Episodic And Semantic Memory

Our long-term memories contain an amazing variety of different kinds of information. As a result, there is a natural temptation to assume there are various long-term memory systems, each of which is specialised for certain types of information. Tulving (1972) argued for a distinction between episodic memory and semantic memory. According to Tulving, episodic memory refers to the storage (and retrieval) of specific events or episodes occurring in a particular place at a particular time. Thus, memory for what you had for breakfast this morning is an example of episodic memory. In contrast, semantic memory contains information about our stock of knowledge about the world. Tulving (1972, p. 386) defined semantic memory as follows:

It is a mental thesaurus, organized knowledge a person possesses about words and other verbal symbols, their meanings and referents, about relations among them, and about rules, formulas, and algorithms for the manipulation of these symbols, concepts, and relations.

As a matter of interest, a distinction closely resembling the one proposed by Tulving (1972) had existed for many years beforehand (Liz Valentine, personal communication). For example, in the 1929 edition of Encyclopaedia Britannica, there is a reference to an individual's memory knowledge, which is "personal and is referred to the past" (p. 233). This is distinguished from other knowledge, which "is not recalled as part of the individual life story. It is not referred to his past and it is impersonal' (pp. 233-234).

Wheeler, Stuss, and Tulving (1997, p. 333) defined episodic memory differently, arguing that its main distinguishing characteristic was "its dependence on a special kind of awareness that all healthy human adults can identify. It is the type of awareness experienced when one thinks back to a specific moment in one's personal past and consciously recollects some prior episode or state as it was previously experienced." They described this form of awareness as autonoetic or self-knowing. In contrast, retrieval of semantic memories does not possess this sense of conscious recollection of the past. It involves instead noetic or knowing awareness, in which one thinks objectively about something one knows.

How do the definitions of episodic and semantic memory offered by Wheeler et al. (1997) differ from those of Tulving (1972)? According to Wheeler et al. (1997, pp. 348-349):

The major distinction between episodic and semantic memory is no longer best described in terms of the type of information they work with. The distinction is now made in terms of the nature of subjective experience that accompanies the operations of the systems at encoding and retrieval.

In spite of the major differences between episodic and semantic memory, there are also important similarities: "The manner in which information is registered in the episodic and semantic systems is highly similar—there is no known method of readily encoding information into an adult's semantic memory without putting corresponding information in episodic memory or vice versa, both episodic and semantic memory obey the principles of encoding specificity and transfer appropriate processing" (Wheeler et al., 1997, p. 333).

Evidence

The key theoretical assumption made by Wheeler et al. (1997) is that episodic memory depends on various cortical and subcortical networks in which the prefrontal cortex plays a central role. Evidence from braindamaged patients and from PET scans has been obtained to test this assumption. For example, Janowsky, Shimamura, and Squire (1989) studied memory in frontal lobe patients. They focused especially on source amnesia, which involves being unable to remember where or how some piece of factual information was learned. This study is relevant, because it can be argued that source amnesia typically reflects a failure of episodic memory. Janowsky et al. (1989) found that frontal lobe patients showed considerable source amnesia, which is consistent with the view that the frontal cortex is involved in episodic memory. Wheeler et al. (1997, p. 338) summarised the findings from frontal lobe patients as follows: "The overall pattern of results is broadly consistent with the hypothesis that damage localised to the prefrontal cortex causes a selective loss in the episodic memory system.The most obvious of the alternative explanations is that the frontal lobes play a critical role in the ability to select and execute complex mental operations."

More convincing evidence comes from PET studies. What was done in these studies was to subtract the image of blood flow in the brain during a semantic memory task from the image of blood flow during a task requiring episodic memory as well as semantic memory. It was assumed that this would reveal those areas of the brain that are active when episodic memory is being used. In 25 out of 26 studies, the right prefrontal cortex was more active during an episodic memory retrieval than during semantic memory retrieval. The same subtraction method was used in 20 studies to identify those brain regions involved in episodic encoding but not in semantic encoding. In 18 out of the 20 studies, the left prefrontal cortex was more active during episodic encoding.

In sum, Wheeler et al. (1997) argued that there are two major differences between episodic and semantic memory. First, episodic memory involves the subjective experience of consciously recollecting personal events from the past whereas semantic memory does not. Second, the prefrontal cortex is much more involved in episodic memory than in semantic memory. Many higher-level cognitive processes take place in the prefrontal cortex, and it is assumed that the "sophisticated form of self-awareness" (Wheeler et al., 1997, p. 349) associated with episodic memory is also a higher-level cognitive process.

