When patients have memory complaints or when professionals discuss memory loss, it is often assumed that memory is a singular trait. In fact, however, there is increasing evidence for several types of memory. The distinction between different memory systems is important because some systems may be affected by aging or by disease, while other systems may remain intact. At least three types of long-term memory representation (conceived by Endel Tulving at the University of Toronto) are important for understanding the differential effects of normal aging versus diseases : episodic memory, semantic memory, and implicit memory.

Episodic memory involves conscious recollection of specific events in your life that occurred in a particular time and place. What were you doing when you heard that John Kennedy was assassinated? When the space shuttle Challenger exploded after liftoff? Where did you spend Thanksgiving last year? When did you last see your spouse? What did you eat for breakfast today? All of these remembrances require episodic memory, which allows us to remember what, when, and where. Episodic memory contains information ranging from a few minutes ago to many years ago.

Semantic memory contains our vocabulary and general knowledge of the world, information that is available independent of time and context. Who was John Kennedy? What do you usually eat for Thanksgiving? What kinds of clothes should you wear to your nephew’s wedding? What is your spouse’s name? Note the difference between these questions and the episodic memory questions in the previous paragraph. In the memory laboratory, we might ask someone to name some fruits (semantic memory) or to recall the names of some fruits from a list presented earlier (episodic memory). Healthy older adults-compared to young adults do experience greater difficulty with episodic memory, but not with semantic memory. Alternatively, patients suffering from Alzheimer’s disease suffer loss of both types of memory.

Implicit memory is the most basic type of memory, as it simply requires a response in the presence of a previously experienced stimulus. At the piano or computer keyboard, our fingers seem to “know” where the keys are. When a traffic light turns red, our right foot goes automatically to the brake pedal. When we see a familiar printed word, its pronunciation is immediately available. The second time we visit a foreign country, the vocabulary and expressions come to mind with greater facility. Priming, then, is quite different from the other types of memory in that it requires no conscious recollection but does reveal the effects of prior experience (i.e., memory). In contrast to episodic and semantic memory which involve “knowing when” or “knowing what,” implicit memory has been characterized as “knowing how.”

For an implicit memory task in the laboratory, an individual, an individual might be asked to engage in word puzzles, with no mention of a memory test. Try to complete the following fragments to form words - D_ N_S_U_, AV _C_D_, T_ QU_L_, A_ R_ V _R_, G_ N_RA_I _N, and E_ E_HA_T - although it’s fairly difficult, when the whole words have been seen previously the number of fragments completed rises dramatically, providing evidence of memory. This form of memory occurs even when individuals don’t remember having seen the very same words. Even more striking. amnesics-who, by definition, have extremely poor episodic memory-perform at the same level as normals when an implicit test is used. For present purposes, this finding is of great interest because it shows that

(1) It is possible to tap information stored in memory not normally available to consciousness

(2) That separate memory systems can be differentially affected by factors such as aging and disease.

We will see evidence that implicit memory is invulnerable to the effects of normal aging and may be spared in Alzheimer’s disease as well.

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