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Three experiments were conducted using a lick-suppression preparation with rats to determine whether temporal and physical context shifts modulate the effectiveness of 2 sequentially trained blocking stimuli. Experiment 1 ascertained that it is possible to obtain blocking by conditioning rats to react to a target cue using 2 different blocking cues, each trained with a single-phase blocking paradigm. Experiment 2 showed that the more recently trained blocking stimulus was more effective (i.e., showed a recency effect) when testing was conducted immediately after training, but a long retention interval attenuated blocking by the most recently trained blocking stimulus and increased blocking by the initially trained blocking stimulus (i.e., a recency-to-primacy shift). This shift from recency to primacy affected in Experiment 2 by varying the retention interval was replicated in Experiment 3 by changing the physical context between training and testing. Taken together, the results indicate that the effectiveness of sequentially trained competing stimuli follows the same recency-to-primacy shift rule that is seen in traditional interference phenomena.
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Recency-to-Primacy Shift in Cue Competition
Olga Lipatova, Daniel S. Wheeler, Miguel A. Vadillo, and Ralph R. Miller
State University of New York at Binghamton
Three experiments were conducted using a lick-suppression preparation with rats to determine whether
temporal and physical context shifts modulate the effectiveness of 2 sequentially trained blocking stimuli.
Experiment 1 ascertained that it is possible to obtain blocking by conditioning rats to react to a target cue
using 2 different blocking cues, each trained with a single-phase blocking paradigm. Experiment 2
showed that the more recently trained blocking stimulus was more effective (i.e., showed a recency
effect) when testing was conducted immediately after training, but a long retention interval attenuated
blocking by the most recently trained blocking stimulus and increased blocking by the initially trained
blocking stimulus (i.e., a recency-to-primacy shift). This shift from recency to primacy affected in
Experiment 2 by varying the retention interval was replicated in Experiment 3 by changing the physical
context between training and testing. Taken together, the results indicate that the effectiveness of
sequentially trained competing stimuli follows the same recency-to-primacy shift rule that is seen in
traditional interference phenomena.
Keywords: blocking, Pavlovian conditioning, retrieval
Pairing a conditioned stimulus (CS) with an unconditioned
stimulus (US) ordinarily results in conditioned responding to the
CS. However, if the pairings occur in the presence of another cue,
responding to the target CS is attenuated. Miller and Matzel’s
(1988) comparator hypothesis offers one account of this sort of cue
competition. The comparator hypothesis states that responding to
a CS during testing is determined by three associations. The first
is the traditional association between the target CS and the US. The
second association is between the target CS and any other salient
stimulus (excluding the US) that was present during training
(called a comparator stimulus). The third association is between
this comparator stimulus and the US. The model posits that when
the target CS is presented at test, it directly activates a represen-
tation of the US based on the strength of the CS–US association.
The presentation of the target CS also activates a representation of
its comparator stimulus, which in turn activates a representation of
the US. The strength of this indirectly activated US representation
is dependent on the product of the strengths of the associations
between the target CS and the comparator stimulus and between
the comparator stimulus and the US. Cue competition occurs
because the indirectly activated representation down modulates
responding to the target CS, which is otherwise dictated by the
strength of the directly activated US representation. A central
assumption of the comparator hypothesis is that cues do not
compete during the acquisition of the associative strength but at
the time of testing. Thus, during a test of the target CS after
training that leads to cue competition, there is a response deficit
due to a failure to express an acquired association, not a deficit in
associative acquisition during training. This account contrasts with
acquisition-focused associative theories of cue competition, such
as that of Rescorla and Wagner (1972), that posit that cue com-
petition occurs during acquisition.
