Forgetting over time in short-term memory. Data from Peterson and Peterson (1959).

mainly affects the recency effect (Glanzer & Cunitz, 1966, see Figure 6.2). The two or three words susceptible to the recency effect may be in the short-term store at the end of list presentation, and thus especially vulnerable. However, Bjork and Whitten (1974) found there was still a recency effect in free recall when the participants counted backwards for 12 seconds after each item in the list was presented. According to Atkinson and Shiffrin (1968) this should have eliminated the recency effect. The findings can be explained by analogy to looking along a row of telephone poles. The closer poles are more distinct that the ones farther away, just as the more recent list words are more discriminable than the others (Glenberg, 1987).

Strong evidence for the distinction between short-term and long-term memory stores comes from the demonstration of a double dissociation with brain-damaged patients. Two tasks probably involve different processing mechanisms if there is a double dissociation, i.e., some patients perform normally on task A but poorly on task B, whereas others perform normally on task B but poorly on task A. Amnesic patients have generally poor long-term memory, but intact short-term memory (see Chapter 7). The reverse problem is relatively rare, but a few such cases have been reported. These cases include KF, a patient who suffered damage in the left parieto-occipital region of the brain following a motorcycle accident. KF had no problem with long-term learning and recall, but his digit span was greatly impaired, and he had a recency effect of only one item under some circumstances (Shallice & Warrington, 1970). However, KF did not perform badly on all short-term memory tasks (see next section).

Peterson and Peterson (1959) studied the duration of short-term memory by using the task of remembering a three-letter stimulus for a few seconds while counting backwards by threes. The ability to remember the three-letter stimulus declined to only about 50% after 6 seconds (see Figure 6.3), showing that information is lost rapidly from short-term memory.

Why does counting backwards cause forgetting from short-term memory? Counting backwards may be a source of interference, or it may divert attention away from the information in short-term memory. Interference and diversion of attention both seem to play a part (e.g., Reitman, 1974). Forgetting from the long-term store involves rather different mechanisms. As is discussed later, it depends mainly on cue-dependent forgetting (i.e., the memory traces are still in the memory system, but are inaccessible).


The multi-store model provided a systematic account of the structures and processes involved in memory. The conceptual distinction between three kinds of memory stores (sensory stores, short-term store, and long-term store) makes sense. In order to justify the existence of three qualitatively different types of memory store, we must show major differences among them. Precisely this has been done. The memory stores differ from each other the following ways:

• Temporal duration.

• Storage capacity.

• Forgetting mechanism(s).

• Effects of brain damage.

Many contemporary memory theorists have used the multi-store model as the starting point of their theories. Much theoretical effort has gone into providing a more detailed account of the long-term store than that offered by Atkinson and Shiffrin (1968, 1971; see Chapter 7).

The multi-store model is very oversimplified. It was assumed that both the short-term and long-term stores are unitary, i.e., that each store always operates in a single, uniform way. Evidence that the short-term store is not unitary was reported by Warrington and Shallice (1972). KF's short-term forgetting of auditory letters and digits was much greater than his forgetting of visual stimuli. Shallice and Warrington (1974) then found that KF's short-term memory deficit was limited to verbal materials such as letters, words, and digits, and did not extend to meaningful sounds (e.g., telephones ringing). Thus, we cannot simply argue that KF had impaired short-term memory. According to Shallice and Warrington (1974), his problems centred on the "auditory-verbal short-term store".

The multi-store model is also oversimplified when it comes to long-term memory. There is an amazing wealth of information stored in our long-term memory, including knowledge that Leonardo di Caprio is a film star, that 2+2=4, that we had muesli for breakfast, and perhaps information about how to ride a bicycle. It is improbable that all this knowledge is stored within a single long-term memory store (see Chapter 7).

Logie (1999) pointed out another major problem with the multi-store model. According to the model, the short-term store acts as a gateway between the sensory stores and long-term memory (see Figure 6.1). However, the information processed in the short-term store has already made contact with information stored in long-term memory. For example, our ability to engage in verbal rehearsal of visually presented words depends on prior contact with stored information concerning pronunciation. Thus, access to long-term memory occurs before information is processed in short-term memory.

Finally, multi-store theorists assumed that the main way in which information is transferred to long-term memory is via rehearsal in the short-term store. In fact, the role of rehearsal in our everyday lives is much less than was assumed by multi-store theorists. More generally, multi-store theorists can be criticised for focusing too much on structural aspects of memory rather than on memory processes.

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