424 Sensory and Working Memory
briefest of memories: the lingering aftereffects of event
presentation.
PROLONGING THE PRESENT:
SENSORY MEMORY
It is easy to demonstrate that the internal experience of a
briefly presented event outlasts the event itself. Twirl a
sparkler on a warm summer night and you’ll see a trail of the
light, perhaps enough to form a rough circle or to attempt the
outlines of a name. The abrupt ending of a symphony, expe-
rienced in a quiet room, leaves an echo that contributes to
the drama of the musical piece. The briefest of touches
can linger, leaving behind a record of the preceding impres-
sion. These are sensory memories: fleeting, raw records of
experience.
There is nothing in the concept of a sensory memory
that is necessarily tied to the passage of time. We can
recognize the sound of a person’s voice, or call to mind two
viewings of the same visual scene, even though months or
years might have passed. Most psychologists, however,
use the term sensory memoryto refer to stimulus persis-
tence, a kind of prolonging of the present. Longer term
modality-specific memories, such as the long-term recall
or recognition of a person’s voice, are generally classified
asperceptualmemories to distinguish them from sensory
persistence per se (e.g., Cowan, 1984, 1988; Massaro,
1975; Massaro & Loftus, 1996). There may be different
forms of short-term sensory persistence. For example,
some have suggested that there are two distinct phases of
sensory storage, one lasting only a few hundred millisec-
onds and a second lasting up to 20 s (e.g., Coltheart, 1980;
Cowan, 1984; Massaro, 1975).
Unfortunately, the distinction between the various forms
of sensory persistence and perceptual memory is not a clean
one and has led to some interpretive problems in the sensory
memory literature. For example, many of the tasks that have
traditionally been used to study sensory persistence may, in
fact, be measuring perceptual memory (see Loftus & Irwin,
1998). Questions have also been raised about the adaptive
value of the persistence process itself (Haber, 1983). In the-
ory, a prolonging process seems clearly adaptive: To perceive
a spoken word, it is necessary to integrate across phonemic
information that is presented sequentially in time (Cowan,
1984; Darwin, 1976); integrating two visual images success-
fully, such as those produced by a rapidly moving object, may
require one to maintain a relatively intact memory for the
initially presented image (Eriksen & Collins, 1967). How-
ever, it is uncertain how much of a role sensory persistence
actually plays in these situations, or whether it plays a role at
all. Some researchers have suggested that sensory persistence
may result from the fact that neural responses are simply
extended in time (Loftus & Irwin, 1998; Francis, 1999). The
fact that the subjective experience of persistence—even its
very presence—depends on factors such as the duration
and intensity of the physical stimulus supports this kind of
explanation.Measuring Sensory PersistenceAs with most psychological phenomena, our understanding
of sensory persistence has been largely defined by measure-
ment techniques. One relatively direct technique, known as a
synchrony judgment task,asks observers to adjust the timing
of an index stimulus, such as an auditory click, until it coin-
cides with the onset or offset of a target stimulus, such as a
light (e.g., Bowen, Pola, & Matin, 1974; Efron, 1970). Ob-
servers are quite accurate at deciding when the target stimu-
lus first appears—its onset—but overestimate its offset by
around 150 ms; in other words, observers think the stimulus
continues for a brief period after it has physically disap-
peared. Again, the extent of the persistence depends on the
intensity and presentation duration of the target stimulus, but
seems to be largely independent of presentation modality. For
brief target presentations, both visual and auditory stimuli
show sensory persistence effects lasting somewhere between
100 and 200 ms.
Comparable results are obtained using a technique called
backward masking(e.g., Massaro, 1970; Turvey, 1973).
Here, visual or auditory stimuli are presented and then fol-
lowed closely by an interfering “mask” from the same pre-
sentation modality. The task is to identify or recognize the
target stimulus. For example, in a simple auditory backward
masking task a high- or low-pitched tone might be presented,
followed at some variable time by another irrelevant but
masking tone; the subject’s task is simply to categorize the
pitch of the first tone (high or low). The common finding is
that recognition or identification performance improves as
the interval between the end of the target stimulus and pre-
sentation of the mask increases, until an asymptote is
reached at around 250 ms (Massaro, 1970; see Figure 15.1).
After 250 ms, further delays in presentation of the mask do
not affect performance very much (see also Massaro & Lof-
tus, 1996).
The 250-ms asymptote is commonly interpreted as the
point beyond which the stimulus no longer persists—that is,
the duration of the sensory memory. Alternatively, 250 ms
could simply represent the point at which the subject has ex-
tracted all the relevant information that he or she needs.