One prediction of the comparator hypothesis is that the post-
training exposure of a comparator cue will result in a recovery
from cue competition because there is a decrease in the associative
strength between the comparator stimulus and the US. This pre-
diction is supported by a number of studies that show a recovery
from cue competition produced by extinguishing the comparator
stimulus after training (e.g., recovery from blocking in Blaisdell,
Gunther, & Miller, 1999; recovery from overshadowing in Kauf-
man & Bolles, 1981; attenuation of conditioned inhibition in Lysle
& Fowler, 1985). In addition, such posttraining extinction could
not only extinguish the association between the comparator stim-
ulus and the US but would also extinguish any within-compound
association between the comparator stimulus and the target stim-
ulus (see Arcediano, Escobar, & Miller, 2005, for evidence of
bidirectional extinction).
Recovery from cue competition after posttraining extinction of
a comparator (competing) stimulus can also be explained by some
contemporary acquisition-focused associative models (e.g., Dick-
inson & Burke, 1996; Van Hamme & Wasserman, 1994). For
example, Dickinson and Burke’s revision of Wagner’s (1981) SOP
model, SOP–Revised (SOP-R), assumes that stimuli are repre-
Olga Lipatova, Daniel S. Wheeler, Miguel A. Vadillo, and Ralph R.
Miller, Department of Psychology, State University of New York at
Binghamton.
National Institute of Mental Health Grant 33881 provided support for
this research. Miguel A. Vadillo was supported by Fellowship BF 101.31
from the Department of Education, Universities and Research of the
Basque Government. We thank Jeffrey C. Amundson, Gonzalo Urcelay,
and Kouji Urushihara for their comments on an earlier version of this
article. We also thank James Esposito for assistance with the collection of
data.
Correspondence concerning this article should be addressed to Ralph R.
Miller, Department of Psychology, State University of New York at
Binghamton, Binghamton, NY 13902-6000. E-mail: rmiller@binghamton
.edu
Journal of Experimental Psychology: Copyright 2006 by the American Psychological Association
Animal Behavior Processes
2006, Vol. 32, No. 4, 396 406
0097-7403/06/$12.00 DOI: 10.1037/0097-7403.32.4.396
396
sented by multiple nodes in memory that exist in one of three states
at any moment in time: an inactive state (I), a highly active state
(A1), and a low activity state (A2). Presentation of an unexpected
stimulus causes a transition from I to A1 for some proportion of
the nodes representing that stimulus, which then decay back
through A2 to I. When a node is activated by an associate of the
stimulus, the node transitions directly from I to A2. The temporal
overlap of nodes in various states affects changes in the associative
connections between these nodes. When nodes of a given CS and
US are concurrently in A1, there is an increment in the strength of
an excitatory connection between them, whereas when nodes of
the cue are in A1 while those of the US are in A2, an inhibitory
connection from the CS to the US is strengthened. An added
assumption unique to Dickinson and Burke’s revision of SOP is
that if nodes of both a CS and a US are in A2, the excitatory
association between the two stimuli will be incremented. Further-
more, if nodes of the CS are in A2 and those of the US are in A1,
the inhibitory association between the CS and US will be incre-
mented. Thus, according to this assumption of SOP-R, when a
competing stimulus is presented alone after single-phase blocking
treatment, the competing CS enters into the A1 state, while the US
and target CS are activated in A2. Thus, incrementing the associ-
ation between the US and the target CS results in retrospective
revaluation (i.e., a recovery of the response to the target stimulus).
In contrast to the many published studies that show evidence of
retrospective revaluation, a number of other experiments have
failed to observe retrospective revaluation (e.g., Holland, 1999).
One reason for this disparity suggested by Holland is that the
mechanisms of overshadowing and blocking are affected by the
conditioning preparation used (appetitive or aversive). However,
this account does not completely address the inconsistencies in the
literature. Many other parameters must play a role in contributing
to the differences in the stimulus selection mechanisms. For ex-
ample, Shevill and Hall (2004), also using a conditioned suppres-
sion procedure, investigated retrospective revaluation. In their
Experiment 1, they expected to observe a recovery of responding
to a target stimulus after extinction of an overshadowing stimulus.
