How is the human brain different from other primate brains?
Let’s start by what is likely common knowledge: humans have a bigger brain- about three times bigger than apes. But as I’ve said before, bigger doesn’t necessarily equate with better. So what else is different in humans?
For starters, comparative studies between non-human primates and humans have revealed differences in the volume and distribution of white matter (the component of the brain that includes glia and myelinated axons which give it its white appearance).
White matter differences can be telling, but it seems like most of the answer lies in the prefrontal cortex- an area in the frontal lobe of the brain that regulates abstract thinking, thought analysis, decision-making, and behavior. For those who are familiar with Freudian concepts, think of it as being similar to the ego- the structure that enables you to balance conflicting thoughts and make decisions based on potential outcomes.
Scientists have found that the human brain not only features larger areas within the prefrontal cortex compared to the chimpanzee brain, but it is also more gyrified. (FYI: gyri are the ridges in the brain that serve to increase the surface area of the brain). In addition, humans have different neurotransmitter innervation of the PFC. For example, humans have a higher number of dopaminergic afferents in layers III, IV and V and a greater density of serotonergic axons in layer IV and V.
Recently, a study led by Semendeferi and colleagues has revealed differences in the spatial organization of neurons in the prefrontal cortex that you can see (first figure above). The region shown above is known as Brodmann area 10 (BA10) and it is thought to contribute to abstract thinking and other complex cognitive processes. As you can see, the neurons in the human brain have more space in between them compared to the neurons in the chimpanzee. This extra space is thought to provide more room for connections between neurons, thus enabling more complex information processing and cognition.
As a control, the researchers studied the organization of other cortical areas such as: primary visual cortex, primary somatosensory cortex and primary motor cortex and found subtle but no large-scale differences between primates and humans (see second diagram).
Who knew that a little space in a higher order structure could make such a difference?
Schoenemann, PT, Sheehan, MJ and LD Glotzer. (2005). Prefrontal white matter volume is disproportionately larger in humans than in other primates. Nature Neuroscience, 8(2): 242-252.
Semendeferi et al. (2011). Spatial organization of neurons in the frontal pole sets humans apart from great apes. Cerebral Cortex, 21(7): 1485-97. doi: 10.1093/cercor/bhq191