Memory is a difficult concept to define. To remember something requires the complex processing of information such as time, place, emotions, or sensory input (sight, smell, sound, touch), in order have the ability to re-create that information at a later time. Scientists have long tried to define memory by using models to describe this process, but some of the most useful memory models have been derived from studies of impaired memory with brain injury patients.
One important theory of memory that has come (at least partially) from brain injury studies, is the model for explicit versus implicit memory. Explicit memory refers to the memories that we can explicitly remember taking part in. For instance, we may have a strong memory of attending a child's graduation. We not only know intellectually that the child has graduated, we can pull up the specific emotions, sights, smells, sounds, time, and place that color that memory.
Implicit memory refers to a much more subtle process-one that is difficult to observe outside a psychology lab. Implicit memory implies that a person has encoded some information in his brain, but without consciously knowing how. The information has gotten in under the radar, in other words. For instance, a psychology lab can flash words very quickly in front of a subject-too quickly for the subject to even perceive-and the subject will be surprised to learn that he can correctly guess the word, even when he has no memory of even seeing it.
In the traumatic brain injury patient, depending on what part of the brain was injured, explicit memory can be impaired. Amnesia-either the loss of memories from the past or the lost ability to learn new information-can occur because of an impaired explicit memory system. Without a specific recollection of memory detail such as sight, sound, smell, emotion, time, or place, the patient has no context to help anchor a memory into reality.
Strangely enough, however, implicit memories are very often intact in brain injury patients. These patients can perform just as well as healthy people at the quickly flashing words experiment described above. The implication of this is that rehabilitation professionals (and family members) can use the intact implicit memory system to help get new information through to the brain injury patient. By repeating a piece of specific information over and over (ie, "The coffee cups are in the right cupboard."), the patient may implicitly remember the information, even if he has no conscious memory of learning it.
Baddeley A. Human Memory, Theory and Practice. Psychology Press Ltd. (2002).