However, they found the opposite results, that is, a small mediated-
extinction effect. Conversely, when they repeated the procedure in
their Experiment 2 using different parameters (serial A–X com-
pound during training), they did observe recovery from overshad-
owing. In summary, the exact conditions and parameters that yield
retrospective revaluation have not yet been identified. Considering
that both effects have been repeatedly demonstrated, this presents
an interesting question for future research. The present research is
not intended to resolve this issue. Instead, we focus on instances of
recovery from blocking instead of mediated extinction.
Most of the research investigating cue competition has involved
competition between a single competing stimulus and a single
target CS. For example, studies of overshadowing (e.g., Pavlov,
1927), blocking (e.g., Kamin, 1969), Pavlovian conditioned inhi-
bition (e.g., Pavlov, 1927), and overexpectation (e.g., Kremer,
1978) all involve situations in which responding to a single target
CS is reduced because it is paired during training with one other
CS that has a strong association with the US. Although less
well-known, some recent studies have investigated the influence of
two competing stimuli that are both trained with a single target
cue. In most of these studies, the two competing stimuli were
trained simultaneously; consequently, there was a direct associa-
tion between them (e.g., Blaisdell, Bristol, Gunther, & Miller,
1998; Stout, Chang, & Miller, 2003; Urcelay & Miller, 2006;
Urushihara, Stout, & Miller, 2004). However, to our knowledge,
only one published study (Grahame, Barnet, & Miller, 1992) has
examined the interaction between two competing stimuli that were
trained sequentially with a single target CS such that there was no
direct association between the two competing stimuli. Competing
stimuli presumably include all CSs present during training of the
target CS. Thus, when a CS is presented alone (i.e., with no other
simultaneously presented punctate stimuli) during training, the
context is the only potentially effective competing stimulus. In
Grahame et al., a target CS was trained sequentially in two distinct
contexts that could have served as two competing stimuli for the
target CS. However, a test of the target stimulus revealed that the
two contexts did not act as competing stimuli. To test the effec-
tiveness of the different contexts, Grahame et al. extinguished the
first-trained context, the second-trained context, or neither context
before testing the target CS in a neutral context. According to the
comparator hypothesis (and select other models, such as SOP-R),
posttraining extinction of a competing context should reduce its
effectiveness. When Grahame et al. extinguished the contexts after
training, only the extinction of the most recently experienced
training context was effective in enhancing responding to the
target CS. That is, animals for which training of the CS occurred
first in one context and then in a second context showed enhanced
fear (i.e., recovery from cue competition) only as a result of
extinguishing the second context, not the first one.
Grahame et al. (1992) seemed to observe a recency effect, that
is, the superior expression of a more recently acquired memory
relative to a memory acquired earlier. Furthermore, the recency
effect seemed to produce retroactive interference, reducing the
influence of the first-experienced competing stimulus. This inter-
pretation of the results of Grahame et al. suggests that the inter-
ference between competing stimuli of a common stimulus resem-
bles interference effects observed in the serial learning literature. A
vast amount of research has been conducted to examine retroactive
and proactive interference effects in learning and memory. One
effect that characterizes interference phenomena in a number of
different fields is the recency-to-primacy shift, which consists of a
shift from better retrieval of memory for a recent training event
(i.e., recency) to the equal or better retrieval of an initial training
event (i.e., primacy) as a result of a delay in testing. Many studies
have been performed in which an improvement in the memory of
humans for initial (i.e., first learned) events was observed with an
increasing delay between learning and test (e.g., Knoedler, Hell-
wing, & Neath, 1999; Neath, 1993; Postman, Stark, & Fraser,
1968). The shift from recency to primacy has been reported not
only in humans using serial lists of words but also with other
species in analogous tasks (e.g., Urushihara, Wheeler, & Miller,
2004; Wright, Santiago, Sands, Kendrick, & Cook, 1985). The
recency-to-primacy shift has also been observed using a variety of
other tasks. For example, recovery over time of the first learned
response to a single cue has been observed in traditional Pavlovian
conditioning. Bouton and Peck (1992) examined the effects of
interposing a retention interval on performance by rats in a coun-
terconditioning paradigm. First they paired a stimulus with a
shock, and then they paired the same cue with food. After training,
397
RECENCY-TO-PRIMACY SHIFT
if they tested after 1 day, the rats showed food-appropriate behav-
ior; however, after 28 days, the tone elicited more shock-
appropriate behavior. Thus, with a delay, there was a shift toward
the earlier learned behavior. More recently, similar recency-to-
primacy shifts have been reported after both extinction and latent
inhibition procedures (e.g., Wheeler, Stout, & Miller, 2005).
