Scientists have long sought to understand the biological basis of thought. In the second century A.D., physician and philosopher Claudius Galen held that the brain was a gland that secreted fluids to the body via the nerves—a view that went unchallenged for centuries. In the late 1800s clinical researchers tied specific brain areas to dedicated functions by correlating anatomical abnormalities in the brain after death with behavioral or cognitive impairments. French surgeon Pierre Paul Broca, for example, found that a region on the brain’s left side controls speech. In the first half of the 20th century, neurosurgeon Wilder Penfield mapped the brain’s functions by electrically stimulating different places in conscious patients during neurosurgery, triggering vivid memories, localized body sensations, or movement of an arm or toe.
In recent years new noninvasive ways of viewing the human brain in action have helped neuroscientists trace the anatomy of thought and behavior. Using functional magnetic resonance imaging, for instance, researchers can see which areas of the brain “light up” when people perform simple movements such as lifting a finger or more complex mental leaps such as recognizing someone or making a moral judgment. These images reveal not only how the brain is divided functionally but also how the different areas work together while people go about their daily activities. Some investigators are using the technology in an attempt to detect lies and even to predict what kinds of items people will buy; others are seeking to understand the brain alterations that occur in disorders such as depression, schizophrenia, autism and dementia.