, we will become conscious of them. In contrast, the locally organized cerebellar cortex, used largely for sensorimotor integration, does not give rise to self-generated or spontaneous activity, and its response to input remains local and non-persistent. Importantly, we generate no subjective record of such local computations.

The complete essay can be downloaded HERE.