The Modular Organization of the Cerebral Cortex: Evolutionary Significance and Possible Links to Neurodevelopmental Conditions

One of my special interests in terms of neuroscience has been systems theory and the hierarchical organization of the brain. I have been specially interested in the emergence of higher cognitive functions and how fingerprints of the same can be gathered from our evolutionary past. I have published many articles/chapters dealing with this theme, the latest just appearing in the Journal of Comparative Neurology.  I hope the reader will enjoy the same.

The Modular Organization of the Cerebral Cortex: Evolutionary Significance and Possible Links to Neurodevelopmental Conditions. Casanova MF, Casanova EL. J Comp Neurol. 2018 Oct 10. doi: 10.1002/cne.24554. [Epub ahead of print] Review.  PMID:30303529

The article deals with the organization of the cerebral cortex and how changes in this organization during brain evolution may have provided a risk factor for neurodevelopmental conditions. The article will be freely available next year and I will provide links to ResearchGate at the proper moment. For those interested in the subject, I am providing a copy of the abstract and introduction in the following paragraphs.

ABSTRACT

The recognition of discernible anatomical regularities that appear to self-organize during development makes apparent the modular organization of the cerebral cortex.  The metabolic cost engendered in sustaining interneuronal communications has emphasized the viability of short connections among neighboring neurons.  This pattern of connectivity establishes a microcircuit which is repeated in parallel throughout the cerebral cortex.  This canonical circuit is contained within the smallest module of information processing of the cerebral cortex; one which Vernon Mountcastle called the minicolumn.  Plasticity within the brain is accounted, in part, by the presence of weak linkages that allow minicolumns to process information from a variety of sources and to quickly adapt to environmental exigencies without a need for genetic change.  Recent research suggests that interlaminar correlated firing between minicolumns during the decision phase of target selection provides for the emergence of some executive functions.  Bottlenecks of information processing within this modular minicolumnar organization may account for a variety of mental disorders observed in neurodevelopmental conditions.

INTRODUCTION

To some extent our understanding of the brain has been dependent on our ability to establish representational maps that communicate spatial information accurately and reproducibly.  Early topographic localization schemes for the cerebral cortex were bottom-up approaches that clustered together microscopic units into larger regions based on characteristics of their somatic morphology, pigment distribution, pathological susceptibility, or myelin architecture. The successful use of defined anatomical characteristics and consequent uniformity of the different parcellation schemes across brains of a given species suggests the presence of discrete scalable modules (Diez et al., 2015; Fischi and Sereno, 2018).  These units or modules lessen the evolutionary cost entailed in the creation of customized regions responsive to ever-changing environmental exigencies. In the end, the presence of homologous modules across different species attests to the evolutionary preference for standardization rather than customization, all at the expense of performance (Kuratani, 2009).

The Nobel laureate Francois Jacob once argued that evolution had “tinkered” with developmental biology. Evolution’s desultory approach explained why, despite the influence of natural selection, there were many homologies, as well as imperfections, across organisms (Jacob, 1977).  Indeed, growth and differentiation across different organisms appears constrained by similar genes within the evo-devo gene toolkit.  From this perspective, the brain is an agglomeration of structural improvisations piled one on top of another through millions of years of evolution with each layer providing for added complexity and a potential bottleneck for information processing (textbox 1) (Linden, 2008; Blazek et al., 2011).  Simply stated, “It must be borne in mind that evolution is a tendentious, almost bureaucratic process. Once something novel has been invented, it’s generally retained and not thrown away. The new is built upon the old, but does not replace it” (Freitas, 2008).  Some complex functions and behaviors are therefore novel outcomes of a system not originally intended for them. (Casanova and Tillquist, 2008). The imperfect disposition[1] of brain parts is only counterbalanced by the presence and versatility of modules whose weak linkages provide for adaptive phenotypic variations (Kirschner an Gerhart, 2006).

Weak linkages explain how different parts of a system are coupled so that changes in one of its modules or compartments do not seriously affect other parts of the system. In addition, weak linkages promote the combination of modules depending on prevailing environmental exigencies. The benefits entailed by having modules with weak linkages, in the midst of an otherwise inefficient system, would make them a conserved property.

The brain according to Kitano (2007), is a modular, weakly linked system that exhibits a clear tradeoff between robustness and fragility. It is therefore unsurprising that Brodal in his classic textbook “Neurological Anatomy in Relation to Clinical Medicine” made the observation that the nervous system is composed of a multitude of minor units, each with its particular structural organization, specific with respect to its finer intrinsic organization as well as with its connections with other units (Brodal, 1981). According to Szentàgothai, the basic reason for looking at the organization of the brain in terms of modules is that it offers a framework for the functional interpretation of structural data (Szentàgothai, 1975).

Footnote:

[1] Throughout evolutionary history more recent levels of anatomical complexity have not resulted from a redesign of the whole brain. This has provided for the preservation of brain regions that are anatomical and functional remnants of our evolutionary past. As an example, the midbrain visual center automatically provides information about objects in our surrounding without the same reaching conscious (cortical) awareness (i.e., blindsight abilities).  Higher layers of anatomical complexity have thus been added to earlier layers without either establishing connections between them nor integrating their functions (Linden, 2007).

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