What can we learn from people with brain damage?
To be sure that a particular region of the brain is critical for some behavior, we need to know what happens when the brain region is no longer functioning. This kind of experiment has been performed in animals, where a part of the brain is inactivated. Such invasive experiments would be unethical in humans, however, and so we must rely on patients who develop brain damage from illness or trauma.
For example, much of what we initially learned about memory came from patient Henry Molaison, also known as H.M. He had a surgical procedure in which his hippocampus was removed to treat intractable epilepsy. After this surgery, he exhibited severe anterograde amnesia, that is, he was unable to create new memories. This provided compelling evidence that the hippocampus is critical for memory. This kind of surgery is no longer performed, but amnesia still occurs naturally as a result of viral encephalitis or other conditions. Patients with amnesia can appear normal in many ways. They still have intact language abilities — able to read, write, and converse. They can also have intact motor skills, such as riding a bike or playing an instrument. These preserved abilities tell us something about brain function too, namely that the damaged region is not critical for these behaviors.