Short-term Memory - Existence of A Separate Store

Existence of A Separate Store

The idea of the division of memory into short-term and long-term dates back to the 19th century. A classical model of memory developed in the 1960s assumed that all memories pass from a short-term to a long-term store after a small period of time. This model is referred to as the "modal model" and has been most famously detailed by Shiffrin. The exact mechanisms by which this transfer takes place, whether all or only some memories are retained permanently, and indeed the existence of a genuine distinction between the two stores, remain controversial topics among experts.

One form of evidence, cited in favor of the separate existence of a short-term store comes from anterograde amnesia, the inability to learn new facts and episodes. Patients with this form of amnesia, have intact ability to retain small amounts of information over short time scales (up to 30 seconds) but are dramatically impaired in their ability to form longer-term memories (a famous example is patient HM). This is interpreted as showing that the short-term store is spared from amnesia and other brain diseases.

Other evidence comes from experimental studies showing that some manipulations (e.g., a distractor task, such as repeatedly subtracting a single-digit number from a larger number following learning; cf Brown-Peterson procedure) impair memory for the 3 to 5 most recently learned words of a list (it is presumed, still held in short-term memory), while leaving recall for words from earlier in the list (it is presumed, stored in long-term memory) unaffected; other manipulations (e.g., semantic similarity of the words) affect only memory for earlier list words, but do not affect memory for the last few words in a list. These results show that different factors affect short-term recall (disruption of rehearsal) and long-term recall (semantic similarity). Together, these findings show that long-term memory and short-term memory can vary independently of each other.

Not all researchers agree that short-term and long-term memory are separate systems. Some theorists propose that memory is unitary over all time scales, from milliseconds to years. Support for the unitary memory hypothesis comes from the fact that it has been difficult to demarcate a clear boundary between short-term and long-term memory. For instance, Tarnow shows that the recall probability vs. latency curve is a straight line from 6 to 600 seconds (ten minutes), with the probability of failure to recall only saturating after 600 seconds. If there were really two different memory stores operating in this time frame, one could expect a discontinuity in this curve. Other research has shown that the detailed pattern of recall errors looks remarkably similar for recall of a list immediately after learning (it is presumed, from short-term memory) and recall after 24 hours (necessarily from long-term memory).

Further evidence against the existence of a short-term memory store comes from experiments involving continual distractor tasks. In 1974, Robert Bjork and William B. Whitten presented subjects with word pairs to be remembered; however, before and after each word pair, subjects had to do a simple multiplication task for 12 seconds. After the final word-pair, subjects had to do the multiplication distractor task for 20 seconds. In their results, Bjork and Whitten found that the recency effect (the increased probability of recall of the last items studied) and the primacy effect (the increased probability of recall of the first few items) still remained. These results would seem inconsistent with the idea of short-term memory as the distractor items would have taken the place of some of the word-pairs in the buffer, thereby weakening the associated strength of the items in long-term memory. Bjork and Whitten hypothesized that these results could be attributed to the memory processes at work for long-term memory retrieval versus short-term memory retrieval.

Ovid J.L. Tzeng (1973) also found an instance where the recency effect in free recall did not seem to result from the function of a short-term memory store. Subjects were presented with four study-test periods of 10 word lists, with a continual distractor task (20-second period of counting-backward). At the end of each list, participants had to free recall as many words from the list as possible. After free-recall of the fourth list, participants were asked to free recall items from all four lists. Both the initial free recall and the final free recall showed a recency effect. These results went against the predictions of a short-term memory model, where no recency effect would be expected in either initial or final free recall.

Koppenaal and Glanzer (1990) attempted to explain these phenomena as a result of the subjects’ adaptation to the distractor task, which therefore allowed them to preserve at least some of the functions of the short-term memory store. As evidence, they provided the results of their experiment, in which the long-term recency effect disappeared when the distractor after the last item differed from the distractors that preceded and followed all the other items (e.g., arithmetic distractor task and word reading distractor task). Thapar and Greene challenged this theory. In one of their experiments, participants were given a different distractor task after every item to be studied. According to Koppenaal’s and Glanzer’s theory, there should be no recency effect as subjects would not have had time to adapt to the distractor; yet such a recency effect remained in place in the experiment.

One proposed explanation of the existence of the recency effect in a continual distractor condition, and the disappearance of it in an end-only distractor task is the influence of contextual and distinctive processes. According to this model, recency is a result of the final items’ processing context being similar to the processing context of the other items and the distinctive position of the final items versus items in the middle of the list. In the end distractor task, the processing context of the final items is no longer similar to the processing context of the other list items. At the same time, retrieval cues for these items are no longer as effective as without the distractor. Therefore, the recency effect recedes or vanishes. However, when distractor tasks are placed before and after each item, the recency effect returns, because all the list items once again have similar processing context.