Cortical Neuron Density, Synapse-to-Neuron Ratio

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Human Uniqueness Compared to "Great Apes": 
Relative Difference
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Relative brain size is often considered as the primary indicator of changes in intelligence between species, and this consideration is extrapolated to human ancestor fossil remains. In addition to changes in brain size, there are many large scale differences in the human brain relative to the great apes, including cortical neuron density and the related neuron/synapse ratio. 

The grey-level index (GLI) is a measurement of the amount of space in a given area of cortex occupied by cell bodies (grey matter). This measure may also give an indication of connectivity since a lower GLI allows more room for connective fibers (white matter). In humans, the GLI in prefrontal cortex is significantly lower than other primates (chimanzees, bonobos, gorillas, orangutans, macaques, and gibbons), which means there is a lower neuron density. In addition, the human GLI across layers II-VI is unimodal, with only small changes between layers. Most of the primates measured were similar, but the gorilla (with the closest GLI value to humans) prefrontal cortex had many peaks and troughs from layers II-VI.

As expected from the GLI values, the density of neurons in the human PFC is almost half that of the chimpanzee (34,014/mm3 compared to 60,468/mm3). Again, gorillas were the most similar to human neuron density at 47,300/mm3, but this similarity is not as meaningful since the cyto-architecture is so different between the two groups.

Possible Selection Processes Responsible for the Difference: 

The prefrontal cortex receives connections from most other regions of the brain in humans. This increased connectivity could have favored a reduction in neuron density.

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  1. Prefrontal cortex in humans and apes: a comparative study of area 10., Semendeferi, K, Armstrong E, Schleicher A, Zilles K, and Van Hoesen G W. , Am J Phys Anthropol, 2001 Mar, Volume 114, Issue 3, p.224-41, (2001)