A popular subject on the big screen and television (especially the daytime variety) is a form of memory loss known as amnesia. Stedman's Medical Dictionary defines amnesia as "a disturbance in the memory of information stored in long-term memory, in contrast to short-term memory, manifested by total or partial inability to recall past experiences." Amnesia is a condition in which someone can't recall stored memories, like their mom's maiden name or what happened last Christmas, but they may recall the knock-knock joke their little brother told them a few seconds ago. This is actually an example of retrograde amnesia.

Types of Amnesia
  • Anterograde amnesia - Inability to remember ongoing events after the incidence of trauma or the onset of the disease that caused the amnesia
  • Emotional/hysterical amnesia - Memory loss caused by psychological trauma; usually a temporary condition
  • Lacunar amnesia - Inability to remember a specific event
  • Korsakoff syndrome - Memory loss caused by chronic alcoholism
  • Posthypnotic amnesia - Memory loss sustained from a hypnotic state; can include inability to recall events that occurred during hypnosis or information stored in long-term memory
  • Retrograde amnesia - Inability to remember events that occurred before the incidence of trauma or the onset of the disease that caused the amnesia
  • Transient global amnesia - Spontaneous memory loss that can last from minutes to several hours; usually seen in middle-aged to elderly people
Source: Stedman's Medical Dictionary

In order to understand how the loss of memory works, it's good to know how we store memories in the first place. If you have read How Your Brain Works, then you know that the human brain is a truly amazing organ. It gives us the power to think, plan, speak and imagine. It also gives us the ability to make and store memories. Physiologically speaking, a memory is the result of chemical or even structural changes in synaptic transmissions between neurons. As these changes occur, a pathway is created. This pathway is called a memory trace. Signals can travel along these memory traces through the brain.

Memory
  • Short-term memory - This refers to memories that last anywhere from a few seconds to a couple of minutes.
  • Intermediate long-term memory - This refers to memories that may last for days or even weeks, but eventually are lost forever (unless they are moved to long-term memory).
  • Long-term memory - This refers to memories that can be recalled for many years (perhaps for an entire lifetime).

There are several different types of amnesia, and they can be caused by such things as disease or head trauma. Sometimes the memory loss associated with amnesia includes everything from a person's past, and other times just bits and pieces are missing. In most cases, amnesia is a temporary condition and is very brief, lasting from a few seconds to a few hours. However, the duration can be longer depending on the severity of the disease or trauma, possibly lasting for a few weeks or even months.

As the amnesiac recovers, he or she usually recalls older memories first, and then more recent memories, until almost all memory is recovered. Memories of events that occurred around the time of the accident or onset of amnesia are sometimes never recovered. The two most commonly discussed forms of amnesia are retrograde amnesia and anterograde amnesia. If someone is suffering from retrograde amnesia, he or she cannot recall memories that occurred before the onset of amnesia. If someone has anterograde amnesia, he or she cannot remember incidents that happen after the onset of amnesia.

Making and storing memories is a complex process involving many regions of the brain, including the frontal, temporal and parietal lobes. Damage or disease in these areas can result in varying degrees of memory loss. Here's a good example of how this works:

In order for short-term memory to become long-term memory, it must go through a process known as consolidation. During consolidation, short-term memory is repeatedly activated -- so much so that certain chemical and physical changes occur in the brain, permanently "embedding" the memory for long-term access. If, during this repeated activation, something interrupts the process -- let's say a concussion or other brain trauma -- then short-term memory cannot be consolidated. Memories can't be "stored" for long-term access. This may be what's going on in anterograde amnesia.

It is believed that consolidation takes place in the hippocampi, located in the temporal-lobe regions of the brain. Medical research indicates that it is the frontal and temporal lobes that are most often damaged during head injury. This is why many people who suffer severe head trauma or brain injury experience anterograde amnesia. If the hippocampi are damaged, the amnesiac will be able to recall older memories, but won't be able to make any new ones.

Here are some interesting links: