Atkinson and Shiffrin (1971)
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First published online 08:30 30th July 2002
This version dated 08:30 30th July 2002
Memory Models Before Atkinson and Shiffrin (1971)
Atkinson and Shiffrin (1971) have been justifiably widely cited for their analysis of the nature and functional layout of the mind's memory systems, but before looking in detail at their work we need firstly to note some of the explanatory problems they faced at the time.
The idea that the brain contains a limited capacity short-term memory (STM) able to "consolidate" a selected portion of its throughput into a much larger capacity long-term memory (LTM) goes back to the end of the nineteenth century (Maudsley, 1876; James, 1890; Müller and Pilzecker, 1900). Such "two-store" models of memory were popular for many decades because they were consistent with "cell assembly" explanations of memory as a physiological process (Lorente de No, 1938; Hebb, 1949): STM could readily be seen as neural electrical activity, and LTM could readily be seen as consequent changes in neural microstructure. Nevertheless, the two-store model had to be forcibly upgraded in 1960, following George Sperling's research into visual iconic memory (Sperling, 1960), and it continued to demand further refinement throughout the 1960s in the light of data from a succession of cleverly designed verbal learning experiments (eg. Peterson and Peterson, 1959; Conrad, 1964; Baddeley, 1966). Here are some of the most influential lines of experimentation from that period:
The Phonological Similarity Effect: The phonological similarity effect refers to an STM impairment when presented with acoustically similar material, that is to say, items which sound alike. It was first detected by Conrad (1964), who found that misrecollections of target letters were more likely to be acoustically similar than not. Thus "D" would be more commonly an error for "B" (with which it rhymes) than for "R" (with which it does not rhyme). Where consonant sequences were to be memorised, Conrad and Hull (1964) found that acoustically similar sequences such as "B-G-V-P-T" were more prone to error than acoustically dissimilar sequences such as "Y-H-W-K-R". The same effect was found where word sequences were to be memorised, with "man-mad-cad-mat-cap" being more prone to error than "pit-day-cow-sup-bar" (Baddeley, 1966).
The Semantic Similarity Effect: The semantic similarity effect refers to an LTM impairment when presented with semantically similar material, that is to say, items which can be associated by their meaning. It was first detected by Baddeley (1966), who found that semantically similar sequences such as "large-great-huge-long-big" were more prone to recall error after 20 minutes than semantically disparate sequences such as "old-wet-strong-thin-deep". He also detected a weak semantic similarity effect in STM.
Atkinson and Shiffrin's (1971) Memory Model
It was this rapidly expanding body of empirical data which Atkinson and Shiffrin set out to make sense of. They published both mathematical and diagrammatic models of memory in the mid-to-late 1960s (Atkinson and Shiffrin 1965, 1968), and then combined the strengths of both approaches in their 1971 model, a model which was so useful as a general purpose summary of what was going on during a memory task that it soon came to be called the "modal" (ie. "popular" or "standard") model (Murdock, 1971). Here is that model:
Atkinson and Shiffrin's (1971) Model of Memory: Here is the model as published. It represents the stages through which sensory information has to go in order to influence behaviour. Three stages are identified, ranged sequentially from left-to-right, as follows:
Sensory Registers: Sensory input is initially detected by an array of sensory registers. These contain memory resources with a lifespan of only a few hundred milliseconds. They include Sperling's iconic memory in the visual modality, plus similar VSTMs in the auditory and other sensory modalities. Each sensory register allows some rudimentary processing to take place before the input is passed to the next stage.
Short Term Store (STS): This contains memory resources with a lifespan of only a few seconds. It is subdivided into a working memory component and a control processes component. The working memory component provides a general purpose processing resource, whilst the control processes have more specific purposes. Four control processes are formally marked on the diagram, but more are mentioned in Atkinson and Shiffrin's supporting text, and others in their 1968 paper. Together, they are responsible for cross-associating current sensory input with the contents of LTM, thus allowing past knowledge to modulate current and future behaviour. Here is the full list:
Long Term Store (LTS): This is a high-volume memory with a potential lifespan of decades. Indeed, so much information can be stored that the main problem is finding the particular bit you are interested in. This is made considerably easier by dividing the content up into logical "subsets", accessed using the STS retrieval strategies described above. "This portrayal of the memory system almost entirely in terms of the operations of the short term store is quite intentional" (p84).
Redrawn from Atkinson and Shiffrin (1971). This graphic Copyright © 2002, Derek J. Smith.
It is important to note that the proposed model is a functional, or logical, model of the layout of the memory system, and not a physical one. It tries to describe what is going on, not where. The authors themselves point out that the physical separation of two mental processes on a model "does not require that the two stores necessarily be in different parts of the brain or involve different physiological structures[; one] might consider the short-term store simply as being a temporary activation of some portion of the long-term store" (p83). Specifically, STS can be seen as a state of excitation within LTS. For an attempt to quantify the flow of information from sensory input via VSTM and STM to LTM, and then out again via motor action, see Frank (1963).
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