Agnosia—literally “forgetting”—is any disorder of the nervous system which inhibits or eliminates an aspect of the senses. As a result, parts of the physical world can’t be detected, i.e., “face blindness.” The symptoms are caused by damage to areas of the brain as well as secondary disorders. In Chapter 1, Dr. P. experiences visual agnosia. The text of Sacks’s book also refers to “auditory agnosia,” “tonal agnosia,” and “auditory verbal agnosia,” describing an inability to hear speech.

Aphasia is a neurological condition that affects a person’s ability to communicate. It often prevents a person from reading and writing or being able to understand written and spoken language. It can be caused suddenly by a stroke or brain injury, or it can be caused more gradually by a tumor. Some people with aphasia can relearn how to use, read, write, and understand language. In Chapter 9, Sacks discusses ways those with aphasia can learn to understand communication based on body language and tone of voice.

Autism is classified as a developmental disorder. Although it can be diagnosed at any age, symptoms of autism usually appear before the age of two. It is characterized by difficulty in communicating with others, having selective interests, engaging in repetitive behaviors, and other symptoms.