Recently, Urushihara, Wheeler, and Miller (2004) found that the
outcome-alone exposure effects, which are considered by most
contemporary learning theories to be a form of competition be-
tween the context and the target cue, also obey the recency-to-
primacy principle. With a sensory preconditioning preparation,
they found that outcome-alone exposure after training (i.e., out-
come postexposure) had a stronger deleterious effect relative to
outcome-alone exposure prior to training (i.e., outcome preexpo-
sure), that is, a recency effect. They also found that when testing
was considerably delayed, outcome-alone preexposure had a stron-
ger deleterious effect than did outcome postexposure (i.e., a
recency-to-primacy shift presumably caused by an increase in
retention interval). Their results can be taken as evidence that the
effect of cue competition is also strongly influenced by differential
retrieval of competing cue– outcome associations caused by vari-
ous factors such as shifts in the temporal context or the presenta-
tion of a priming cue. These results are problematic for both the
acquisition-focused theories and the comparator hypothesis and
suggest the importance of investigating the mechanism of differ-
ential retrieval of acquired associations with a common element.
The previously mentioned studies all examined the effect of
retention interval on sequentially learned information. However,
there is also a literature demonstrating that changes in physical
context between training and testing can affect recency-to-primacy
shifts. For example, Bouton and Ricker (1994) trained and extin-
guished conditioned responding in rats in one context and then
tested them in the same context or a different context. They found
a recovery of conditioned responding when testing occurred out-
side of the training and extinction context (AAB renewal), that is,
a recency-to-primacy shift based on a change in physical context.
Bouton (1993) proposed that renewal is similar to spontaneous
recovery from extinction, which occurs when a long period of time
passes after extinction treatment (Pavlov, 1927). He suggested that
an instant in time can be regarded as an element of the context,
thereby creating a spatiotemporal context. Presumably any appre-
ciable change in the spatiotemporal context that follows sequential
learning will result in a recency-to-primacy shift. In the present
research, we examined this assertion with respect to both temporal
and physical contexts for sequentially trained competing stimuli.
On the basis of Grahame et al.’s (1992) and Urushihara,
Wheeler, and Miller’s (2004) data and Bouton’s (1993) theory, we
further investigated the effect of differential retrieval of competing
associations in a cue competition situation. Unlike Grahame et al.,
the present study focused on blocking by two discrete stimuli
rather than two contexts. Our fundamental question was whether a
recency-to-primacy shift could be observed with respect to the
effectiveness of sequentially trained competing stimuli. Thus, we
compared the effectiveness of two competing stimuli on respond-
ing to a target cue and investigated how the relative effectiveness
of each cue changed as a function of increasing the retention
interval or changing the physical context between training and
testing. Because studies conducted in our laboratory have shown
that overshadowing (which is the simplest cue competition treat-
ment) wanes quickly as a function of the number of training trials
when many training trials are presented (Stout, Arcediano, Esco-
bar, & Miller, 2003), we used an alternative cue competition
paradigm less vulnerable to trial number, single-phase blocking. In
a single-phase blocking procedure, trials in which the blocking
stimulus (A) is paired alone with the US are interspersed with
reinforced pairings of the blocking and target CSs (AX–US; Wag-
ner, 1969). Thus, in the present research, we used two sequential
single-phase blocking procedures to determine whether a recency-
to-primacy shift would occur with respect to the effectiveness of
the competing stimuli. Extrapolating from Grahame et al. and
Urushihara, Wheeler, and Miller, we anticipated that there would
be a shift in the effective competing stimulus from the most recent
one to the earliest after a shift in either the temporal or the physical
context, as is observed in interference between simple excitors.
