Since retrograde amnesia relates to memory for events that took place when brain function was unimpaired, it clearly cannot be ascribed to failure of registration—with the exception, perhaps, of the very brief permanent amnesias following electroconvulsive shock or head injury. Retrograde amnesia otherwise…

…or disease are forgotten, and retrograde amnesia, in which events preceding the causative event are forgotten. In rare instances, an individual may experience both anterograde and retrograde amnesia at the same time.

…almost always show evidence of retrograde amnesia that can span as little as a few weeks past to as much as 15 or 20 years before onset of the disorder. These extensive retrograde amnesias are seldom total or uniform, and “islands” of memory often can be found by persistent interrogation.…

…are also apt to show retrograde amnesia, which may extend over brief or long periods into the past, the duration seeming to depend on factors such as severity of injury and age.