The computer model represented an enormous change in the way we thought about memory. It completely refocused memory research. Instead of looking at retention by an undifferentiated memory system, cognitive psychologists began to examine the properties of different storage structures. For a while, most of this research was focused on the sensory registers and primary memory.
At the lowest end of the multistore model of memory lie the sensory
registers -- one for each sensory modality: the icon, the
echo, and other
analogous sensory memories, in which information arriving from
the sensory
surfaces is held for a brief time before being copied to primary
memory.
We
know
this from experiments employing a paradigm devised by George
Sperling (1960), in
which subjects were briefly presented (for a few hundred
milliseconds) with a
3x4 visual array of letters. After the array disappeared,
there was a
retention interval of only 0-5 seconds, and then the subjects
were asked to
report the contents of the array.
The implication was that the entire array was actually represented in iconic memory, because the subjects could report accurately the contents of any randomly selected row -- indicating that the contents of all the rows were available. However, in the whole report condition, subjects could not extract all the information in the array before it rapidly decayed.
A similar experiment was
performed in the auditory domain -- the echo
-- by Darwin, Turvey, and Crowder (1972). In this case,
the subjects were
presented with 3 lists of 3 items each, for a total of 9 items,
presented over
stereo headphones in such a way that each list was perceived as
coming from a
different part of space -- left, middle, and right.
Performance in
whole-report and partial-report conditions, with the partial
reports cued
visually, paralleled those obtained by Sperling.
Apparently, the function of the sensory registers is to hold briefly presented stimulus information long enough to allow attention to be directed to it. In this way, the sensory registers exemplify an important function of memory, which is to free behavior from control by whatever stimulus happens to be present in the environment (because, in this case, the stimulus is no longer present).
This is all very well and good, and the Sperling experiment stimulated several years' worth of research activity on the part of a number of investigators. But it's not clear how useful the sensory registers are. In the real world, stimuli usually remain present in the environment for longer than just a few hundred milliseconds -- and this ecological fact may obviate any need for the sensory registers. As Ralph Haber once put it, in a famous paper, iconic memory may only be useful for reading at night in a lightning storm!
Still the sensory registers are memory storage structures --
the point where
sensory information first makes contact with the cognitive
system. And
they do give an organism the opportunity to react to very brief
events.
At the highest end of the multistore model is a long-term store or long-term memory, also known as secondary memory -- a term that, as noted, was adopted from William James. In theory, secondary memory is the permanent repository of all stored knowledge, both declarative and procedural. For our purposes, these three terms are interchangeable. And, also in theory, the representations in secondary memory reflect considerable processing and transformation. Atkinson and Shiffrin proposed that the capacity of secondary memory was essentially unlimited. Information might be lost from the sensory registers or from primary memory through decay or displacement, but there is essentially no forgetting from secondary memory. Once encoded, memories remain available in storage, and "forgetting" is a matter of gaining access through retrieval processes.
Primary
memory, as conceived in the multistore model, is a
short-term store located
between the sensory registers and secondary memory. For
our purposes, we
may use the terms primary memory, short-term memory,
and short-term
store interchangeably.
In the multistore model, information in the
sensory registers is
not simply subject to attentional selection it is subject to
some actual
information processing as well.
Here is where the dotted arrow in A&S's diagram comes in. In the theory, knowledge of meaningful patterns is stored in secondary memory. But pattern recognition requires direct contact between the sensory registers and secondary memory. In the Atkinson & Shiffrin model, both feature detection and pattern recognition are unconscious -- precisely because they skirt primary memory, which is the (implied) locus of consciousness in the modal model.
In any event, information in primary memory is subject to further, more extensive processing, as it makes further contact with information stored in, and retrieved from, secondary memory.
In the original theory, information was represented in memory in acoustic form -- as, for example, the pronunciation of the letters, numbers, and words in a display or list studied by the subject; or, put another way, as the name of an item, or a description of what the name sounds like.
Primary memory has limited capacity, as reflected in George Miller's famous "magical number 7, plus or minus 2". That is, primary memory can hold about 7 items of information.
There's a story here. Miller made the capacity of short-term memory famous, but -- as he acknowledged -- it wasn't his discovery. Credit for that goes to John E. Kaplan (1918-2013), a psychologist and human-factors researcher who spent his career at Bell Laboratories, the research arm of AT&T. Prior to the 1940s, telephone exchanges typically began with the first two letters of a familiar word, followed by 5 digits -- as in the Glenn Miller tune, "Pennsylvania 6-500" (the actual telephone number of the Pennsylvania Hotel, often claimed as the oldest telephone number in New York City), or the film Butterfield 8 (after an exchange serving the upscale Upper West Side of Manhattan. But the telephone company (there was only one at the time!) worried about running out of phone numbers, and so switched to an all-digit system. This raised the question of how many digits people could hold in memory. Kaplan did the study, and determined that the optimal number was 7. (Kaplan also determined the optimal layout for the letters and numbers on the rotary, and then the touch pad telephone -- reversing the conventional placement on a hand calculator or computer keyboard -- but that's another story).
