P Through T: “Short-Term Memory and Long-Term Memory Are Still Different”
A commonly expressed view is that short-term memory (STM) is nothing more than activated long-term memory. If true, this would overturn a central tenet of cognitive psychology—the idea that there are functionally and neurobiologically distinct short- and long-term stores. Here I present an updated case for a separation between short- and long-term stores, focusing on the computational demands placed on any STM system. STM must support memory for previously unencountered information, the storage of multiple tokens of the same type, and variable binding. None of these can be achieved simply by activating long-term memory.
Oberauer depicts his model as having three concentric regions—activated LTM, the direct access region, and the focus of attention, all of which select a different subset of LTM representations (shown by the lines and nodes). However, while the diagram and the term region imply that the direct access region is simply a subset of activated LTM, the operations taking place within the direct access region involve much more than activation. According to Oberauer “the region of direct access is a mechanism for establishing and holding temporary bindings between contents. . . . By supporting arbitrary bindings between virtually any content with any context, this system enables the compositionality of thought that many theorists regard as a hallmark of human cognition: we can create an unlimited number of different ideas by freely combining content elements into new structures.” The direct access region is therefore much more than just an activated region of LTM. It must have all of the computational machinery needed to support all aspects of higher cognition, and to have the same functionality as the central executive and episodic buffer components of the Baddeley and Hitch WM model. As with Cowan’s model, it is not clear whether activated LTM alone plays an essential part in the model’s operation, or in its ability to retain information over the short-term. The fact that Oberauer’s model needs to be supplemented with a binding mechanism reinforces the argument that a model of STM needs more than just activated LTM to make it work. Indeed, the big challenge for any model that might be based purely on activation of LTM would be to make it work. The possibility that an STS might be based on pointers rather than copies has a number of interesting implications. If the pointers address representations in LTS then this would greatly complicate the relationship between STS and LTS. However, pointers might also operate within the bounds of a self-contained modality-specific STS. As we will see, the advantage of this option is that it can make very efficient use of the available storage capacity. Far from blurring the lines between STS and LTS, consideration of how a pointer system might work actually reinforces the case for modality-specific short-term stores.
How many phases of a memory are there? In a naïve view of memory, it could be made all of one cloth. Some people have a good ability to capture facts and events in memory, whereas others have less such ability. Yet, long before there were true psychological laboratories, a more careful observation must have shown that there are separable aspects of memory. An elderly teacher might be seen relating old lessons as vividly as he ever did, and yet it might be evident that his ability to capture the names of new students, or to recall which student made what comment in an ongoing conversation, has diminished over the years. The scientific study of memory is usually traced back to Hermann Ebbinghaus (1885/1913 translation), who examined his own acquisition and forgetting of new information in the form of series of nonsense syllables tested at various periods upto 31 days. Among many important observations, Ebbinghaus noticed that he often had a “first fleeting grasp … of the series in moments of special concentration” (p. 33) but that this immediate memory did not ensure that the series had been memorized in a way that would allow its recall later on. Stable memorization sometimes required further repetitions of the series. Soon afterward, James (1890) proposed a distinction between primary memory, the small amount of information held as the trailing edge of the conscious present, and secondary memory, the vast body of knowledge stored over a lifetime. The primary memory of James is like the first fleeting grasp of Ebbinghaus. The Industrial Revolution made some new demands on what James (1890) called primary memory.
Usually, short-term memory plays a vital role in shaping our ability to function in the world around us, but it is limited in terms of both capacity and duration. Disease and injury can also have an influence on the ability to store short-term memories as well as convert them into long-term memories. As researchers continue to learn more about factors that influence memory, new ways of enhancing and protecting short-term memory may continue to emerge.
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