HOW MEMORY WORKS

Gary S. Moak, M.D.

“How can Mom have Alzheimer’s Disease?  She remembers things that happened thirty years ago better than I do.”   Family members of patients with Alzheimer’s disease and related disorders frequently are puzzled by this apparent paradox, that someone who cannot recall what happened five minutes ago can remember what happened decades ago.  To understand this common observation, it is necessary to appreciate that memory is not a simple function of the mind but, rather, highly complex.  In fact, there even are different types of memory that serve different functions and are affected differently in brain diseases.  In a 2005 article in the New England Journal of Medicine, Andrew Budson, M.D. and Bruce Price, M.D. described what is known about the workings of memory in the brain.  This article summarizes this information.

Memory is a way for the brain to hold onto information for later use.  Many people are aware of the distinction between short-term and long-term memory.  Memory is more complicated than that, however.  There are at least four types of memory and they represent a collection of mental abilities that depend on networks of systems within the brain.  Thus, different types of brain disorders can cause different types of memory impairment.

Episodic Memory  Episodic memory is the type of memory with which most people are familiar.  This is the ability to recall personal experiences such as where you grew up or what you had for breakfast.  Mental disorders that affect episodic memory follow a highly predictable pattern.  The most recent memories are the most vulnerable while remote memories are spared.

Episodic memory depends primarily on structures in the temporal lobes called the hippocampus.  The frontal lobes also have a critical role in registration and retrieval of episodic memory.  Thus, the hippocampus can be thought of as a file cabinet for episodic memory while the frontal lobes are the file clerk.  Two common mental disorders of later life that affect memory are Alzheimer’s disease and depression.  In Alzheimer’s Disease, memory capacity is lost – the file cabinet is damaged.  In depression memory retrieval is affected – the file clerk is “out sick”.  Diagnosing and effectively treating depression is critical because depression may be a totally curable cause of memory impairment.

Semantic Memory   Semantic memory refers to one’s store of general knowledge of facts and concepts.  In includes knowing that the sky is blue and that George Washington was the first president of the United States.  Scientists believe that semantic memory is located in an area called the inferolateral temporal lobe of the brain.  Alzheimer’s Disease is the most common condition to affect semantic memory.  People with this disorder may have mixed difficulty with both episodic and semantic memory.  How much each is affected may vary from person to person.  Strokes and head injuries that involve the inferolateral temporal lobe are other common causes of problems with semantic memory.

Procedural Memory   Procedural memory is the ability to remember rules for how to do things.  Unlike episodic and semantic memory, which are conscious functions, procedural memory often occurs outside of conscious awareness.  Abilities such as driving or playing cards depend on procedural memory.  Areas of the brain involved in procedural memory include the supplementary motor area, the basal ganglia, and the cerebellum.  These areas tend not to be affected in Alzheimer’s disease, whereas in Parkinson’s  disease procedural memory often is affected.  Other conditions that can affect procedural memory include strokes, hemorrhages, and depression.

Working Memory   Working memory is the ability to keep in mind information you need to use.  When you look up a telephone number and try to keep it in your mind long enough to dial it, you are using your working memory.   If you happen to remember the number the next day, it has become part of your episodic memory.  Working memory depends on a network of systems in the brain that are important in paying attention.  Thus, working memory can be affected by mental disorders that affect attention, such as attention deficit disorder and depression.  Alzheimer’s disease, stroke, and head injuries also can affect it.

Other General Patterns   In right handed people, the left side of the brain is more linguistic and mathematical while the right side of the brain is more visual and musical.  People with left brain injuries may be able to recall a song’s melody but not its lyrics.  They may be able to find their way from one place to another but not describe in words how they did it.

Conclusion   How can someone be able to play gin rummy, and win, yet not remember having played at all just ten minutes later?  The answer lies in the types of memory at work.  The card player needs procedural memory for the rules of the game and working memory to keep his/her hand in mind as well as the cards already played.  Episodic memory and semantic memory are not required.  In order to remember having played afterward, episodic memory is needed.

Caregivers commonly are bewildered by such inconsistent patterns of memory impairment.  It can increase the frustration and stress they feel in living with and caring for someone with a mental disorder that affects memory.  By understanding these different types of memory impairments, family members can feel less perplexed and confused.

 

Budson A, Price B.  New England Journal of Medicine 2005; 352: 692-699

 

Moak Center for Healthy Aging ©

Updated January 2009