Sometimes the best way to learn how the brain works is to watch what happens when it goes awry. When one part—a clump of neurons or a brain-building gene—doesn’t do what it is supposed to, the brain may fail in an illuminating way. Its failure may even expose some of the hidden foundations of the mind.
Neuroscientists have recently become fascinated with a particularly telling pair of rare brain disorders. One was identified in 1965 by English physician Harry Angelman, who was struck by the faces of three children he treated. These children were always smiling and often laughing. This disorder, now known as Angelman syndrome, affects around 1 in 20,000 children. Along with the smiles and laughs come other symptoms, some of which overlap those of severe autism. Many children with Angelman syndrome never learn to speak or read. They also keep their bodies in motion, often flapping their hands. When they nurse they suckle desperately, thrusting out their tongue.
Another similarly rare condition, called Prader-Willi syndrome, produces a different set of symptoms. Babies with Prader-Willi nurse very little—so little that they often have to be tube fed. However, once Prader-Willi children get to be a few years old, they develop an insatiable appetite. They will try to get around any obstacle put between them and food. Their fierce hunger is driven by a malfunctioning hypothalamus, a region deep in the brain that governs hunger and growth. Instead of autism, many people with Prader-Willi syndrome develop schizophrenia by adulthood, hearing voices and generating paranoid delusions.
