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Module 20
The Foundations of Memory
211
When exposed to this pattern of letters for just one twentieth of a second, most
people could recall only four or fi ve of the letters accurately. Although they knew
that they had seen more, the memory of those letters had faded by the time they
reported the fi rst few letters. It was possible, then, that the information had initially
been accurately stored in sensory memory. But during the time it took to verbalize
the fi rst four or fi ve letters, the memory of the other letters faded.
To test that possibility, Sperling conducted an experiment in which a high,
medium, or low tone sounded just after a person had been exposed to the full pat-
tern of letters. People were told to report the letters in the highest line if a high tone
was sounded, the middle line if the medium tone occurred, or the lowest line at the
sound of the low tone. Because the tone occurred after the exposure, people had to
rely on their memories to report the correct row.
The results of the study clearly showed that people had been storing the com-
plete pattern in memory. They accurately recalled the letters in the line that had been
indicated by the tone regardless of whether it was the top, middle, or bottom line.
Obviously, all the lines they had seen had been stored in sensory memory. Despite
its rapid loss, then, the information in sensory memory was an accurate representa-
tion of what people had seen.
By gradually lengthening the time between the presentation of the visual pattern
and the tone, Sperling was able to determine with some accuracy the length of time
that information was stored in sensory memory. The ability to recall a particular row
of the pattern when a tone was sounded declined progressively as the period between
the visual exposure and the tone increased. This decline continued until the period
reached about one second in duration, at which point the row could not be recalled
accurately at all. Sperling concluded that the entire visual image was stored in sensory
memory for less than a second.
In sum, sensory memory operates as a kind of snapshot that stores information—
which may be of a visual, auditory, or other sensory nature—for a brief moment in
time. But it is as if each snapshot, immediately after being taken, is destroyed and
replaced with a new one. Unless the information in the snapshot is transferred to
some other type of memory, it is lost.
Short-Term Memory
Because the information that is stored briefl y in sensory memory consists of repre-
sentations of raw sensory stimuli, it is not meaningful to us. If we are to make sense
of it and possibly retain it, the information must be transferred to the next stage of
memory: short-term memory. Short-term memory is the memory store in which
information fi rst has meaning, although the maximum length of retention there is
relatively short (Hamilton & Martin, 2007).
The specifi c process by which sensory memories are transformed into short-term
memories is not clear. Some theorists suggest that the information is fi rst translated
into graphical representations or images, and others hypothesize that the transfer
occurs when the sensory stimuli are changed to words (Baddeley & Wilson, 1985).
What is clear, however, is that unlike sensory memory, which holds a relatively full
and detailed—if short-lived—representation of the world, short-term memory has
incomplete representational capabilities.
In fact, the specifi c amount of information that can be held in short-term memory
has been identifi ed as seven items, or “chunks,” of information, with variations up
to plus or minus two chunks. A chunk is a meaningful grouping of stimuli that can
be stored as a unit in short-term memory. According to George Miller (1956), a chunk
can be individual letters or numbers, permitting us to hold a seven-digit phone
number (such as 226-4610) in short-term memory.
But a chunk also may consist of larger categories, such as words or other mean-
ingful units. For example, consider the following list of 21 letters:
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