The Man Who Mistook his Wife for a Hat was written by Oliver Sacks, a neurologist noted for his work with Alzheimer's disease, schizophrenia, retardation and autism. He has written eight other books, the best known of which is Awakenings which was adapted into a movie starring Robin Williams.
Dr. Sacks divides the book into several vignettes about different patients he treated in his years at the Albert Einstein College of Medicine. The patient who he refers to as Dr. P, an accomplished musician and teacher, had a serious neurological disorder. It started out looking like Alzheimer's as he became unable to recognize the faces of his students. As the disease progressed, he sometimes mistook fire hydrants and parking meters for people and stopped to talk with them. When Dr. Sacks examined Dr. P for the first time, he found his reflexes slightly abnormal on the left side. After scratching the sole of his left foot to test his reflexes, he instructed Dr. P to put his shoe back on. He couldn't do it, because he could recognize neither his foot nor his shoe. When the examination was over, Dr. P reached over to put his hat on, but instead of a hat, he tried to lift off his wife's head.
Dr. Sacks visited Dr. P at home to see how he functioned in a familiar environment. Dr. P could identify shapes and cartoons, but he couldn't recognize people on television or photographs of family. Dr. Sacks took a rose out of his lapel and asked Dr. P to identify it. He could not name it, but he was able to describe its dimensions, shape and color. However, when he smelled it, he immediately knew it was a rose. From this, Dr. Sacks determined that the problem was in his parietal and occipital lobes where visual processing occurs. He had become completely unable to visualize, even internally. When asked to close his eyes and describe a familiar market square, he correctly described everything on the right side, but nothing on the left. When asked about Anna Karenina, he could recount plots, but omitted any mention of visual characteristics.
Dr. P functioned on a daily basis by using his musical talents. He sang to himself to keep his focus. He sang eating songs, dressing songs and bathing songs, but if he was interrupted he just stopped. He didn't know his clothes or his own body.
At the time, Dr. Sacks thought his patient had a tumor or degenerative process in the visual parts of his brain. At the time of his writing he was not aware of the studies on visual agnosia, which comes from the Greek word meaning "lack of knowledge". It describes the inability to recognize objects using a given sense, in Dr. P's case, his sight. Patients with object agnosia usually have sustained damage to their occipital lobe or inferotemporal cortex, located at the back of the cerebral hemisphere where you receive and process visual information. Damage here can blind a person even though the eyes may be perfectly healthy.
Dr. Sacks' final recommendation to Dr. P was to continue making music and live his life as normally as he could. Dr. P continued to teach at a university and played music (and did a lot of singing) until his death.
Dr. Sacks describes many other unusual instances of neurological disorders in his book, which, without his study in the field, might have been incorrectly categorized as schizophrenia or Alzheimer's.