This relatively simple behavior is reflected in how the neurons are wired. The neurons in one region mostly make short connections to a neighboring region. They carry signals through the brain like a bucket brigade from the sensory cortices to the motor cortices.
The bucket brigade begins to take shape when mammals are embryos. Different regions of the brain release chemical signals, which attract developing neurons.
“They will tell a neuron, ‘You’re destined to go to the back of the brain and become a visual neuron,’ for example,” Krienen said.
After mammals are born, their experiences continue to strengthen this wiring. As a mammal sees more of the world, for example, neurons in the visual cortex form more connections to the motor cortices, so that the bucket brigade moves faster and more efficiently.
Human brains are different. As they got bigger, their sensory and motor cortices barely expanded. Instead, it was the regions in between, known as the association cortices, that bloomed.
Our association cortices are crucial for the kinds of thought that we humans excel at. Among other tasks, association cortices are crucial for making decisions, retrieving memories and reflecting on ourselves.
Association cortices are also unusual for their wiring. They are not connected in the relatively simple, bucket-brigade pattern found in other mammal brains. Instead, they link to one another with wild abandon. A map of association cortices looks less like an assembly line and more like the Internet, with each region linked to others near and far.