In addition to short-term and working mem­ory, we can further divide memory systems into explicit memory (sometimes called declarative), which is intentional and conscious remembering of information that is learned and remembered at a specific point in time, and implicit memory (some­times called procedural memory), which involves retrieval of information without conscious or inten­tional recollection. This distinction is important because research during the last 20 years shows that aging impacts these two forms of memory very differently. Overall, implicit memory shows much smaller age differences than explicit memory (Mitchell & Bruss, 2003). The distinction between the explicit and the implicit memory system is important and needs to be taken into account when evaluating the degree to which performance on a particular task involves intentional (explicit) or nonintentional (implicit) memory retrieval (Old & Naveh-Benjamin, 2008).

Implicit Memory. Implicit memory is a facilitation or change in task performance that is attributable to having been exposed to information at some earlier point in time, but which does not involve active, explicit memory. A good example of implicit memory is a language task such as stem completion. In a stem completion task, you would be required to complete a word stem with the first word that comes

to mind (for instance, con_____ ). Previously, you may

have been shown a list of words that contained a valid completion of the stem (such as contact). If you have seen valid completions of the stems, you are more likely to use them later to complete the stems than you are to make up a different one (such as contest). This facilitation is called priming (Zacks et al., 2000). The memory aspect of the task is that you remember the stem completion you were shown; the implicit part is remembering it without being told to do so.

Research on implicit memory focused on demon­strating that it was a separate process from explicit memory. For example, whereas explicit memory performance is affected by having people think about the meaning of the items, implicit memory is not, and people with amnesia show severe prob­lems on explicit memory tests but often perform similarly to normal people on implicit memory tests (Schacter, 2000; Yu & Yang, 2003). These findings suggest that implicit memory may be an excep­tion to the general finding of age-related decline in long-term memory for new information (Lazzara, Yonelinas, & Ober, 2002; Mitchell & Bruce, 2003).

However, there are some qualifications to this conclusion. Age differences on implicit memory tests either are not there or are notably smaller than age effects on more explicit memory tasks (Fleischman & Gabrieli, 1998). When age differences do appear, they are almost always in favor of younger adults, though the age difference is smaller than for explicit memory tests (LaVoie & Light, 1994). In addi­tion, some differences exist among various types of implicit memory tests. The most important dis­tinction has been between perceptually based and conceptually based tests (Mitchell & Bruss, 2003; Roediger & McDermott, 1993). Perceptually based implicit memory tasks rely on processing the physi­cal features of a stimulus; an example would be to process whether a word appears in lowercase or uppercase letters. Conceptually based implicit memory tasks rely on the semantic meaning of the items, such as whether the word is a verb. Depending on what people are asked to process (i. e., physical features or semantic meaning), performance on the tests differ (Toth, 2000).

The difference between the types of tests has important implications for age differences (Mitchell & Bruss, 2003). Results from several studies reveal a mixed pattern of age differences in implicit mem­ory; some studies find no differences on concep­tual tasks and find age differences on perceptual tasks (Lazzara et al., 2002), whereas others find the reverse pattern (e. g., Dennis, Howard, & Howard,

2003) . Furthermore, others find that older adults show greater conceptual priming whereas younger adults do not (Multhaup, Hasher, & Zacks, 1998).

However, overall evidence suggests equal age effects in the two types of priming (Fleischman & Gabrieli, 1998; Lazzara et al., 2002).

What accounts for the sparing of implicit memory with age? Recent research in cognitive neuroscience (see Chapter 2) suggests that there is differential age-related deterioration in the brain underlying more explicit forms of memory (e. g., frontal-striatal system) as opposed to those underlying implicit memory (e. g., cerebral neocortex) (Lazzara et al., 2002; Prull et al., 2000; Tulving & Schacter, 1990). Behaviorally oriented researchers also point out that when age-related decline is evident in implicit memory tasks, it is probably because the tasks are contaminated with some explicit memory demands (McCarley et al., 2004; Mitchell & Bruss, 2003).