The Evolution of the Human Forebrain occurs through the processes of Inputs via Environmental Factors and Phylogenetic Age. Here, The cytoarchitectonic subdivisions of both the thalamus and the neocortex are topographically defined in terms of the variables of phylogenetic age and input specificity. The cortical and thalamic parcellations of Brodmann, von Economo and Hassler are each quantitatively correlated to a specific Cartesian coordinate value designating discrete levels for both age and input basic parameters. The variable of phylogenetic age is represented in the cortex by the five circumferential growth rings demonstrated by Sanides, plus an additional growth ring detected intermediate to the fifth and sixth age levels and designated as “prekoniocortex.” The paleocortex and the archaecortex are the two primordial neocortical precursors that form the mammalian neocortex. In contrast to the arrangement in the planar cortex, six phylogenetically distinct “growth shells” are detected in the three-dimensional thalamus and are designated after the corresponding schematic levels of Rolf Hassler’s paradigm of hexapartition of unit-thalamic inputs. The subthalamus and the epithalamus analogously represent the primordial diencephalic precursors of the mammalian dorsal thalamus, Both the neocortex and the dorsal thalamus evolved in response to the necessity for a more comprehensive blending of environmental inputs from differing neuraxial levels. Unlike the age variable, the parameter of input specificity is most readily apparent in the dorsal thalamus; which is the site of termination for each major forebrain input. Accordingly, the fourteen individual units of the parameter of input specificity are designated after each of the specific environmental input classifications projecting discretely to circumscribed thalamic sectors, An identical complement of input parameter levels also occurs in the cortex by way of thalamic relay across the internal capsule. Furthermore, each thalamic nucleus of specific parameter coordinates directs its main projection to cells of the cortex displaying identical coordinate values, establishing forebrain interconnectivity as an additional function of the dual parameter paradigm.
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