Extended neocortical maturation time encompasses speciation, fatty acid and lateralization theories of the evolution of schizophrenia and creativity

Abstract

I suggest that the extended maturation time of some regions of the human neocortex is the uniquely human factor which allows the development of language, creativity and madness. The genetic event or events which contribute to the long delay to final maturation occurred at or after the speciation of Homo sapiens sapiens. Neocortical growth may follow the previously detailed "balloon model", which suggests that intra- and subcortical myelin production during development physically stretches each local area of the cortex tangentially to the pial surface, thereby causing neuronal columns to become more disjoint and more functionally independent, thereby increasing the functional capacity of the area [Seldon HL. Does brain white matter growth expand the cortex like a balloon? Hypothesis and consequences. Laterality 2005;10(1):81-95]. This occurs in addition to Hebbian synaptic modeling. Therefore, the size and functional capacity of each cytoarchitectonic area of each individual adult neocortex are the outcomes of partly deterministic (e.g., genetic) and partly statistical growth processes with numerous factors including environmental stimuli and fatty acid content in diets. The possible functional capacity and variation among growth outcomes increase with the length of time allowed to "finalize" synaptic weights, myelination and other plastic processes. For example, acquisition of quite differing linguistic skills becomes possible only in Homo sapiens because of the extended, decades-long plasticity of temporal lobe areas; in contrast, tactile skills vary little among human races and cultures, or even among higher primates, because of the faster maturation of the somatomotor areas. Some of the statistically extreme variations of the neocortical growth processes lead to abnormal cognition and behavior called "madness" or "genius". This maturation hypothesis overcomes some problems with those based purely on fatty acid metabolism or on functional asymmetry (non-human species show functional asymmetry, but no language. Neanderthals had brains comparable in size and shape to ours, but failed to develop language or creativity). This hypothesis implies that the search for genetic factors should include those which influence the temporal regulation of neuronal and myelin growth, but it also allows the development of unusual creativity or madness as a statistical extreme in the absence of any deterministic factors. It has implications about our attitudes toward mental "disorders" and about potential approaches to treating some of them - for example, attempting communication and conditioning via those senses and cortical areas which show less variation and are less affected.