Figure 614

Methods of testing for proactive and retroactive interference.

studies of eye-witness testimony, in which memory of an event is interfered with by post-event questioning (see Chapter 8).

It used to be thought that forgetting was due more to retroactive interference than to proactive interference. The position changed, however, with the publication of an article by Underwood (1957). He reviewed studies on forgetting over a 24-hour retention interval. About 80% of what had been learned was forgotten in one day if the participants had previously learned 15 or more lists in the same experiment, against only 20-25% if no earlier lists had been learned. These findings suggested that proactive interference can have a massive influence on forgetting.

There is a potential problem with many of these studies. The learning of each successive list was equated, in that all lists were learned to the same criterion (e.g., all items correctly recalled on an immediate test), but the participants reached the criterion more rapidly with the later learning lists. Thus, they had less exposure to the later lists than to the earlier ones, and this may explain some of the apparent proactive interference. Warr (1964) equated the amount of exposure to the learning material on all lists, and found the forgetting rate was only modestly affected by the number of lists previously learned. However, Underwood and Ekstrand (1967) obtained substantial proactive interference in a study in which the learning rate did not increase over lists. Thus, proactive interference is a genuine phenomenon.

Evaluation

As proactive and retroactive interference have both been shown numerous times, why does interference theory no longer enjoy the popularity it once did? There are three main reasons. First, interference theory is uninformative about the internal processes involved in forgetting. Second, it requires special conditions for substantial interference effects to occur (i.e., the same stimulus paired with two different responses), and these conditions may be fairly rare in everyday life. Third, associations learned outside the laboratory seem less liable to interference than those learned in it. Slamecka (1966) obtained free associates to stimulus words (e.g., colour-red). Then the stimulus words were paired with new associates (e.g., colour-yellow). This should have caused retroactive interference for the original association (e.g., colour-red), but it did not.

Cue-dependent forgetting and context-change theory

According to Tulving (1974), there are two major reasons for forgetting. First, there is trace-dependent forgetting, in which the information is no longer stored in memory. Second, there is cue-dependent forgetting, in which the information is in memory, but cannot be accessed. Such information is said to be available (i.e., it is still stored) but not accessible (i.e., it cannot be retrieved).

Tulving and Psotka (1971) compared the cue-dependent approach with interference theory. There were between one and six word lists, with four words in six different categories in each list. After each list had been presented, the participants free-recalled as many words as possible. That was the original learning. After all the lists had been presented, the participants tried to recall the words from all the lists that had been presented. That was total free recall. Finally, all the category names were presented, and the participants tried again to recall all the words from all the lists. That was total free cued recall.

There was strong evidence for retroactive interference in total free recall, as word recall from any given list decreased as the number of other lists intervening between learning and recall increased (see Figure 6.15). This finding would be interpreted within interference theory by assuming that there had been unlearning of the earlier lists. However, this interpretation does not fit with the findings from total cued recall. There was essentially no retroactive interference or forgetting when the category names were available to the participants. Thus, the forgetting observed in total free recall was basically cue-dependent forgetting.

The studies of cue-dependent forgetting we have considered so far have involved external cues (e.g., presenting category names). However, cue-dependent forgetting has also been shown with internal cues (e.g., mood state). Information about current mood state is often stored in the memory trace, and there is more forgetting if the mood state at the time of retrieval is different. The no tion that there should be less forgetting when the mood state at learning and at retrieval is the same is known as mood-state-dependent memory (see Chapter 18). Ucros (1989) reviewed 40 studies, and concluded there is reasonable evidence for mood-state-dependent memory. The effect is stronger when the participants are in a positive than a negative mood, and it is stronger when they try to remember personal events rather than information lacking personal relevance.

Tulving developed the notion of cue-dependent forgetting in his encoding specificity principle (Wiseman & Tulving, 1976, p. 349): "A to-be-remembered (TBR) item is encoded with respect to the context in which it is studied, producing a unique trace which incorporates information from both target and context. For the TBR item to be retrieved, the cue information must appropriately match the trace of the item-in-context." Tulving (1979, p. 408) put forward a more precise formulation of the encoding specificity principle: "The probability of successful retrieval of the target item is a monotonically increasing function of informational overlap between the information present at retrieval and the information stored in memory." For the benefit of any reader wondering what on earth "monotonically increasing function" means, it refers to a generally rising function that does not decrease at any point. Thus, memory performance depends directly on the similarity between the information in memory and the information available at retrieval. As we will see shortly, there is much support for this principle.

Original learning, total free recall, and total free cued recall as a function of the number of interpolated lists. Data from Tulving and Psotka (1971).

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