Short-Term or Working Memory 429operating laws or solves problems that cannot be solved by
the mechanisms governing long-term retention. The sugges-
tion that most, if not all, short-term forgetting is caused by
proactive interference—the same kind of interference that
controls much of long-term retention—diminishes the ratio-
nale for rejecting a single-system view (see Melton, 1963).
Other early work further supported the case for interference.
Murdock (1961) found less forgetting when one word, rather
than three, was used as the to-be-remembered stimulus (see
Figure 15.2); in addition, Melton reported dramatically dif-
ferent short-term forgetting functions for lists varying from
one to five consonants (a form of within-trial interference that
he termed “intra-unit” interference). Others went on to show
that even retroactive interference could play a role under
some circumstances: For instance, more forgetting is found
when items are presented aloud and the intervening distractor
activity is also auditory (e.g., Proctor & Fagnani, 1978).
However, the fact that interference is operative in short-
term memory environments does not rule out decay; both
decay and interference might be involved. Indeed, this was
the position advocated a decade later by Baddeley and Scott
(1971). They noted that in the Keppel and Underwood (1962)
study, as well as in other studies documenting little effect of
forgetting on the first trial (e.g., Cofer & Davidson, 1968),
performance tended to hover near or at the ceiling. Thus, they
argued, there might have been forgetting, but it was masked
by the high performance levels. To get performance off the
ceiling, they increased the length of the memory list from the
standard three items to five items in one condition and seven
in another. Under these conditions, significant forgetting was
found on the first trial, but it appeared to reach asymptotic
levels by around 5 or 6 s. Moreover, no differences were
found in the slope of the forgetting curves as a function of list
length, and the asymptotic levels of performance were sig-
nificantly above the levels normally found when multiple tri-
als are tested in a session. This suggested that some sort of
decay operates early in the retention interval but is complete
by around 5 s; it also suggested that interference, particularly
from prior trials, must cause the bulk of the forgetting found
later in the retention interval.
The conclusions reached by Baddeley and Scott (1971)
have largely dominated the field for the past three decades.
Most researchers believe that interference plays a significant
role in short-term forgetting—in fact, interference is ac-
knowledged to cause most, if not all, of the forgetting in the
Brown-Peterson task—but few have completely rejected the
concept of decay. As I discuss later, the case for decay was
strengthened initially by the discovery of time-dependent
limitations in short-term memory capacity (the word length
effect; Baddeley, Thomson, & Buchanan, 1975). Moreover, it
was subsequently shown that short-term forgetting is nearly
complete after only a second or two if the recall test appears
unexpectedly; in the traditional Brown-Peterson procedure,
subjects expect the recall test and, consequently, may engage
in elaborative processes that enhance long-term memory for
the list items (see Healy & Cunningham, 1995; Muter, 1980;
Sebrechts, Marsh, & Seamon, 1989). In the absence of elab-
orative processing, which enables a kind of back-up recall
from long-term memory, one is forced to rely exclusively on
the fragile activity trace, which decays rapidly—in a second
or two—in the absence of rehearsal.Temporal DistinctivenessAccording to the standard model, as just discussed, decay of
the activity trace is largely complete after only a few seconds.
Interference, particularly proactive interference, is then
largely responsible for any further forgetting that occurs
during a retention interval. But what specific interference
mechanisms are involved? One common assumption is that
subjects are able to retrieve just-presented items from long-
term memory, after the activity trace has decayed, but suc-
cessful retrieval requires discriminating correct list targets
from incorrect alternatives. Items from earlier trials, as well
as extraexperimental items to a certain extent, form a noisy
background against which the correct item must be selected.
There is a reasonable amount of evidence indicating that
the mechanism for trace discrimination involves temporal or
positional information (see G. D. A. Brown, McCormack, &
Chater, 2001; Neath & Crowder, 1990). After all, time-of-
occurrence information, when available, provides a foolproof
method for distinguishing items from Trial N from those oc-
curring on the previous trial, N–1. Various studies have
shown that forgetting in short-term memory environments
depends importantly on the temporal spacing of items within
a list, the length of the retention interval, and the temporal
spacing between trials in the session. It is not time per se that
predicts retention, but rather the temporal relations among
the items in the experimental session.
In a slight variation of the typical Brown-Peterson task,
Turvey, Brick, and Osborn (1970) asked different groups of
subjects to count backward as distractor activity for either 10,
15, or 20 s. Remarkably, no retention differences were found
among these groups, despite the retention interval differences
(see also Greene, 1996). Of main interest, though, was a crit-
ical trial in which all groups were switched to the same 15-s
distractor interval. Correct recall dropped in the 10-s group
(from .33 to .20), stayed roughly constant in the 15-s group
(.30 to .28), and actually improvedin the 20-s group (.30 to
.38). Notice that the passage of time—and therefore the