Short-Term or Working Memory 427recency advantage is reduced or eliminated. In this case it
was assumed that the suffix, because of the limited capacity
of PAS, interfered with the echoic traces for the last list item
or two, eliminating the recency boost.
Initially, a great deal of evidence accumulated supporting
the PAS account. For example, delaying the suffix by a few
seconds typically reduces its interfering effect, supporting the
proposal that the contents of PAS have a useful lifetime of
only a few seconds. In addition, the destructive power of the
suffix depends importantly on its acoustic, rather than its se-
mantic or categorical, similarity to the list items (J. Morton,
Crowder, & Prussin, 1971). What is stored in PAS, according
to the model, is a raw, uncategorized sensory trace; conse-
quently, one would expect the modality and suffix effects to
show sensitivity only to acoustic variables. Crowder (1978)
further showed that if list items are comprised of homo-
phones presented aloud (plus visually for identification),
such as pare, pair, pear,no enhanced recency is found. In this
case the lingering echoic information, although still stored in
PAS, is nondiscriminative acoustically: It cannot be used se-
lectively to correct the short-term memory records for re-
cency items (see Nairne, 2001, for a fuller discussion).
In the 1980s, however, support for the PAS model was
weakened considerably by the demonstration of modality ef-
fect patterns for nonacoustic presentation modes. For exam-
ple, sharp recency effects were obtained for lip-read stimuli
(Campbell & Dodd, 1980) and occurred when subjects silently
mouthed visual stimuli (Greene & Crowder, 1984; Nairne &
Walters, 1983). Neither lipreading nor mouthing involves
sound, thus precluding a role for PAS, yet both produced
serial position curves that mimicked those found for auditory
presentation. The suffix effect was also discovered to be sensi-
tive in some cases to conceptual attributes (Ayres, Jonides,
Reitman, Egan, & Howard, 1979; Neath, Surprenant, &
Crowder, 1993), and to last over intervals considerably longer
than a few seconds (Watkins & Watkins, 1980). Although
various attempts were made to rescue the PAS model from
conflicting data (see Greene, 1992), the account generally
has fallen into disfavor (see Neath, 1998). Instead, both the
modality and suffix effects are now widely believed to be
short-term memory phenomena, although some form of resid-
ual perceptual memory may play an important role (see
Nairne, 1988, 1990).
SHORT-TERM OR WORKING MEMORY
Whereas sensory memories tend to be veridical copies of the
environment, short-term memories comprise the stuff of im-
mediate experience. Consider the process of remembering a
telephone number as you cross the room. The numbers, no
longer physically present, remain active in consciousness be-
cause you engage in a process of internal repetition (in what
appears to be a kind of inner voice). If you are distracted prior
to reaching the phone, or fail to rehearse, the numbers are
likely to vanish, leaving you with considerable uncertainty—
a number here or there perhaps, but little or no confidence
about the final order.
This description, which corresponds to subjective experi-
ence, actually represents the standard way that most memory
researchers think about remembering over the short term
(see Nairne, 2002). Permanent knowledge structures are
activated, creating short-term memories, which renders the
activated information immediately and directly recallable.
Because of inherent attentional and resource limitations (see
the chapter by Egeth & Lamy in this volume), only a certain
number of items can be refreshed, through rehearsal, prior to
loss, creating the familiar limitations in memory span (e.g.,
Miller, 1956). The whole process is somewhat akin to a jug-
gler’s attempt to maintain a set of plates in the air: The ca-
pacity of the juggler is determined by how well he or she can
counteract the forces of gravity by effectively retossing each
plate before it hits the ground (also see Nairne, 1996).
The standard model of short-term memory successfully
explains a wide range of empirical data, everything from the
recency effect in free recall to the intricacies of immediate
serial recall (see Healy & McNamara, 1996, for a review). In
recent years, various researchers have attempted to formalize
the mechanics of how storage is controlled in the form of
computer simulation models, and I review some of these
models later in the chapter. However, questions remain about
the proper interpretation of how we remember over the short
term. Not all researchers accept the standard juggler model of
short-term memory, choosing instead to opt for general
mnemonic principles that apply over both the short and the
long term (e.g., Melton, 1963; Nairne, 2002). In the fol-
lowing sections, I review the empirical data base on short-
term retention with an eye toward shedding light on these
controversies.Forgetting Over the Short TermAny discussion of short-term memory is properly begun with
the topic of forgetting. As noted previously, it is the fact that
we forget rapidly over the short term that produces the famil-
iar limitations in memory span. The concept of a short-term
memory capacity, in effect, is meaningful only because we
typically fail to remember certain portions of a presented
memory list. For many years, immediate retention was largely
ignored by memory researchers. Instead, the focus was placed