Experiment 1
Experiment 1 was preliminary to Experiments 2 and 3. It was
designed to determine if two successively trained competing stim-
uli would successfully block responding to a target CS in a
single-phase blocking paradigm. That is, in Phase 1, we trained the
target cue (X) with one competing stimulus (A) in a single-phase
blocking design, and then, in Phase 2, we trained the same target
cue with a different competing stimulus (B) in a single-phase
blocking design (see Table 1). Assuming blocking was observed in
Experiment 1, we planned to use this preparation in Experiments
2 and 3 to examine the potential shift in the effective competing
stimulus from CS B (most recent relative to testing) to CS A
(earliest) as a function of test context.
Method
Subjects
The subjects were 12 male (305–350 g) and 12 female (215–240 g)
Sprague–Dawley, experimentally naive, young adult rats bred in our col-
ony. The rats were randomly assigned to one of two groups (blocking and
control; for each group, n 12), counterbalanced for sex. Subjects were
individually housed and maintained on a 16:8-hr light– dark cycle. Exper-
imental sessions occurred roughly midway through the light portion of the
cycle. Subjects had free access to food in the home cages. Prior to initiation
of the experiment, water availability was progressively reduced to 30 min
per day, provided approximately 2 hr after any scheduled treatment.
Table 1
Design of Experiment 1
Group
Phase 1,
Blocking A,
3 days
Phase 2,
Blocking B,
3 days Test
Blocking 12A/3AX 12B/3BX X
Control 12C/3AX 12D/3BX X
Note. A, B, C, and D competing stimuli; X target cue; ⫹⫽
footshock. Numbers next to the pairings indicate total number of trials in
that phase. Slashes indicate interspersed trials.
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LIPATOVA, WHEELER, VADILLO, AND MILLER
Apparatus
Because Experiment 3 required two distinct contexts, six identical copies
of each of two different types of experimental chambers were used for
training in all experiments. Chamber Rectangular (R) was a clear, Plexi-
glas, rectilinear chamber, measuring 23.0 8.5 12.5 cm (length
width height). The floor was constructed of 0.48-cm diameter stainless
steel rods, spaced 1.5 cm apart, center to center. The rods were connected
by NE-2H neon bulbs that allowed a constant-current footshock to be
delivered by means of a high-voltage alternating current (AC) circuit in
series with a 1.0-M resistor. Each copy of Chamber R was housed in a
separate light- and sound-attenuating environmental isolation chest, which
was dimly illuminated by a 2-W (nominal at 120 VAC) incandescent bulb
driven at 60 VAC. This house light was mounted on the ceiling of the
environmental chest approximately 26 cm from the center of the experi-
mental chamber.
Chamber V-shaped (V) was a 22.1-cm long box in the shape of a vertical
truncated V (25.3 cm height, 21.3 cm wide at the top, 5.1 cm wide at the
bottom). The floor and long sides were constructed of stainless steel sheets,
the short sides were constructed of black Plexiglas, and the ceiling was
constructed of clear Plexiglas. The floor consisted of two parallel metal
plates, each 2.0 cm wide, with a 1.1-cm gap between them, which permit-
ted the delivery of constant-current footshock. Each V-shaped chamber was
housed in its own environmental isolation chest, which was dimly illumi-
nated by a 7.5-W (nominal at 120 VAC) incandescent house light driven at
60 VAC mounted on an inside wall of the environmental chest approxi-
mately 30 cm from the center of the experimental chamber. The light
entering the animal chamber was primarily that reflected from the roof of
the environmental chest, which was white sound-attenuating material. The
light intensities in the two types of chambers (R and V) were approximately
equal because of the differences in opaqueness of the walls.
