Enjoying a piece of music or recognizing the face of a loved one seems like a simple, instantaneous process. But like all things associated with the brain, they aren't. Both these activities arise from complex, interconnected networks in the brain—networks that we normally take for granted, until some sort of illness or trauma disrupts them. As Columbia University neurologist, physician, and author Oliver Sacks has documented throughout his career, damage to the brain can render even the most quotidian of tasks impossible, making one unable to perceive music, unable to recognize a spouse's face, or—even stranger—convinced that one's spouse is an alien impostor! 

The Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) classifies common mental illnesses into broad groups, including anxiety disorders, mood disorders, psychotic disorders, dissociative disorders, and personality disorders. But the spectrum of brain disorders is too diverse to fit neatly within these categories. There are many disorders—often caused by brain damage or degenerative brain disease—that defy these normal categories. And Dr. Sacks is their undisputed expert. 

His most famous work, "The Man Who Mistook His Wife for a Hat," takes its name from one of the case studies it presents. Because of acute brain damage to his occipital and temporal lobes, one of Sacks's patients suffered from visual agnosia, the inability to recognize common visual stimuli. At one point during a session, the patient reached over and grabbed his wife's head, attempting to lift it towards his head as if it were his hat. The problem was not with his vision but with his brain, where visual input is processed; he could see his wife's face perfectly fine, could pick out and describe details, like the shape of her nose of the color of her eyes. But he was unable to add these constituent parts together to create a unified image of her face. The only way he knew that she was his wife was by recognizing her voice. 

In his Big Think interview, Sacks recounts some of the more bizarre cases from his most recent book "Musicophilia," which focuses exclusively on disorders affecting the ability to perceive or create music. Sacks describes one patient who was normal in every way except that she could not recognize music, a condition called amusia. "When she was asked to sing, she really didn't know what was meant," says Sacks. "People were puzzled because she wasn't deaf. She could speak perfectly well and hear their speech and ambient noises, but she seemed to have no idea what music was about." 

There are two basic categories of musical perception, which, if disrupted, can lead to a form of amusia: one involves the recognition of melodies, the other the perception of rhythm or time intervals. "Impairments of melody usually go with right-hemisphere lesions, but representation of rhythm is much more widespread and robust and involves not only the left hemisphere, but many subcortical systems in the basal ganglia, the cerebellum, and other areas," says Sacks. In this case, the patient's problem was clearly of the first order: she could not hear tones or semi-tones and therefore could not hear the intervals that compose a melody. Another of Sacks's patients experienced disturbing auditory hallucinations, as if a radio were playing loudly in her head but she could not shut it off. 

V.S. Ramachandran a neurologist and director of the Center for Brain and Cognition at UC San Diego, is also known for his expertise in rare brain disorders. His book "Phantoms in the Brain" catalogues some of the outlandish cases he's encountered during his career. One of the "rarest and most colorful" syndromes he discusses is the "Capgras delusion," in which an otherwise lucid person comes to regard close acquaintances, spouses, or parents as impostors—often aliens or robotic clones.