Chimpanzee brains are asymmetrical in key areas and their handedness reflects it

The hippocampus skews ’right,’ especially in males, plus the righty-lefty distinction may go back 5 million years

New MRI-based studies present more evidence that the brains of chimpanzees are human-like in terms of the relationships among brain asymmetry, handedness and language, according to research undertaken at the Yerkes National Primate Research Center in Atlanta. Understanding our evolutionary cousins helps us to understand what makes us human. Two related reports appear in the December issue of Behavioral Neuroscience, which is published by the American Psychological Association (APA).

In the first study, Hani Freeman, BA, Claudio Catalupo, PhD (also with Georgia State University), and William Hopkins, PhD (also with Berry College), took magnetic resonance images of 60 chimpanzees to measure the anatomy of two key structures in their brains’ limbic systems, an early-evolving central region that includes the hippocampus and amygdala. In the MRI pictures, the hippocampus (which regulates learning and consolidation of spatial memory, mood, appetite and sleep) was asymmetrical, its right half significantly larger than its left. This asymmetry was bigger in males. These findings are consistent with studies of human hippocampi, which are also asymmetrical. At the same time, just as in humans, the amygdalas of the chimps were symmetrical.

Studies such as this confirm that human and chimp brains are not only asymmetrical, but asymmetrical in the same way. The findings echo previous looks at the non-limbic parts of chimpanzee brains, which also appear human-like in their patterns of asymmetry. This fact, especially if studied in the context of functional behaviors that reflect asymmetries, may help scientists get a better fix on the evolution of the limbic system in all primates, including humans.

Says Hopkins, “The limbic system asymmetries advance the position that asymmetries are fundamental aspects of the nervous system of all primates, and apply to more primitive systems in the brain.” The asymmetries influence behavior. Given the new findings about chimps and previous findings that the limbic system affects human facial expression and emotions, it now seems more clear why across primates, says Hopkins, the left half of the face – controlled by the right side of the brain — is more emotionally expressive. In addition, a right-dominant hippocampus would explain apes’ well-developed spatial memory. Again, that parallels how the right hippocampus in humans is involved in spatial memory.

In a second study, Hopkins and Cantalupo report the first-ever evidence of an association between hand preference and asymmetries in three areas of the brain cortex in chimps. Observing 66 chimps, they correlated asymmetries in brain anatomy with three measures of handedness: Simple reaching (which hand chimps used to pick up a raisin thrown into the cage), two-handed feeding (which hand chimps used to feed themselves chunks of fruit while holding the whole piece, such as a banana, in the other hand), and a measure of coordinated bimanual actions (which hand chimps used to fish peanut butter from a plastic tube with a finger).

Left-handed and right-handed chimps differed relative to the asymmetries in two primary motor areas, the planum temporale and the precentral gyrus. Say the authors, the results “challenge the long-held belief that the neurobiological substrates for handedness are unique to humans.” Just as in humans, neuroanatomy governs whether a chimp becomes a lefty or a righty. Hopkins points out that chimps are also strongly right-handed for manual gestures and throwing, a clue to the origins of more general right-hand dominance in both chimps and humans.

This second study also confirms that handedness goes way back. Its findings, say the authors, “suggest that the neurobiological basis for handedness evolved as early as five million years ago and emerged independent of systems associated with language and speech.”
The findings mesh with other recent human evidence that handedness has nothing to do with asymmetry in language-related cortical areas. The Yerkes chimps showed no links between handedness and the classic left-side “language” areas. Hopkins says, “Many studies document a correlation between handedness and lateralization, but these are only correlations. It doesn’t mean that being right-handed causes a person to be left-hemisphere dominant for language, or vice versa. Rather, these two abilities might be assigned to the same sides but independently of each other.” The findings about chimps support the hypothesis that a third, undiscovered “sidedness” factor may account for both handedness and language-related dominance.

To further understand language functions relative to brain asymmetries, the Yerkes team will soon begin using another brain-imaging technology, PET scans (positron emission tomography), to evaluate brain regions that are active when chimpanzees manually gesture and/or vocally communicate.