Each chamber (R and V) was also equipped with three 45- speakers
widely separated on the inside walls of the environmental chest. Each
speaker could deliver a different auditory stimulus. One speaker mounted
on the right sidewall was used to deliver a complex tone stimulus (800 and
1,000 Hz) 8 dB above background. A second speaker mounted on the back
sidewall of the experimental chamber was used to deliver a click stimulus
(6/s) 8 dB above background, which served as Stimulus X. A third speaker
mounted on the left sidewall of the chamber was used to deliver a white
noise stimulus 8 dB above background. Additionally, a SonAlert (Mallory
SonAlert Products, Inc., Indianapolis, IN) mounted on each environmental
chest was able to deliver a high-frequency (approximately 1,900 Hz) tone
8 dB above the background sound level. Ventilation fans in each enclosure
provided constant 74-dB background noise. Each chamber could also
provide a flashing-light stimulus (0.17 s on, 0.17 s off). In Chamber R, the
flashing light was provided by a 25-W bulb (nominal at 120 VAC) driven
at 60 VAC, whereas the flashing light in Chamber V was provided by a
100-W bulb (nominal at 120 VAC) driven at 60 VAC. The bulbs were
located on the back wall of each environmental chest. Because of differ-
ences in the opaqueness of the experimental walls, these two stimuli
produced approximately equal illumination of the R and the V chambers.
All CS durations were 5 s except during testing. The white noise and
SonAlert served as Stimuli A and C, counterbalanced. The tone and
flashing light served as Stimuli B and D, counterbalanced. The uncondi-
tioned stimulus was a 0.7-mA, 0.5-s footshock.
Each chamber (R and V) could be equipped with a water-filled lick tube
that extended 1 cm from the rear of a cylindrical niche that was 4.5 cm in
diameter, left–right centered in one short wall with its axis perpendicular to
the wall and positioned with its center 4.25 cm above the floor of the
chamber. Each niche had a horizontal infrared photobeam horizontally
traversing it parallel to the wall on which the niche was mounted, 1 cm in
front of the lick tube. To drink from the tube, subjects had to insert their
heads into the niche, thereby breaking the infrared photobeam. Thus, we
could record when subjects had their heads in the niche to access the water
tube. Ordinarily, they broke the beam only when they were drinking.
Disruption of ongoing drinking by a test stimulus served as our dependent
variable. Chambers R and V were counterbalanced within groups and each
subject was trained and tested exclusively in one chamber.
Procedure
Acclimation. On Day 1, each subject was exposed to the experimental
context for 60 min with the lick tube present. No nominal stimuli were
presented. At the end of this session, the lick tube was removed.
Blocking by A (Phase 1). On each of Days 2– 4, subjects in the
blocking group were exposed to one reinforced presentation of Stimulus A
simultaneously paired with Stimulus X (AX–US) and four reinforced
presentations of Stimulus A alone (A–US). The subjects in the control
group received similar treatment, except that Stimulus C instead of Stim-
ulus A was reinforced alone (C–US and AX–US). On Day 2, the reinforced
presentation of the compound AX occurred 41 min into the 60-min session
and the 4 reinforced presentations of stimulus A or C alone occurred 8, 19,
30, and 50 min into the 60-min session. On Day 3, the reinforced presen-
tation of the compound AX occurred 30 min into the 60-min session and
the 4 reinforced presentations of Stimulus A or C alone occurred 8, 19, 41,
and 50 min into the 60-min session. On Day 4, the reinforced presentation
of the compound AX occurred 19 min into the 60-min session and the 4
reinforced presentations of Stimulus A or C alone occurred 8, 30, 41, and
50 min into the 60-min session. In all cases, the CS or CSs were imme-
diately followed by a US presentation.
Blocking by B (Phase 2). On each of Days 5–7, subjects in the
blocking group were exposed to one reinforced presentation of Stimulus B
simultaneously paired with Stimulus X (BX–US) and four reinforced
presentations of Stimulus B alone (B–US). The subjects in the control
group received similar treatment, except that Stimulus D instead of Stim-
ulus B was reinforced alone (D–US and BX–US). The reinforced presen-
tation of the compound BX on Days 5, 6, and 7 occurred at the same times
as the compound AX occurred on Days, 2, 3, and 4, respectively, into the
60-min session. Similarly, the four reinforced presentations of Stimulus B
or D alone on Days 5, 6, and 7 occurred at the same times as the Stimulus
A or C occurred on Days 2, 3, and 4, respectively, into the 60-min session.
