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Review
. 2016 Jan 5;371(1685):20150180.
doi: 10.1098/rstb.2015.0180.

Neuronal Factors Determining High Intelligence

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Free PMC article
Review

Neuronal Factors Determining High Intelligence

Ursula Dicke et al. Philos Trans R Soc Lond B Biol Sci. .
Free PMC article

Abstract

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.

Keywords: brain size; cortex size; high intelligence; information processing capacity; mammals; number of cortical neurons.

Figures

Figure 1.
Figure 1.
The relationship between brain weight (ordinate, grams) and body weight (abscissa, kilograms) in 200 vertebrate taxa in double-logarithmic presentation. Light grey circles: bony fishes; light grey triangles: reptiles; grey triangles: birds; dark grey circles: mammals except primates; grey squares: primates; and encircled grey squares: Homo sapiens. Adapted from [1]; modified after [15].
Figure 2.
Figure 2.
The relationship between brain weight and body weight in mammals in double-logarithmic presentation. Some species of shrews, mouse, dog, horse and African elephant have ‘average’ brain weights; accordingly their data points lie exactly on the regression line. Chimpanzees, humans, but also dolphins have brain weights above average; some bat species, hedgehog, pig, hippopotamus, blue whale and sperm whale have brain weights below average. Adapted from [11]; modified after [11].

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