March 18, 2002 What separates us from other primates? Many researchers believe that it's our brain's large frontal cortex that makes us "King of the Jungle." The frontal cortex is involved with reasoning, planning, abstract thought and other complex cognitive functions in addition to motor function. However, a new study using magnetic resonance imaging (MRI) of different primate brains tells a different story: our frontal cortex is NOT proportionately larger than that of the great apes. Scientists from the Department of Anthropology at the University of California in San Diego and the Department of Neurology at the University of Iowa compared the cortical volume of the frontal lobe in: Humans Great Apes (Chimpanzees, Orangutans, Gorillas, Bonobos) Lesser Apes (Gibbons) Monkeys (Rhesus, Cebus) The scientists found that humans had the largest total frontal cortex volume in any of the primates (volume range: 238.8 cm3 to 329.8 cm3). Orangutans and chimpanzees had the next largest volume of frontal cortex, followed by the gibbons and monkeys. However, when the proportion of frontal cortex to the entire cerebral cortex was compared, the values for humans (36.4%-39.3%) overlapped with those of the great apes (35%-38.7%). In other words, the proportion of frontal cortex to the rest of the cerebral cortex in humans and great apes is similar. Both humans and great apes have relatively larger frontal cortices compared to those of the lesser apes and monkeys. The researchers suggest several reasons why their results are different from previous experiments: They studied more brains from the great apes than any other published study. Previous experiments measured only one or two brains from a particular species. Earlier studies estimated the size of the frontal cortex by examining the surface of the brain. The new study used more accurate MRI brain imaging to calculate brain volume. Earlier studies examined "postmortem" (after death) brain specimens. It is possible that chemicals used to preserve these specimens causes the brains to shrink in size. By using MRI methods, the researchers in the new study were able to study brain size in living animals!

Volume of the Frontal Cortex Relative Size of the Frontal Cortex

It's Not Just Size! So, it appears that there are no differences in the relative size of the frontal cortex in humans and great apes. Size of the frontal cortex, though, may NOT tell the whole story. The frontal cortex is a large area with many different functions. There are many questions about the frontal cortex that remain unanswered: 1. Are all parts of the frontal cortex the same in humans and the great apes? 2. Is the organization of frontal cortex the same at the cellular level? 3. Do humans and great apes have similarities or differences in the way the frontal cortex is connected to other brain areas? Moreover, the entire cerebral cortex (frontal, parietal, occipital, temporal lobes) of humans is much larger in proportion to body size than that of other primates. This increased connectivity and total cortex size may provide humans with added cognitive abilities.

References: Semendeferi, K., Lu, A., Schenker, N. and Damasio, H. Humans and great apes share a large frontal cortex. Nature Neuroscience, 5:272-276, 2002. Mammalian Brain Collection - The brain images on this Neuroscience for Kids page have been adapted from those at the Mammalian Brain Collection.

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