The brain has a way of blocking out traumatic incidents in order to allow the person time to deal with the events. The memory is not lost, simply filed away in a secret compartment until it feels the person is able to cope with the trauma.
If someone sustained a head injury during an auto accident they may remember what happened the day before the accident, but not remember anything just prior to, during or right after the accident. The brain has saved the person the pain of reliving the event until he/she has time to adjust to the trauma.
When a patient is diagnosed with retrograde amnesia (RA) the doctor usually means they are not able to recall incidents or things they have learned before the injury or the disease began. Incidents that occurred closer to the time of the injury or disease, and things in the more distant past are not completely forgotten. In extreme cases, the individual may completely forget who he or she is, but that is rare.
Amnesia can also be selective. It could affect a person’s ability to remember certain events related to a specific incident – such as a recent car accident.
RA commonly affects the temporal lobes, hippocampus and the diencephalons. The parts of the brain are commonly associated with autobiographical memory and declarative memory. Semantic or episodic memories make up the declarative (otherwise known as explicit) memory, which is one of the two major memory divisions that can consciously be recalled, such as facts and knowledge. The other memory division is known as non-declarative (or procedural) memory, which refers to unconscious memories such as skills (like riding a bicycle).
The main function of the hippocampus is to take short-term memory and move it to long-term memory. It takes everything we have learned or experienced through our five senses (touch, smell, hearing, sight and taste) and interprets this information before it sends them along. The hippocampus deals with memory consolidation, in particular episodic memory. Patients who have damaged their hippocampus are able to understand concepts, cannot process personal information.
The temporal lobes are important to understanding semantic memory (basic concepts) and processing facts through the brain. Damage to this area of the brain will affect verbal communication and comprehension, and the patient could experience long-term damage.
If the right side of the temporal lobe is affected, the patient has difficulty in naming ordinary objects, and has trouble with verbal memory and non-verbal activities – such as drawing or music.
Scientists do not fully understand the role of the diencephalons and the surrounding areas in memory, but they do believe this portion of the brain is involved in remembering and recalling personal emotions, time, place and objects (episodic memory).
On the positive side, the brain tries to repair itself by rerouting the connections to the brain in order to let another portion take over (plasticity). The other portions of the brain take over to compensate for the damaged areas, or until there is sufficient healing for it to return to normal.
About the author:
Ron White is a two-time U.S.A. Memory Champion and memory training expert. As a memory keynote speaker he travels the world to speak before large groups or small company seminars, demonstrating his memory skills and teaching others how to improve their memory, and how important a good memory is in all phases of your life. His CDs and memory products are also available online at BrainAthlete.com.
Sources:
Wikipedia – Memory consolidation: http://en.wikipedia.org/wiki/Memory_consolidation; declarative memory: http://en.wikipedia.org/wiki/Declarative_memory;
The Human Memory – Retrograde Amnesia: http://en.wikipedia.org/wiki/Retrograde_amnesia
Memory Loss & The Brain: http://www.memorylossonline.com/glossary/retrogradeamnesia.html