Psychogenic amnesia, also known as functional or issociative amnesia, is a disorder characterized by abnormal memory functioning in the absence of structural brain damage or a known neurobiological cause; severe cases are very rare. It is defined by the presence of retrograde amnesia or the inability to retrieve stored memories and events leading up to the onset of amnesia and an absence of anterograde amnesia or the inability to form new long term memories. In most cases, patients lose their autobiographical memory and personal identity even though they are able to learn new information and perform everyday functions normally. Other times, there may be a loss of basic semantic knowledge and procedural skills such as reading and writing.