We have more information than ever about the brain—how neurons talk to one another, which cells correspond to specific tasks, where cells are located—but the big picture, how the organ functions as a whole, remains a puzzle. Inspired by a 2016 Carlsberg Academy workshop titled "How Does the Brain Work?," Neuron asked theoretical and experimental neuroscientists for their ideas on this question. The result is a collection of Perspectives covering such topics as the neural control of movement, learning, memory, and social interaction that surveys what we know and what hypotheses have yet to be tested.

The collection offers a flavor of ongoing work on theory-based research,and hopes to inspire further collaboration among theorists and experimentalists.

For instance, one contribution from Riitta Hari of Aalto University in Finland, explores the speed at which the brain processes information and how it constrains the way we act in the world. Hari has studied timing in the brain, where even the thickest and fastest neurons send information at about 200 miles an hour. For context, a copper wire can transmit data ten million times faster. Thicker nerve fibers would be faster, but they would take up so much space that a newborn's head would be too large to pass through the birth canal. Instead, evolution has responded by making brain areas communicate with a dense network of thin connections.

Because of the brain's sluggishness, Hari proposes that what we perceive is actually an individual "caricature world" that can be captured quickly with minimal sensory input and without too much brain processing.

"We think there is some kind of real world, but everybody gets out of that world something a little bit different because of their different senses and individual histories," she says. "We behave in this world on autopilot, with certain neurons alerting us to react to deviations of our expectations. However, the individual caricature worlds are similar enough for people to share their own realities."

In addition to timing, Hari emphasizes the tight brain-environment connection for the organization of the human brain. "Simple physical laws, such as Newton's second law of motion (force equals mass times acceleration), that govern our world also have had an imprint on the organization of our brains," Hari says. For example, wind moves small leaves much faster than it moves the large branches of a tree. Thus, brain areas processing small visual features have evolved to be more sensitive to faster changes in the environment than brain areas that process large visual objects. In other words, space and time are connected in the brain in a similar manner as they are connected in the real world.

"When we think about how the human brain works, we have to consider the brain-environment connection: we can't just take snapshots of a person in a scanner; we have to follow what happens from one moment to the next," she says. "This is also true for social interaction, another essential factor that has shaped the human brain."