436 Sensory and Working Memory
the phonological loop, such as articulatory suppression and
the word length effect, also affect one’s ability to learn novel
phonological forms, such as those required in the learning of
a second language (see Baddeley et al., 1998). The learning
of new words by children can also be predicted reasonably
well by nonword repetition, a task that is assumed to tap
functioning of the phonological loop. Finally, patients who
show severe impairments in short-term memory tasks but
show generally intact long-term memory and learning appear
to have a selective deficit in the long-term learning of phono-
logical information (Papagno, Valentine, & Baddeley, 1991).
Baddeley et al. (1998) argue that the phonological loop may
have evolved primarily to store unfamiliar sound patterns
during time periods when more permanent memory records
are being constructed.
The Visuo-Spatial Sketchpad
Whereas the phonological loop handles the temporary storage
of verbal and acoustic information, the visuo-spatial sketch-
pad controls short-term processing and retention of visuo-
spatial material. Like the phonological loop, the sketchpad
probably has separable components, controlling visual, spa-
tial, and possibly kinesthetic information (Baddeley, 2000;
Baddeley & Logie, 1999). The visual component, which
helps to retain visual patterns, is known as the visualcache;
the capacity to remember sequences of spatial movements is
attributed to an innerscribe(see Logie, 1995).
Most of the research investigating the sketchpad has em-
ployed dual-task methodologies. The goal is to demonstrate
selective interference, thereby dissociating the capacity to re-
tain visual, spatial, or verbal information, or combinations of
these types of information. If subjects are asked to learn a list
of words using an imagery mnemonic, which presumably
taps the sketchpad more than the phonological loop, perfor-
mance is hurt by the concurrent requirement to track a mov-
ing spot of light; the same tracking task has little, if any,
effect on performance when subjects use a verbal-based rote
learning strategy (Baddeley & Lieberman, 1980). Changing,
but irrelevant, visual materials have been shown to disrupt
the short-term retention of visual information (e.g., Quinn &
McConnell, 1996); the retention of spatial patterns can also be
selectively disrupted by spatial movements during a retention
interval (see Baddeley & Logie, 1999). Collectively, these
dissociations bolster the case for proposing separate storage
mechanisms for verbal, spatial, and visual information.
However, at this point, there is no firm consensus on the
inner workings of the sketchpad. It is unclear how dissocia-
ble the visual cache and the inner scribe will turn out to be, or
the extent to which the different components draw on the
same cognitive resources. Questions have also been raised
about the relationship between mental imagery, in general,
and operation of the sketchpad. Some evidence suggests that
the two are dissociable. For instance, patients have been dis-
covered who perform poorly on mental imagery tasks (such
as mental rotation) but handle the short-term retention of
visuo-spatial information quite well (N. Morton & Morris,
1995). In addition, some concurrent tasks, such as arm move-
ments, that selectively disrupt the retention of spatial patterns
have little effect on the performance of mental imagery tasks
(Baddeley & Logie, 1999).The Episodic BufferThe working memory model has been enormously influential
as an explanatory heuristic. It successfully ties together a
wide range of standard laboratory phenomena, as well as data
gathered from developmental and neurological studies. The
model does have inherent problems, however, which it shares
with other implementations of the standard juggler model
(see Nairne, 2002, for a full discussion). For example, as
noted earlier, word duration is probably not the important
controlling factor in the word length effect. Words matched
for pronunciation duration, but differing along other di-
mensions such as lexicality, regularly lead to memory span
differences. The working memory model has no obvious
mechanism to handle such effects. Moreover, the phonologi-
cal similarity effect is assumed to result from confusions
among representations in the phonological store, but no
mechanism has ever been offered to explain exactly how
these confusions arise. If items are immediately available by
virtue of their residence in the store, why do the confusions
occur? Is there some cue-based retrieval mechanism in place
that can explain phonological confusions as well as other
cue-driven immediate retention effects?
Another issue that has particularly troubled Baddeley is the
question of how verbal information is stored temporarily when
the phonological loop is unavailable. For example, under artic-
ulatory suppression, immediate memory performance is im-
paired, but only slightly: That is, performance might drop from
a span of seven items to a span of five (Baddeley, Lewis, &
Vallar, 1984). Given that the phonological loop is filled to ca-
pacity by the suppression activity, how are these items being
stored? One possibility is the central executive, but Baddeley
has assumed that the central executive performs no storage
function (see Baddeley, 2000). Another possibility is the
sketchpad, but verbal materials show little sensitivity to visual
similarity under articulatory suppression. As Baddeley has re-
cently stated, “the data suggest the need for some kind of
‘back-up’ store that is capable of supporting serial recall, and