Is the human brain, with all its problem-solving prowess and creative ability, powerful enough to understand itself? Nothing in the known universe (with the exception of the universe itself) is more complex; the brain contains about 100 billion nerve cells, or neurons, each of which can communicate with thousands of other brain cells.
Because we primates are primarily visual creatures, perhaps the best way for us to make sense of the brain is to see it clearly. That has been the goal for 125 years, since the Spanish scientist Santiago Ramon y Cajal began using a stain that marked individual neurons. He peered through a microscope at the stained cells and the branchlike projections with which they connected to other neurons. "Here everything was simple, clear and unconfused," he wrote of his observations, the beginning of modern neuroscience.
Scientists have since devised methods for determining the specific tasks in which different brain regions specialize - for example, some neurons, devoted to processing sight, detect only horizontal lines, while others sense danger or produce speech. Researchers have created maps delineating how brain regions not adjacent to one another are connected by long tracts of cellular projections called axons.