Through the 20th century, neuroscientists gradually tracked particular brain functions to specific regions. Sadly, the most efficient way to determine such a linkage was to find a patient with an injury to a specific region of his brain, and then to establish which of the patient's faculties were impaired.

One of these patients was H.M., an epileptic who went under the surgeon's knife at the age of 27--the doctors stopped his seizures, but they also cut into a brain region called the hippocampus and inadvertently destroyed his ability to form new memories. For the rest of his life, until he died in 2008 at the age of 82, he could only hold a thought for about 20 seconds. Yet H.M. could clearly remember things from before his surgery, and could also form new "motor memories"--for example, after many repetitions he performed a difficult drawing task with ease, but had no memory of doing it before. Such results helped researchers understand the role of the hippocampus, which is shown in cross-section at the bottom half of this 2005 image.