ShortTerm Memory and Working Memory

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Chapter 6
Memory
eyes fixate at one point for about one-fourth of a second and then rapidly jump to a
new position. You perceive smooth motion because you hold the scene in your visual
sensory register until your eyes fixate again. Similarly, when you listen to someone
speak, your auditory sensory register allows you to experience a smooth flow of information, even though there are actually short silences between or within words.
The fact that sensory memories fade quickly if they are not processed further is actually an adaptive characteristic of the memory system (Baddeley, 1998). You simply cannot deal with all of the sights, sounds, odors, tastes, and touch sensations that come to
your sense organs at any given moment. Using selective attention, you focus your
mental resources on only part of the stimuli around you, thus controlling what information is processed further in short-term memory.
Short-Term Memory and Working Memory
The sensory registers allow your memory system to develop a representation of a stimulus. However, they can’t perform the more thorough analysis needed if the information is going to be used in some way. That function is accomplished by short-term
memory and working memory.
Short-term memory (STM) is the part of your memory system that stores limited amounts of information for up to about eighteen seconds. When you check the
newspaper or an on-screen guide for the channel number of a TV show you want to
watch and then keep that number in mind as you switch channels, you are using shortterm memory. Working memory is the part of the memory system that allows us to
mentally work with, or manipulate, the information being held in short-term memory. When you mentally calculate what time you have to leave home in order to have
lunch on campus, return a library book, and still get to class on time, you are using
working memory.
Short-term memory is actually a component of working memory, and together these
memory systems allow us to do many kinds of mental work (Baddeley, 2003; Engle &
Oransky, 1999). Suppose you are buying something for eighty-three cents. You go
through your change and pick out two quarters, two dimes, two nickels, and three pennies. To do this you use both short-term memory and working memory to remember
the price, retrieve the rules of addition from long-term memory, and keep a running
count of how much change you have so far. Now try to recall how many windows there
learn are on the front of the house or apartment where you grew up. In answerby
this question, you probably formed a mental image of the building. You
doing ing
used one kind of working-memory process to form that image, and then you
maintained the image in short-term memory while you “worked” on it by counting the
windows. So working memory has at least two components: maintenance (holding
information in short-term memory) and manipulation (working on that information).
2
selective attention The process of
focusing mental resources on only part
of the stimulus field.
short-term memory (STM) A stage of
memory in which information normally
lasts less than twenty seconds; a component of working memory.
working memory Memory that allows
us to mentally work with, or manipulate, information being held in shortterm memory.
Encoding in Short-Term Memory The encoding of information in short-term
memory is much more elaborate and varied than encoding in the sensory registers
(Brandimonte, Hitch, & Bishop, 1992). Acoustic coding (by sound) seems to dominate.
This conclusion comes from research on the mistakes people make when encoding
information in short-term memory. These mistakes tend to involve the substitution of
similar sounds. For instance, Robert Conrad (1964) showed people strings of letters
and asked them to repeat the letters immediately. Among their most common mistakes
was the replacement of the correct letter with another that sounded like it. So if the
correct letter was C, it was often replaced with a D, P, or T. The participants made these
mistakes even though the letters were presented visually, without any sound. Studies in
several cultures have also shown that items are more difficult to remember if they sound
similar. For example, native English speakers do less well when they try to remember
a string of letters like ECVTGB (which all have similar sounds) than when trying to
remember one like KRLDQS (in which there are different sounds).
Encoding in short-term memory is not always acoustic, however. Information in
short-term memory also can be encoded visually, semantically, and even kinesthetically
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Storing New Memories
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Source: Howard (1983).
FIGURE
6.3
The Capacity of Short-Term
Memory
Here is a test of your immediate memory span (Howard,
1983). Ask someone to read to
you the numbers in the top row at the rate
of about one per second; then try to repeat
them back in the same order. Then try the
next row, and the one after that, until you
make a mistake. Your immediate memory
span is the maximum number of items you
can repeat back perfectly.
doing
2
learn
by
(in terms of physical movements; Best, 1999). In one study, deaf people were shown a
list of words and then asked to immediately write down as many as they could remember (Shand, 1982). When these participants made errors, they wrote words that are
expressed through similar hand movements in American Sign Language, rather than
words that sounded similar to the correct words. Apparently, these individuals had
encoded the words on the basis of the movements they would use when making the
signs for them.
How much information can you
hold in short-term memory? The simple test presented in Figure 6.3 will help you determine your immediate memory span, which is the largest number of items you can
recall perfectly after one presentation. If your memory span is like most people’s, you
can repeat six or seven items from the test in this figure. And you should come up with
about the same result whether you use digits, letters, words, or virtually anything else
(Pollack, 1953). George Miller (1956) noticed that many studies using a variety of tasks
showed the same limit on the ability to process information. This “magic number,”
which is seven plus or minus two, appears to be the immediate memory span or
capacity of short-term memory, at least in laboratory settings. In addition, the “magic
number” refers not only to discrete elements, such as words or digits, but also to meaningful groupings of information, called chunks.
To see the difference between discrete elements and chunks, read the
learn following letters to a friend, pausing at each dash: FB-IAO-LM-TVQ-VCBby
doing MW. The chances are very good that your friend will not be able to repeat
this string of letters perfectly. Why? There are fifteen letters, which exceeds most people’s immediate memory span. Now, give your friend the test again, but group the letters like this: FBI-AOL-MTV-QVC-BMW. Your friend will probably repeat the string
easily. Although the same fifteen letters are involved, they will be processed as only five
meaningful chunks of information.
Storage Capacity of Short-Term Memory
2
Chunks of information can be quite complex. If you
heard someone say, “The boy in the red shirt kicked his mother in the shin,” you could
probably repeat the sentence perfectly. Yet, it contains twelve words and forty-three letters. How can you repeat the sentence so effortlessly? The answer is that you are able
to build bigger and bigger chunks of information (Ericsson & Staszewski, 1989). In this
case, you might represent “the boy in the red shirt” as one chunk of information rather
than as six words or nineteen letters. Similarly, “kicked his mother” and “in the shin”
represent separate chunks of information.
Learning to use bigger and bigger chunks of information can improve short-term
memory. In fact, children’s short-term memories improve partly because they
gradually become able to hold as many as seven chunks in memory and also because
they get better at grouping information into chunks (Servan-Schreiber & Anderson,
1990). Adults also can greatly increase the capacity of their short-term memory by
using more efficient chunking. For example, after extensive training, one college student increased his immediate memory span from seven to eighty digits (Neisser,
2000a). So although the capacity of short-term memory is more or less constant
(from five to nine chunks of meaningful information), the size of those chunks can
vary tremendously.
The Power of Chunking
immediate memory span The maximum number of items a person can
recall perfectly after one presentation
of the items.
chunks Stimuli that are perceived
as units or meaningful groupings of
information.
Duration of Short-Term Memory Why don’t you remember every phone number you ever called or every conversation you ever had? The answer is that unless you
do something to retain it, information in short-term memory is soon forgotten. This
feature of short-term memory is adaptive because it gets rid of a lot of useless information, but it can also be inconvenient. You may have discovered this if you ever
looked up a phone number, got distracted before you could call it, and then forgot
the number.