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The Evolution of the Human Forebrain

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Published on Jan 3, 2011

Announcing the introduction of the first "Periodic Table for the Human Forebrain," which enjoys similar advantages to the dramatic influence that the Periodic Table of the Elements has enjoyed with respect to Chemistry and Physics. This respective neural counterpart imparts a crucial sense of systematic order and purpose to the fragmented state of affairs currently prevailing within the neurosciences.
The cerebral cortex represents the most logical initiation point for such an innovation, celebrated as the crowning culmination of human forebrain evolution. This radical expansion of the neocortex is observed to occur in a discrete pattern suggestively termed cortical growth rings. The general pattern of neural evolution specifies that older structures are periodically modified to create newer functional areas, with the precursor circuitry also preserved, all persisting side by side. The stepwise repetition of these processes over the course of mammalian evolution ultimately accounts for the six sequential age levels of cortical evolution, schematically depicted. This representation depicts the entire surface of the cortex folded flat so that the medial and sub-temporal surfaces are fully exposed.

Returning to the dual parameter grid, the two fundamental variables defining forebrain evolution are the parameters of phylogenetic age and input specificity. The precise number of elementary levels has accurately been determined for both basic forebrain parameters. Sanides (1972) proposed that the human cortex evolved as a sequence of five concentric growth rings comprising a medio-lateral hemisphere gradient. Furthermore, the interoceptive, exteroceptive and proprioceptive input categories each project to their own four-part complex of cortical bands that (when taken collectively) define an antero-posterior hemisphere gradient.

The evolutionary gradient extending from the archaecortex via the cingulate gyrus is essentially the top half of the dual parameter grid. Here, when the para-coronal variable of phylogenetic age is plotted as the ordinate and the para-sagittal parameter of input specificity charted as the abscissa in a Cartesian coordinate system, the resulting dual parameter grid depicted is spatially oriented in a pattern analogous to the standard cortical representation.

The evolutionary gradient extending from the paleocortex via the insular lobe is the bottom half of the dual parameter grid. This version similarly shows that the human cortical parcellation schemes of Brodmann (1909) and von Economo (1929) correlate topographically on essentially a one-to-one basis with the dual parameter grid.

Each cortical area described by Brodmann and von Economo corresponds to schematically unique age/input parameter coordinates. Furthermore, each affiliated thalamic nucleus of specific age and input coordinates projects principally to that cortical area comprising identical pair-coordinate values, implying that the evolution of both the dorsal thalamus and the cortex are similarly defined in terms of the specifics for the dual parameter grid. More details posted at forebrain.org

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