Mind

Neurology advanced during the 1700s and early 1800s, as researchers discovered electricity's role in the nervous system and mapped out reflex pathways between the spine and limbs. But Willis's most ambitious project—to work out the foundations of the mind—did not see major advances until the mid-1800s. The huge technical challenge of studying many aspects of human cognition certainly was responsible for some of the delay. But the Cartesian dualism that lingered into the nineteenth century also acted as a brake on progress. Many researchers continued to believe that the mind was a unitary, immaterial entity. It was therefore impossible to discover its components, as had been done with the heart or the lungs. According to one prominent French neurologist in the 1800s, to divide the soul was to deny it.

A growing number of scientists rejected this claim during the nineteenth century. The concept that humans were the product of evolution—first broached in the 1700s and brought to fruition by Charles Darwin (1809–1882) in the following century—implied that the faculties of the mind were the product of evolution as well. Scientists began to divide the soul, as it were, and in the process they established the foundations of cognitive neuroscience. The work of two researchers in particular, Pierre-Paul Broca (1824–1880) and Frans Cornelius Donders (1818–1889), illuminate the scientific shift that occurred in the mid-1800s.

In 1861, the French physician Broca treated a man who suffered a stroke. The patient could understand language but could not speak, except for one sound, "tan." (The patient became known as Tan.) After Tan's death, Broca followed Willis's example and autopsied his patient's brain. Tan's brain was damaged in the left frontal lobe. Other patients with the same difficulty in speaking exhibited damage in the same place. Broca demonstrated that a restricted part of the brain was responsible for a restricted aspect of human mental life—specifically, the ability to produce speech.

At the same time that Broca was doing this work, the German ophthalmologist Donders was dissecting the mind in a radically different—yet complementary—way. In the early 1800s, physiologists and physicists began to study the performance of people in simple tasks, such as recognizing colors and shapes. These tests, the researchers hoped, would reveal the inner workings of the mind without any recourse to anatomical details. In the 1860s Donders performed one of the most elegant of these tests. He first measured how long it took for people to react to seeing a light come on. In his second experiment, one of two differently colored lights could turn on, and his subjects had to indicate which color had come on. He found that it consistently took 50 milliseconds longer to discriminate colors than to perceive the presence of a light. Essentially, Donders was doing in time what Broca was doing in space: He was isolating and studying a specific mental function.

In the twenty-first century, cognitive neuroscientists continue to employ the methods of Broca and Donders, albeit with more sophisticated technology.