Remember those little arrows that Santiago Ramon y Cajal added to his drawings to indicate the flow of information through the neurons? With that detail, Portraits of the Mind states, "Cajal established the overarching conceptual framework under which we approach the brain--the circuit." But understanding the patterns of connections between brain cells and regions is no small task. Even our fastest supercomputers can't yet simulate the interactions between the billions of neurons in a human brain.

This image from 1994 illustrates the brain as a circuit diagram and focuses on a region called the amygdala, which plays an important role in fear and other emotions. It summarizes the connections between the amygdala (in the center) and many different areas of the cerebral cortex, including regions responsible for reasoning and high-level decision making. This wiring diagram makes it clear that emotions and reasoning are bound strongly together.