428 Sensory and Working Memory
on multitrial learning, particularly paired-associate and serial
learning, in the interest of specifying the conditions of trans-
fer and interference (see Osgood, 1953). Single-trial immedi-
ate serial recall, which today reigns as the prototype of the
short-term memory task, went largely unstudied for the first
half of the twentieth century.
The situation changed dramatically around 1960 with the
introduction of the Brown-Peterson technique, developed
independently by John Brown (1958) and Lloyd and
Margaret Peterson (Peterson & Peterson, 1959). In the origi-
nal Peterson procedure, trials consisted of the presentation
and recall of single consonant trigrams (e.g., CHJ) following
a distractor-filled retention interval. Retention intervals var-
ied randomly from 3 to 18 seconds and, importantly, the sub-
ject was required to count backwards aloud (by threes)
throughout the interval to prevent rehearsal. The striking find-
ing was that consonant trigrams, presented singly, were es-
sentially forgotten after a retention interval of 18 s. Murdock
(1961) extended the procedure to word recall and found sim-
ilar results: nearly complete forgetting of a list of three words
after approximately 18 s of counting (see Figure 15.2).
The Peterson finding was newsworthy for a number of rea-
sons: First, most researchers were surprised to find any signif-
icant forgetting after such a short retention interval, especially
given that the memory load was well below span. Second, the
activity filling the retention interval—counting backward—
lacked formal similarity to the to-be-remembered stimulus
items (letters or words). Circa 1960, the main mechanism for
forgetting was assumed to be interference, and not much inter-
ference was expected to occur between highly dissimilar
materials. (There was some phonemic similarity between
the letters and the digits, although this fact was not widely
appreciated at the time.) Finally, the negatively accelerated
form of the short-term forgetting curve showed a marked sim-
ilarity to long-term forgetting functions (Ebbinghaus,
1885/1964), suggesting that it might be possible to study re-
tention at a more fine-grained level (Slamecka, 1967).Decay Versus InterferenceAmong the more interesting implications of this rapid short-
term forgetting, as noted by J. Brown (1958) and others (e.g.,
Broadbent, 1958), was the possibility that autonomous decay
might be responsible for the loss. The notion that mnemonic
information is lost spontaneously with the passage of time
(e.g., as in Thorndike’s law of disuse) had largely fallen out of
favor among psychologists, at least for long-term retention,
because (a) memory sometimes improves with the passage of
time (e.g., spontaneous recovery, reminiscence, or both) and
(b) forgetting depends so critically on the nature of interfering
material. John McGeoch’s famous analogy was of an iron bar
left out to rust in open air: Rust accumulates with time, but it
is the processes that operate in time (i.e., oxidation), not
time per se, that are ultimately responsible for the changes
(McGeoch, 1932). The fact that significant forgetting could
occur in the absence of interference resurrected the concept
of decay and bolstered the novel idea that short-term reten-
tion might be mediated by its own unique operating system.
As noted, retroactive interference could be easily dis-
missed as the source of short-term forgetting in the Brown-
Peterson task, because counting and letters or words are highly
dissimilar, but proponents of decay were forced to acknowl-
edge that proactive interference might be responsible for at
least some of the loss. Proactive interference is the interfer-
encethatpriorinformation,suchastheitemsontrialN–1,
imposes on the retention of current trial information (trial N).
Peterson and Peterson (1959) checked for proactive interfer-
ence in their experiments but failed to find any support for it
(in fact, performance actually improved from early to late in
the session). However, in a landmark study, Keppel and
Underwood (1962) eliminated practice trials and focused only
on the very first trial. No proactive interference is possible
on the first trial in a session, because there is no prior trial
information, and Keppel and Underwood found almost no for-
getting, regardless of the length of the retention interval. Dif-
ferences between a short and a long retention interval began to
emerge only on the second or third trial in the session, when,
presumably, proactive interference was able to kick in.
Theoretically, the Keppel and Underwood (1962) findings
are critical: If you believe in a separate short-term memory
system, distinct from long-term memory, then it is important
to show that short-term memory follows its own uniqueFigure 15.2 Proportion correct recall performance in a Brown-Peterson
short-term retention task. The data are shown separately for lists containing
one word, three words, or three consonants and are plotted as a function of
the length of the distractor period (after Murdock, 1961).