Atkinson-Shiffrin memory model

The Atkinson-Shiffrin model, Multi-store model or Multi-memory model is a psychological model proposed in 1968 as a proposal for the structure of memory. It proposed that human memory involves a sequence of three stages:


 * 1) Sensory memory (SM)
 * 2) Short-term memory (STM)
 * 3) Long-term memory (LTM)

Summary
The multi-store model of memory is an explanation of how memory processes work. You hear and see and feel many things, but only a small number are remembered. The model was first described by Atkinson and Shiffrin in 1968.

Sensory memory
The sense organs have a limited ability to store information about the world in a fairly unprocessed way for less than a second. The visual system possesses iconic memory for visual stimuli such as shape, size, colour and location (but not meaning), whereas the hearing system has echoic memory for auditory stimuli. Coltheart et al (1974) have argued that the momentary freezing of visual input allows us to select which aspects of the input should go on for further memory processing. The existence of sensory memory has been experimentally demonstrated by Sperling (1960) using a tachistoscope.

Short-term memory
Information selected by attention from sensory memory, may pass into short term memory (STM). This allows us to retain information long enough to use it, e.g. looking up a telephone number and remembering it long enough to dial it. Peterson and Peterson (1959) have demonstrated that STM last approximately between 15 and 30 seconds, unless people rehearse the material, while Miller (1956) has found that STM has a limited capacity of around 7±2 ‘chunks’ of information. STM also appears to mostly encode memory acoustically (in terms of sound) as Conrad (1964) has demonstrated, but can also retain visuospatial images.

Long-term memory
LTM provides the lasting retention of information and skills, from minutes to a lifetime. Long term memory appears to have an almost limitless capacity to retain information, but it could never be measured as it would take too long. LT information seems to be encoded mainly in terms of meaning (semantic memory) as Baddeley has shown, but also retains procedural skills and imagery.

Evidence
In a type of memory test called a free recall task, subjects memorize a list of words and then repeat them. When they are allowed to repeat the words immediately after memorizing them, subjects recall the last few words in the list much better than words in the middle. But if subjects are delayed before they are given a chance to repeat the words, this effect disappears. This effect is called the recency effect. Glanzer and Cunitz (1966) conducted this experiment and Atkinson and Shiffrin used this as evidence that the words at the end were still being held in the short-term memory.

In another version of the free recall task, subjects were given a list of words that sounded similar (like cat, map, man, mat). On these tests, subjects performed very poorly. When they were given lists of words that sounded unrelated, but had similar meanings, their immediate recall was normal. But when asked to recall the similar words after a delay, suddenly they performed much better than they did after delays when the words were unrelated. Atkinson and Shiffrin believed this to show that short term memory uses acoustic information, but long term memory encodes things based on their meaning.

Amnesics with impaired long-term memory systems have been given similar tasks. They tend to perform just as well as healthy control subjects, which led Atkinson and Shiffrin to believe this was still more evidence for separate short-term and long-term memory systems.

Linearity
Some may argue that the Multi-Store model is much too linear, and does not accommodate for the subdivisions of STM and LTM memory stores -- particularly, its structure does parallel well within the neurological explanations of where and how memory is stored; the model suggesting that memory would be purposely disregarded by physiological processes and stored in a linear memory sequence -- only being able to handle or deal with memory which has been "added" to this stream, which is juxtaposed to the composition of neurons including axons and dendrites.

The concept of the "stream of memory" in this model also has internal criticism, which would mean that by definition, its own stream of memory was inconstant and often discarded for newer information with relatively no emphasis or importance on the information, which precedes it. A supposed example of this was given in the control tests for the studies; the asymptote of the data largely revealing that the primary and recency areas of data were well remembered, overshadowing the asymptote. While this may be an affirmation of the decay of memory and to a small extent, the idea of 3 separate areas for memory storage would be juxtaposed to the principle of intensity and rate of firing within neurons, as well as the idea of the "Ionised sodium gate" model of action potentials.

In the case of sensory memory, the model does not acknowledge the asynchronous nature of the neural activity which occurs between anatomical structures, such as the relationship between a neuron and a motor end-plate - an example of this would be the reference to sensory memory being used to perform physical processes such as motor function, which suggests that once an action is performed, it is remembered for 3 seconds and then begins a process of rapid decay.

Monolithicity
While the model deals with the several forms of memory in its model, it does not take into account the way in which the information is presented, nor does it take into account biological, or internal factors which may interfere with an individual's ability to respond or understand the experiment - including an individual's cognitive ability, or previous experience with learning techniques.

Whilst there is studies to suggest that some people, such as Clive Wearing have limited memory capacity, it is not enough evidence in itself to suggest that the brain has 3 separate memory stores within its structure. The reasoning is that whilst these cases can be somewhat explained by the multi-store model, other cases such as those of autistic savants, completely disband the theory of repetition and rehearsal within the multi store model, due to their ability to be able to recall precise figures with clarity and lucidity without the need for rehearsal; showing no process of decay, nor any other factors mentioned in the original model.

Had the model included internal factors which influenced each stage of the memory process, it would have been somewhat more credible in explaining such phenomena.

Later Developments
This model provided an important framework for learning and memory theories to evolve from, but a number of problems with it have been cited since. Since each element in the model builds off the one preceding, it cannot explain the rare situations where short-term memory is impaired, but long-term memory is not. According to this model, information that can't make it through short-term memory has no way to become encoded in long-term memory.

Atkinson and Shiffrin also refrain from proposing any mechanisms or processes that might be responsible for encoding memories and transferring them between the three systems. The model is a hypothetical layout of the function of memory systems, but not in any way representative of a physical "map" of memory systems.

Many newer models have been created that can better account for these other characteristics, and a tremendous body of research on the physical layout of memory systems has emerged. As the oldest and simplest model, this is can no longer be considered entirely accurate or comprehensive.

The rehearsal loop also must be included in the transfer of memory into LTM from the STM, it is said that for things to be transferred correctly they must negate the rehearsal loop to ensure full remembrance.

Reference
Atkinson, R.C. & Shiffrin, R.M. (1968) Human memory: A proposed system and its control processes. In K.W. Spence and J.T. Spence (Eds.), The psychology of learning and motivation, vol. 8. London: Academic Press.