A morphogenetic quantitative approach to species-typical behaviour

The numerical ratio of the allocortical, late-myelinized infra-temporal sector (Flechsig) to the neocortical convexity of the temporal lobe in the human brain is proposed as a quantitative invariant of the human species, called "Neocorticalisation Index NI". Its morphogenetic nature derives from (1) the dual, that is the reptilian-palaeomammalian origin of the primate cortex (Kappers 1909, Vogt 1910. Kuhlenbeck 1924, Filimonoff 1947, Northcutt 1967, Stephan 1975). The two layers differentiate at independent rates forming a transitional belt, called "periallocortex" (limbic) and "mesocortex" (insular). They approach the most the general cyto- and myeloarchitectonic scheme of Brodmann (1909) and C. and O. Vogt (1919), and in the human brain are the latest to reach maturity (Flechsig 1920, Kahle 1969, Sanides 1975); (2) measurements in fossilized human brain endocasts (Taung, Rhodesian man) reported earlier (Irsigler 1984a and b); (3) clinico-pathological evidence gained from cases of the Pick-type of presenile dementia confirming the anthropogenetic character of the frontotemporal "basal forebrain" (Spatz and his school since 1937, Lüers 1947, Jakob 1979). The latter is considered to be in the mammals the major counterpart of the hormonally and reciprocally controlled reptilian forebrain (Schepers 1948, Sanides 1970, MacLean 1978). The results of NI measurements in 128 human brains are tabulated and interpreted in the light of species-typical behaviour. It turns out that the left hemisphere is the one possessing the greater share of allocorticity showing precocious development in the non-European (Negro) infant. There are gender-related differences of the NI parameters. The Indo-Europeans (Caucasians) undergo, in their ontogeny, a kind of "secondary" allocorticalisation in both hemispheres which, hypothetically, may be correlated with the "substantializing" (reifying) property inherent in their languages.