Compare humans to other mammals and a distinguishing feature stands out: our large, cavernous craniums, and the densely folded brains stuffed into them. The human brain is more than triple the size of the brain of chimpanzees, our closest relatives. In particular, it’s the cerebral cortex—the wrinkled outer layer of the brain—that sets us apart. Whales and elephants also have big brains, but they can’t match our cortex in the sheer number of neurons and billions of connections among them.
It’s obvious that our big brains are responsible in some way for enabling the unique things that humans do: developing languages, music, and art; using sophisticated tools and technologies; forming complex societies. But what is it about a bigger brain that makes these feats possible?
Randy Buckner, professor of psychology and of neuroscience, and his former student Fenna Krienen, Ph.D. ’13, have proposed a hypothesis to explain how the evolution of a large cortex may have enabled the distinct cognitive skills that humans display. The key is not just size but organization. As the human brain swelled, they argue, the cells in newly evolved areas were increasingly freed from constraints that patterned the simpler connections in other areas, and thus able to connect to each other in more complex ways that enabled new kinds of thinking.
The human cortex can be divided into two types of regions. One type includes the sensory and motor cortices, which process bodily sensations such as olfactory, gustatory, auditory, and visual information, and also control movements. These regions evolved early and are therefore similar in all mammals. But humans also possess a great deal of brain mass interspersed among these motor and sensory areas. Called “association cortices,” those intervening areas are responsible for many of the tasks we associate with higher-level thought.
We know from animal studies that the flow of information in the sensory and motor areas follows a relatively simple, stepwise path. When visual information comes in from the retinas, for example, it’s handed off from neuron to neuron through the visual cortex according to a chain of command. The raw sensory data are processed in more complex ways as they make their way to different areas.