In all cases, the CS or CSs were immediately followed by a US
presentation.
Reacclimation. On Days 8 and 9, to stabilize baseline drinking rates
that might have been perturbed by fear conditioning, all subjects received
a daily 60-min session with no nominal stimuli being presented.
Test. On Day 10, all subjects were tested for suppression to CS X in an
11-min test session. During this session, there was a 10-min continuous
presentation of X alone starting on completion of the first 5 cumulative
seconds of licking. Thus, all rats were drinking at the moment of CS X
onset. Time to complete the first 5 cumulative seconds of drinking pro-
vided a pre-CS score. The time to complete another 5 cumulative seconds,
this time in the presence of the CS, provided a CS score. As is the custom
of our laboratory, animals with pre-CS scores greater than 60 s were
eliminated from the study because of their exhibiting excessive fear of the
context. On the basis of this criterion, 1 subject from each of the two
groups was excluded from the analyses. All scores were capped at 10 min
and subjected to a log (base 10) transformation to improve the normality of
the within-group distributions and allow the use of parametric statistics.
Results and Discussion
Figure 1 shows that the blocking group suppressed less during
the presentation of CS than did the control group. A t test per-
formed on the transformed pre-CS scores showed no effect,
t(20) 0.81, p .43; therefore, any differences in responding to
the CS were not appreciably influenced by differences in fear of
399
RECENCY-TO-PRIMACY SHIFT
the context. A t test analysis of the transformed latencies in the
presence of the CS revealed an effect of blocking, t(20) 5.98,
p .01. Thus, we observed a cue competition effect with two
different competing stimuli sequentially presented in two phases of
single-phase blocking. However, an alternative interpretation of
these results is possible. In this experiment, the blocking group
received training with two stimuli in addition to X (A–US and
AX–US followed by B–US and BX–US), while the control group
received four additional stimuli during training (C–US and
AX–US followed by D–US and BX–US). Thus, the stronger
responding to X in the control group compared with the blocking
group may have been a result of greater generalization from the
training stimuli instead of the effect of blocking. Although such an
explanation of the results of Experiment 1 is possible, it does not
explain the results of Experiments 2 and 3.
Experiment 2
The purpose of Experiments 2 and 3 was to examine the effects
of posttraining extinction of the blocking CSs A or B as a function
of the retention interval (Experiment 2) and changes in the phys-
ical context (Experiment 3) between blocking treatment and ex-
tinction treatment. In the framework of the comparator hypothesis,
we assessed relative value as a comparator stimulus of A and B by
observing the effects on responding to X as a consequence of
posttraining extinction of these cues. In Experiment 2, we assessed
the effect of a temporal context switch on the identity of the
effective competing stimulus. In this experiment, we used the
single-phase blocking paradigm that was used in Experiment 1.
Table 2 summarizes the design of Experiment 2. In Phase 1, the
blocking occurred with competing stimulus A and in Phase 2,
blocking occurred with a second, different competing stimulus, B.
After training, half of the groups received a short retention interval
of 1 day, and the other half received a long interval of 22 days. In
both the short condition and the long condition, one group expe-
rienced extinction of the first competing stimulus (Ext A), another
group experienced extinction of the second competing stimulus
(Ext B), and a control group did not experience extinction of either
stimulus (No Ext). Both extinction and testing occurred after the
retention interval. If posttraining extinction of one blocking stim-
ulus but not the other had substantial effect on responding to the
target cue, we could conclude that that blocking stimulus was
responsible for the weak responding observed without the extinc-
tion treatment. On the basis of the findings of Blaisdell et al.
