Neuronal factors determining high intelligence

Abstract

One contribution of 16 to a discussion meeting issue 'Homology and convergence in nervous system evolution'. Many attempts have been made to correlate degrees of both animal and human intelligence with brain properties. With respect to mammals, a much-discussed trait concerns absolute and relative brain size, either uncorrected or corrected for body size. However, the correlation of both with degrees of intelligence yields large inconsistencies, because although they are regarded as the most intelligent mammals, monkeys and apes, including humans, have neither the absolutely nor the relatively largest brains. The best fit between brain traits and degrees of intelligence among mammals is reached by a combination of the number of cortical neurons, neuron packing density, interneuronal distance and axonal conduction velocity-factors that determine general information processing capacity (IPC), as reflected by general intelligence. The highest IPC is found in humans, followed by the great apes, Old World and New World monkeys. The IPC of cetaceans and elephants is much lower because of a thin cortex, low neuron packing density and low axonal conduction velocity. By contrast, corvid and psittacid birds have very small and densely packed pallial neurons and relatively many neurons, which, despite very small brain volumes, might explain their high intelligence. The evolution of a syntactical and grammatical language in humans most probably has served as an additional intelligence amplifier, which may have happened in songbirds and psittacids in a convergent manner.

Introduction to ‘Homology and convergence in nervous system evolution’

“…An altogether different level of behavioural analysis tackles questions about neurological constituents that contribute to the evolution of intelligence [37]. The authors of this contribution emphasize that it is not simply brain size that should be taken into consideration; indeed, such consideration can be misleading.…”

Human high intelligence is involved in spectral redshift of biophotonic activities in the brain

“…Early studies have proposed that brain size and the degree of encephalization [encephalization quotient (EQ)] might be related to the evolution of animal intelligence, including that of human beings (2-4), but, so far, the relationship between relative brain size and intelligence is inconclusive, and EQ is also not the best predictor of intelligence (1,(5)(6)(7). Communications and information-processing capacity between neurons in neural circuits play an important role in the realization of various neural functions, such as sensorimotor control, learning and memory, consciousness, and cognition.…”

Is Cetacean Intelligence Special? New Perspectives on the Debate

“…For example, cetacean's modest use of tools when compared to those of primates. The fact that a greater proportion of cetacean's cerebral cortex when compared to great apes and other mammals of similar brain sizes is occupied by white matter rather than by gray matter (where neuronal computation occurs), and particularly an unfavourable combination of high interneuronal distance plus low axonal conduction velocity has lead to the scientific judgement that cetacean's brains have less computational power [2,98,99]. Further it has lead to the belief that there is not a single behavioral achievement that has not also been shown in several other species [18,20].…”

“…However, concerning the assumption that cetacean's brains have less computational power, the study by Dicke and Roth [99] identifies the principal factors determining general information processing capacity in the brain, and thus intelligence among mammals, as a combination of the number of cortical neurons, the number of cortical synapses, and processing speed. Addressing these in turn, firstly, it is important to note that processing speed provides a necessary but not sufficient condition for intelligence [100,101].…”