430Sensory and Working Memory
Figure 15.3Aschematic outline of the three conditions in the Turvey,
Brick, and Osborn (1970) study. P1 represents list items presented on trial N;
P2 represents list items presented on trial N+1; R2 represents the point of
recall for items presented on trial N+1. Recall improves as the ratio of the
interpresentation interval (P1–P2) to the retention interval (P2–R2) increases
(see text).
opportunity for decay—was equated across the groups on the
critical 15-s trial, yet performance depended critically on the
timing of prior trials.
According to distinctiveness accounts, memory improves
to the extent that target items can be easily discriminated
from the memories created by prior trials. As with most per-
ceptual comparisons, the discrimination process is assumed
to be relative: As time passes, temporally distant items seem
more similar and become harder to discriminate, just as tele-
phone poles watched from a moving car appear to merge
together with increasing distance traveled (Crowder, 1976).
Thus, two items separated by 5 s become harder to discrimi-
nate after 20 s than after 10 s. This relationship can be
expressed easily in terms of a ratio: Discriminability is pro-
portional to the interitem interval (the period separating the
two items in question) divided by the retention interval (the
time separating the most recent item from the point of test).
Now reconsider performance in the Turvey et al. (1970)
experiment (see Figure 15.3). Prior to the 15-s critical trial,
the intertrial intervals remained constant within the session
(10, 15, or 20 s), creating a discriminability ratio of 1.0 for
each group. As noted above, no retention differences were ac-
tually found among these groups, despite the different reten-
tion intervals, providing strong support for the distinctive-
ness account. On the critical trial, however, the ratios change
differentially across the groups, in the direction predicted
bythe data. More specifically, the discriminability ratio de-
clines inthe 10-s group (10/15= 0.67), leading to poorer
retention performance, and increases in the 20-s condition
(20/ 15 = 1.33), leading to improved performance; the ratio
remains at 1.0 in the 15-s condition, and no performance
changes were recorded (15/ 15 =1.0).
The Turvey et al. (1970) data are particularly important
because they show that short-term memory performance can
decrease, stay the same, or even increase depending on tim-
ing variables. Comparable results have been found in other
contexts: Neath and Knoedler (1994), for example, found
that memory for early items in a list sometimes improves as
the length of the retention interval increases (see also Wright,
Santiago, Sands, Kendrick, & Cook, 1985). According to dis-
tinctiveness accounts, early items become relatively more
discriminable as all list items recede backward into the past
(see also Bjork, 2001). No simple version of decay theory can
handle such findings; the passage of time, it turns out, fails
as a general predictor of short-term memory performance
(except, perhaps, for the first few seconds of a retention
interval).Capacity LimitationsObviously, to the extent that items are rapidly forgotten over
the short term, for whatever reason, there will appear to be
fundamental limitations in memory capacity. From the per-
spective of the standard juggler model outlined earlier, the
storage capacity of short-term memory is determined by the
trade-off between decay and the rate of internal rehearsal.
The number of items that can be recalled correctly, in order,
on at least half of the trials (i.e., memory span) is determined
by the number of items that can be rehearsed within the time-
limited decay window. That number, as noted by Ebbinghaus
(1885/1964) and others (e.g., Miller, 1956), tends to be
around seven plus or minus two unrelated items.
It is misleading, though, to think about capacity limita-
tions in short-term memory simply in terms of items, particu-
larly in terms of a number such as seven plus or minus two.
First, as Miller (1956) showed, it is not really the number of
nominal items but rather the number of functional “chunks”
that influences span. We can remember seven unrelated
letters, seven unrelated words, or even seven unrelated
sentences with somewhat comparable degrees of efficiency
(although see Cowan, 2001, for evidence that the limit may
actually be closer to four). More importantly, though, a
strong argument can be made that some other variable—
perhaps time—truly controls retention. Holding the number
of to-be-remembered chunks constant, immediate retention
can vary dramatically as a function of item characteristics.
For example, it turns out that memory span is well predicted
by the length of time needed to say items aloud or repeat
items aloud in succession.The Limits of TimeIn a seminal article, Baddeley et al. (1975) found that lists of
short words lead to better immediate serial recall than lists of
long words, even though the number of chunks (i.e., words)
is constant across conditions. Of course, short and long words
can differ in a number of ways (e.g., number of letters, sylla-
bles, and so on), so intraunit interference could conceivably