As you read these words, your eyes scan the page, picking up patterns to which your mind assigns meaning. Meanwhile your heart contracts and relaxes, your diaphragm rises and drops to control your breathing, your back muscles tense to maintain your posture, and a thousand other basic tasks of conscious and subconscious life proceed, all under the coordinated control of roughly 86 billion neurons and an equal number of supporting cells inside your skull. To neuroscientists like us, even the simple act of reading a magazine is a wondrous feat—as well as an example of perhaps the hardest problem in science today: in truth, we cannot yet fully explain how the human brain thinks and why the brain of a monkey cannot reason as we do.
Neuroscientists have intensely studied the human brain for more than a century, yet we sometimes still feel like explorers who have landed on the shores of a newly discovered continent. The first to arrive plotted the overall boundaries and contours. In the early 1900s German scientist Korbinian Brodmann sliced up human brains and placed them under his microscope to examine the cerebral cortex—the exterior layers of gray matter that handle most perception, thought and memory. He parceled this cortex into several dozen regions based on the topology of the organ and how the cells in each area appear when labeled with various stains.