434 Sensory and Working Memory
proactive interference. On interference trials, one or more
items in the first list were drawn from the same taxonomic
category as items in the second list; on control trials the items
were unrelated across the lists. When subjects were tested im-
mediately, Tehan and Humphreys found no evidence for
proactive interference (recall of control trials was equal to in-
terference trials), but a significant control advantage emerged
when a 2-s distractor task (reading digits aloud) occurred
prior to recall.
If short-term memories are immediately available in con-
sciousness, requiring no cue-based retrieval, then there is no
reason to expect interference from a prior trial: One does not
need to discriminate current items from previous items be-
cause the former are still active in awareness. However, im-
munity from interference turns out to be illusory. Semantic
similarity between the first and second lists fails to hurt per-
formance only because, on immediate tests, subjects can use
residual phonemic information from the second list as a cue
to help discriminate second- from first-list items. If the first
and second lists contain items that rhyme, immunity disap-
pears and significant proactive interference is found on the
immediate test (see Tehan & Humphreys, 1995, 1996, 1998).
Presumably, these subjects still have residual phonemic in-
formation available at test, but that information no longer
uniquely specifies items from the second list.
Existing evidence strongly supports the idea that imme-
diate retention is cue-driven. First, as described earlier, imme-
diate recall is very sensitive to item characteristics, such as
word frequency, lexicality, and concreteness. Most re-
searchers assume, as a result, that the short-term activity trace
must be interpreted prior to recall—that is, the short-term
activity trace acts essentially as a cue to guide retrieval. Sec-
ond, there is a considerable literature on release from proac-
tive interference that confirms the importance of cues in
immediate retention. In a typical release experiment, succes-
sive lists are drawn from the same conceptual class (e.g., fruit).
Recall gets worse over trials, presumably because people have
a difficult time discriminating items on the current trial from
conceptually similar items that occurred on previous trials. On
the release trial, however, list items are drawn from a new con-
ceptual class (e.g., moving from fruit to animals) and perfor-
mance improves substantially (see Wickens, 1970).
This effect is most commonly interpreted to mean that
people are using conceptual class as a cue to guide short-term
recall; the effectiveness of this cue, in turn, hinges on its abil-
ity to predict current trial information. When successive lists
are presented from the same conceptual class, the cue be-
comes overloaded (Watkins & Watkins, 1975) which means
it starts to predict many items, especially those from previous
trials. On release trials, however, the distinctive power of the
cue is regained: It now uniquely specifies information from
the current trial, and performance improves. Particularly
strong support for this interpretation comes from experiments
in which the nature of the retrieval cue is manipulated at test,
after the critical list has actually been presented. It is possible
to record significant levels of release, at test, if discriminating
cues are provided (see Dillon & Bittner, 1975; Gardiner,
Craik, & Birtwistle, 1972).
Diagnostic evidence for cue-driven retention also comes
from the study of errors in immediate recall. When mis-
takes are made, the errors that occur tend not to be random,
but rather follow certain patterns. For example, when a list
item is recalled in an incorrect serial position, it is typically
placed in a nearby position (e.g., the third item on the list is
placed incorrectly in the second or fourth serial position).
Recorded error gradients are systematic, showing that incor-
rect item placements drop off regularly as distance from the
original position of occurrence increases (e.g., Healy, 1974).
If lists are grouped, and people wrongly place an item from
one group into another group, the item tends to be put in an
identical relative serial position (e.g., Henson, 1999). Finally,
when people intrude an item from a previous list, it is likely
to have occurred at the same serial position in that list (Estes,
1991; Henson, 1996). Data of this sort indicate that people
are not simply outputting activated items from short-term
memory, but rather may be using some kind of position cue to
help decide what occurred on the just-presented list.
Clearly, any full understanding of short-term memory will
require some specification of how short-term memories are
retrieved and translated into performance. The idea that im-
mediate retention is cue-driven is appealing, in particular, be-
cause it is consistent with how most researchers conceive of
long-term retention (see the chapter by Roediger & Marsh in
this volume). All forms of remembering are cue-driven, al-
though the nature of the cues, as well as the mechanics of the
retrieval process, may differ between the short and the long
term. This means that any empirical dissociations between
short- and long-term retention, by themselves, will not be di-
agnostic of separate short- and long-term memory systems.
One could still hold to a single system view, in which all
forms of remembering are guided by the same set of general
operating principles, and simply attribute the dissociations to
differences in the composition of active retrieval cues (see
Nairne, 2002).The Working Memory ModelIn the remainder of the chapter, I discuss some popular theo-
retical conceptions of the short-term memory system, begin-
ning with the working memory model championed by Alan