However, Miller noted that the effective capacity of primary memory can be increased by chunking, or grouping related items together. Thus, the 18-item string
LBJIRTUSALSDFBICIA
would, ordinarily, be too long to keep in primary memory. But if chunked as follows, the whole thing fits very nicely:
LBJ IRT USA LSD FBI CIA.
So, the capacity of primary memory is roughly 7 chunks, not 7 items. If the chunks are big enough -- yes, you can chunk chunks -- primary memory can hold an immense amount of information.
Of course, chunking in primary memory depends a lot on knowledge stored in secondary memory. You have to recognize a string like LBJ as meaningful -- in fact, the initials of an American president -- before you can chunk it (IRT is the acronym for a subway line in New York City). In fact, the whole 18-letter string is the chorus of a song, "Initials", from HAiR: The American Tribal Love-Rock Musical (lyrics by James Rado & Gerome Ragni): once you've heard the song, you never forget the letters.
The distinction between primary and secondary
memory is supported by the serial-position curve,
which plots the probability of recalling any individual item
as a function of its position in the study list. The
serial-position curve is typically bowed, meaning that items
at the beginning and the end of the list have a higher
probability of recall than
items in the middle.
A common interpretation of the primacy and
recency effects is
that they reflect the operations of two different memory
stores:
This interpretation is supported by two lines of experimental evidence:
Slowing
the pace of list presentation increases the primacy effect,
but has no effect on
recency. Apparently, increasing the spacing between
items gives the
subject more time to rehearse each item, and more opportunity
to recode items
into secondary memory.
Increasing
the
retention interval, and filling the retention interval with
distracting
material, reduces the recency effect but has no effect on
primacy. The
distraction effectively displaces items from primary memory,
but has no effect
on items already encoded into secondary memory.
The distinction between primary and secondary
memory is also
supported by neuropsychological evidence from brain-damaged
patients.
Both the experimental and clinical evidence indicates that primary and secondary memory can be dissociated:
However, the strict separation of primary and secondary memory can also be challenged on empirical grounds. For example, primary memory is sometimes referred to as immediate memory. The idea is that items in primary memory are already in consciousness, and immediately available for various operations. By contrast, items in secondary memory appear to be in a latent state: they must be retrieved and brought into consciousness. But it turns out that items in primary memory also need to be retrieved.
This
was shown clearly by a classic memory-scanning
experiment by Sternberg
(1960). In this experiment, subjects were presented with
sets of 1 to 6
digits, which they were asked to hold in primary memory (note
that 6 digits is
well within the normal memory span). Then, they were
presented with a
probe digit, and asked to say whether the probe was in the
memorized list.
This is of course an extremely simple tasks, and subjects
rarely make any errors
in it. But it turns out that the search process takes
time. the more
items there are in the memory set, the longer it takes
subjects to say
"yes" or "no". Thus, items in primary memory are not
immediately available after all. Primary memory must be
searched, just
like secondary memory.
As another example, it turns out that there are
serial-position
curves in retrieval from secondary memory.
Thus, primacy and recency effects do not necessarily reflect retrieval from different memory stores.
What Murdock (1967) called "the modal model of memory" -- the division of memory into three classes of storage structures, and a set of control processes which moved information between them -- had enormous heuristic power. It represented a huge change in the way we thought about memory -- it both exemplified and consolidated the cognitive revolution in the study of memory; and it generated a large number of clever experiments intended to explore the properties of the various storage structures (especially the icon, the echo, and primary memory) and control processes (especially attention). But it also very quickly began to decline.
As noted earlier, the sensory registers are hardly memories at all.
And then there were the doubts raised about the structural distinction between short-term primary memory and long-term secondary memory. Some of these had been raised by Melton (1963), a leading figure in the verbal-learning tradition, even before the modal model had been clearly articulated. But other doubts quickly followed (Murdock, 1972; Craik & Lockhart, 1973; Wickelgren, 1974).
The case against a structural distinction was
put forth most
strongly by Craik and Lockhart (1972), in their analysis of
the capacity of
primary memory. Certainly primary memory seems
to be capacity
limited, s in Miller's "magical number 7". But the
precise
nature of this limitation is unclear.
And then there is what they called the paradox of chunking. Chunking seems to increase the capacity of primary memory, but chunking also relies on knowledge in semantic memory -- it goes beyond a mere acoustic representation, and includes the results of meaning analysis -- like the meaning of trigrams like LBJ and IRT. This begs the question of coding differences between primary and secondary memory. Primary memory is supposed to represent information in acoustic form -- or, at least, some kind of physical code. But with chunking, the information is clearly not in a purely acoustic (or other physical) form any longer.
