Cognitive Processes 411schedule,retrospective memory would consist of retaining
the memory of poor reward or nonreward over the retention
interval, utilizing that memory to determine whether re-
sponding on the current trial should be fast or slow. Employ-
ingprospective memory,the animal would determine at the
time of reward or nonreward whether it should run fast or
slow on the subsequent trial, thus making it unnecessary to
retain the memory of reward or nonreward over the retention
interval.
Memory may be analyzed in terms of three stages. The
first,encoding,refers to the stage in which the memory is
formed. The second, retention,refers to the persistence of the
memory over time. The third, retrieval,refers to recall of the
stored memory. We may fail to remember because of poor re-
trieval cues. This occurs when the cues at retrieval differ
from the cues that accompany storage. Interferencemay also
be responsible for forgetting. In proactive interference, mem-
ory for material learned earlier may interfere with material
learned later. In retroactive interference, material learned
later may interfere with material learned earlier.
A popular procedure employed to study animal memory (it
is sometimes used with people as well) isdelayed matching to
sample(DMTS). In this procedure a subject (say, a pigeon) is
initially trained to peck each of three keys arranged in a hori-
zontal row on the wall of an apparatus called anoperant
chamber. A typical trial begins by exposing a stimulus—
the sample stimulus, say, a horizontal line on a white back-
ground—on the center key, with the side keys being blank.
After the pigeon has observed the horizontal line, or sample
stimulus, for some period (or has pecked it), the center key
goes blank. There then ensues the retention interval in which
all three keys remain blank. When the appropriate retention
interval has elapsed the side keys are illuminated, one with
the horizontal line and the other with a vertical line. These are
called the comparison stimuli. A correct response, which may
produce food reward, consists of pecking the side key that
contains the comparison stimulus matching the sample—
in the present example, the horizontal line. The horizontal
and vertical lines may be presented equally often as samples
in an irregular fashion over trials. The positions of the com-
parison stimuli are varied irregularly over trials such that
each may appear equally often on the right key and on the
left key.
Both retroactive and proactive interferences have been
demonstrated in the DMTS situation. Retroactive interfer-
ence has been investigated as follows. Typically the chamber
is dark during the retention interval of a DMTS task. If, after
the sample stimulus is removed, the chamber is illuminated,
correct responding may decrease substantially. Proactive
interference is a major factor in DMTS (see, e.g., Wright,
Urcuioli, & Sands, 1986). For example, memory is much bet-
ter when many rather than few sample stimuli are used. This
is because presenting only a few sample stimuli increases
proactive interference. In an interesting experiment employ-
ing monkeys, when trial unique stimuli were employed, re-
tention was good even at a 24 hr interval.
Several models of Pavlovian conditioning emphasized
memory. In an early model suggested by Wagner (1976), re-
hearsal of the conditioned stimulus was stressed. According
to Wagner, surprising events are better rehearsed and thus
better remembered than expected events. For example, on the
first trial, a surprising tone may be strongly rehearsed to-
gether with the subsequent shock because of surprise. This
would lead to a strong increment in the capacity of the tone to
signal shock. On subsequent trials in which shock is expected
following tone, and thus surprise is reduced, little or no in-
crease in learning may occur.
A person may be asked to remember, say, 12 items con-
sisting of 3 items in each of four different categories: flowers,
foods, furniture, and animals. On outputting the items the
person may do so by category: 3 flowers, followed by 3 ani-
mals, and so on. Organization processes of this sort are of
concern in animal memory. For example, in a study by
Roberts (1998), a 12-arm radial maze, which consists of a
central platform with a number of arms branching out at
equal angles, was baited with four each of three different
types of food, always in the same arms over successive trials.
For example, cheese might be placed in arms 1, 3, 5, and 8 on
successive trials. The rats learned to take the food items in a
particular order, each of the four preferred foods first, the four
least preferred foods last. The ability of rats to employ one
entire series of items to predict another series of items cor-
rectly, as considered earlier, is another example of complex
organization processes in rats.
Animals such as birds, rats, and monkeys have been
shown to possess highly impressive memories—impressive
from the standpoint of retaining a considerable amount of in-
formation over long intervals (consider Clark’s nutcracker),
and from that of being able to organize discrete events into
useful wholes (identified as chunksin the “Serial Learning”
section). Memory investigations are perhaps as illuminating
as any other in suggesting that animals are not merely passive
learners but actively process information.Spatial LearningAnimals may have to move around in space for a variety of
reasons: to find mates, to forage for food, to escape predators.
Thus, spatial learning is of vital importance to a wide variety
of animals. Spatial learning is an area of intense investigation