Evaluation

The theoretical views of Wheeler et al. (1997) represent an advance in our understanding of long-term memory. In particular, the notion that there is a major distinction between episodic and semantic memory seems plausible. However, there are some doubts about the strength of the empirical support for the distinction. As Wheeler et al. (1997) themselves pointed out, the finding that patients with damage to the frontal lobes show impaired episodic memory is open to various interpretations. One possibility is that the actual processes involved in episodic memory are specifically affected by the brain damage. Another possibility is that the effects of frontal lobe damage are more general (e.g., loss of some higher-level cognitive processes). As a result, such brain damage disrupts the performance of numerous kinds of cognitive tasks, including those involving episodic memory.

What about the findings from PET studies? As Wheeler et al. (1997) pointed out, the validity of the subtraction method used in the PET studies depends on three key assumptions:

1. The two tasks being compared differ with respect to only one component (e.g., presence vs. absence of episodic memory).

2. Subtraction permits the isolation of this component.

3. The brain regions associated with the component can be identified by PET scans.

Unfortunately, there is no easy way to show that these assumptions are justified. However, the great consistency of the findings from the PET studies across several different tasks and measures provides reasonable evidence that the prefrontal cortex is involved in episodic memory.

According to Wheeler et al. (1997), there is an important distinction between autonoetic or self-knowing awareness (found in episodic memory) and noetic or knowing awareness (found in semantic memory). However, there are some doubts about the value of this distinction, especially when applied to amnesic patients (see later in the chapter).

What remains for the future is to consider more closely the relationship between episodic and semantic memory. Research so far has focused on the differences between episodic and semantic memory, in spite of the fact that there are several similarities and interconnections between them.

IMPLICIT MEMORY Definitions

Traditional measures of memory (e.g., free recall; cued recall; and recognition) involve use of direct instructions to retrieve information about specific experiences. Thus, they can all be regarded as measures of explicit memory (Graf & Schacter, 1985, p. 501): "Explicit memory is revealed when performance on a task requires conscious recollection of previous experiences." In recent years, researchers have become much more interested in understanding implicit memory (Graf & Schacter, 1985, p. 501): "Implicit memory is revealed when performance on a task is facilitated in the absence of conscious recollection." The terms "explicit memory" and "implicit memory" tell us nothing about memory structures, and relatively little about the processes involved. In other words, they are mainly descriptive concepts.

Evidence

In order to understand what is involved in implicit memory, we will consider a study by Tulving, Schacter, and Stark (1982). Initially, they asked their participants to learn a list of multi-syllabled and relatively rare words (e.g., "toboggan"). One hour or one week later, they were simply asked to fill in the blanks in word fragments to make a word (e.g., _ O _ O _ GA _). The solutions to half of the fragments were words from the list that had been learned, but the participants were not told this. As conscious recollection was not required on the word-fragment completion test, it can be regarded as a test of implicit memory.

There was evidence for implicit memory, with the participants completing more of the fragments correctly when the solutions matched list words. This is known as a repetition-priming effect, and is found when the processing of a stimulus is faster and/or easier when it is presented on more than one occasion. A sceptical reader might argue that repetition priming occurred because the participants deliberately searched through the previously learned list, and thus the test actually reflects explicit memory. However, Tulving et al. (1982) reported an additional finding that goes against that possibility. Repetition priming was no greater for target words that were recognised than for those that were not. Thus, the repetion priming effect was unrelated to explicit memory performance as assessed by recognition memory.

This finding suggests that repetition priming and recognition memory involve different forms of memory. Tulving et al. (1982) also found that the length of the retention interval had different effects on recognition memory and fragment completion. Recognition memory was much worse after one week than after one hour, whereas fragment-completion performance was unchanged (see Figure 7.1).

Process-dissociation procedure

In terms of the definition of implicit memory, it is important to ensure that effects on memory performance are shown in the absence of conscious recollection. This is easier said than done. The usual method is to ask the participants at the end of the study about their awareness of any conscious recollection. However, participants may forget, or the questioning may be insufficiently probing. Jacoby, Toth, and Yonelinas (1993) devised the process-dissociation procedure as a way of measuring the respective contributions of explicit and implicit memory processes to performance on a test of cued recall. A list of words was presented (e.g., "mercy"), and there were two conditions at the time of the test:

• Inclusion test: participants were told to complete the cues or word stems (e.g., "mer_") with list words they recollected, or failing that with the first word that came to mind.

Performance on fragmentcompletion and recognition memory tests as a function of retention interval. Adapted from Tulving et al. (1982).

• Exclusion test: participants were instructed to complete the word stems (e.g., "mer_") with words that were not presented on the list.

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