And
then there is the function of forgetting from primary memory,
famously mapped
out by Brown (1958) and Peterson and Peterson (1959) in what
has come to be
known as the Brown-Peterson paradigm. These
investigators presented
subjects with a consonant trigram such as BNX.
Then, they asked the
subject to count backwards by 3s from a three-digit number,
such as 417,
for an interval of 3 to 18 seconds, and then to report the
trigram. They
observed extremely rapid forgetting: Without distraction,
virtually everyone
remembered the three letters (though, you do have to wonder
about those 5% who
didn't!). But , after only 18 seconds of distraction,
retention dropped to
less than 20% -- a figure that Brown and Peterson attributed
to retrieval from
secondary memory.
The Brown-Peterson function is consistent with the rapid forgetting claimed for primary memory, but there's a problem. To see what the problem is, try the Brown-Peterson experiment on a friend. Give him a consonant trigram, such as BNX, and have him count backwards by 3s from the number 417 for 18 seconds, and then ask him to write down the trigram. He almost certainly got it right. And if you try it out on your entire dormitory, you might get the occasional instance of forgetting, but nothing like the 80%+ observed by Brown and Peterson.
But that's not exactly how Brown and Peterson did their experiments. Instead of having only a single trial per subject, and averaging over subjects, they gave each subject many trials, and averaged across trials, and then across subjects. Apparently, the Brown-Peterson function is observed only after subjects have gone through many trials.
To
check this, Keppel and Underwood (1962) analyzed the
Brown-Peterson function on
individual trials, with retention tested after 3, 9, and 18
seconds. On
the first trial, there was essentially no forgetting, even
after 18 seconds of
distraction. On later trials, however, they observed a
progressive
increase in forgetting. They concluded that the
Brown-Peterson function
represents the build-up of proactive inhibition.
Apparently, items from earlier trials were apparently interfering with items on later trials. But those items from the early trials are no longer in primary memory -- it doesn't have enough capacity. Thus, the Brown-Peterson experiment, which seemed to confirm rapid forgetting from primary memory, isn't really about primary memory after all, because the items that cause the forgetting aren't being held there.
Interestingly,
we
can also show release from proactive inhibition by
changing the
category of the item being memorized. Wickens (1972)
gave subjects a
series of three trials, with all items drawn from the same
category -- e.g., 3
consonants. In this phase of the experiment, he observed
a progressive
buildup of PI, just as in Keppel and Underwood (1962).
On the 4th trial,
Wickens shifted the item type for half the subjects -- e.g.,
from 3 consonants
to 3 digits. In this condition, PI disappeared, and
recall returned to the
level observed on Trial 1. As the 5th and 6th trials
continued with the
new item type, PI continued to build up again -- again, just
as in Keppel and
Underwood.
Release from PI shows that the interference effect is controlled by the categorization of the item. But these categories are a matter of secondary memory, which go way beyond any acoustic representation. Forgetting in the Brown-Peterson paradigm is not a function of primary memory; rather, the buildup and release of PI must reflect interference from secondary memory. This phenomenon provides further evidence that the forgetting function determined in the Brown-Peterson paradigm doesn't bear on the nature of primary memory, separate from secondary memory.
The general thrust of these kinds of experiments is to cast doubt on the structural distinction between primary (short-term) and secondary (long-term) memory -- which is the sine qua non of the multistore model.
Still,
there is something special about primary memory, which is that
we can
deliberately maintain items in an active state by means of
rehearsal.
Baddeley and Hitch (1974) proposed that we rename this memory
structure working
memory -- because this is where we perform cognitive
activities.
According to Baddeley and Hitch, working memory consists of a
central
executive which controls several slave systems:
The slave systems maintain items in an active state. These are very transient memories: once rehearsal stops, the items disappear from working memory; and newly incoming information will replace old information.
So what's the difference between working memory and primary memory? While primary memory was considered as an intermediary between the sensory registers and secondary memory, this is not the case for working memory. Baddeley and Hitch propose that information is processed directly into secondary memory, with no need for an intermediate way-station like primary memory. Then, when the cognitive system wants to use stored information, it copies it into working memory.
John Anderson (1983) has proposed an expanded
conception of
working memory, which he considers to consist of those
representations --
whether acquired through perception or retrieved from memory
-- that is
currently in a high state of activation. Other
representations are also
part of working memory:
Baddeley and Hitch's conception of working memory has become extremely popular among cognitive psychologists, and has stimulated cognitive neuroscientists to search for the neural substrates of working memory in the prefrontal cortex.
This page last modified 05/27/2014.