(1999) and Grahame et al. (1992), we anticipated that after a short
retention interval, only the subjects that received extinction to the
second competing stimulus (Ext B) would show a recovery from
blocking. Therefore, a strong response to the blocked CS was
expected in Group Short–Ext B relative to Group Short–No Ext
and Group Short–Ext A. Moreover, if the effectiveness of a com-
peting stimulus is subject to a recency-to-primacy shift as a con-
sequence of a long retention interval as suggested by Urushihara,
Wheeler, and Miller (2004), in the long condition, only the group
that received extinction to the first competing stimulus A should
show a recovery from blocking. Such a result would indicate that
the effective competing stimulus reflects a primacy effect after a
long retention interval.
Method
Subjects and Apparatus
The subjects were 36 male (213–334 g) and 36 female (160 –233 g)
Sprague–Dawley, experimentally naive, young adult water-restricted rats
(n 12 per group) bred in our colony. Apparatus and Phases 1 and 2 of
treatment were identical to those used in Experiment 1. The white noise
Figure 1. Mean times to complete 5 cumulative seconds of drinking on
presentation of the target conditioned stimulus (X) in Experiment 1. See
Table 1 for treatments of the two groups. Error bars denote standard errors
of the means.
Table 2
Design of Experiment 2
Group
Phase 1,
Blocking A,
3 days
Phase 2,
Blocking B,
3 days
Phase 3,
retention
interval
Phase 4,
extinction Test
Short–Ext A 12A/3AX 12B/3BX 1 day 400 A X
Short–Ext B 12A/3AX 12B/3BX 1 day 400 B X
Short–No Ext 12A/3AX 12B/3BX 1 day Context X
Long–Ext A 12A/3AX 12B/3BX 22 days 400 A X
Long–Ext B 12A/3AX 12B/3BX 22 days 400 B X
Long–No Ext 12A/3AX 12B/3BX 22 days Context X
Note. A and B competing stimuli; X target cue; ⫹⫽footshock; Short 1-day retention interval; Long
22-day retention interval; Ext extinction. Numbers next to the stimuli indicate the total number of trials or days
in that phase. Slashes indicate interspersed trials.
400
LIPATOVA, WHEELER, VADILLO, AND MILLER
and tone served as Stimuli A and B, counterbalanced. The click train
served as Stimulus X.
Procedure
Acclimation and blocking by A (Phase 1) and B (Phase 2). On Days
1–7, acclimation, Phase 1 (single-phase blocking by A), and Phase 2
(single-phase blocking by B) were conducted in the same manner as in
Experiment 1. There were no blocking control groups in Experiment 2; that
is, all subjects received the blocking treatments.
Retention interval (Phase 3). Over Days 8 –28, subjects in the long
condition stayed in their home cages and received no experimental treat-
ment except for 30 s of handling three times per week.
Extinction (Phase 4). On each of Days 8 and 9, subjects in Group
Short–Ext A were exposed to 200 nonreinforced presentations of Stimulus
A, 1 every 36 s in the 120-min session. Subjects in Group Short–Ext B
were exposed daily to 200 nonreinforced presentations of Stimulus B, on
an average interval of 1 every 36 s in the 120-min session. Group Short–No
Ext was exposed daily to context alone for 120 min. The total number of
extinction trials was based on the results of Blaisdell et al. (1999), which
demonstrated recovery from single-phase blocking with a total of 200
nonreinforced presentations of the competing stimulus. We gave twice that
number of extinction trials to assure robust recovery. At the end of the
extinction treatment, the lick tubes were restored to the apparatus. On Days
29 and 30, subjects in Groups Long–Ext A and Long–Ext B received
extinction treatment with A and B, respectively, that was identical to that
previously received by Groups Short–Ext A and Short–Ext B.
Reacclimation. On Days 10 and 11, the subjects in the short condition
were reacclimated to stabilize baseline drinking rates as in Experiment 1.
Subjects in the long condition received identical treatment on Days 31
and 32.
Test. On Day 12, subjects in the short conditions were tested for
suppression to CS X as in Experiment 1. On Day 33, subjects in the long
condition were similarly tested. Subjects that took longer than 60 s to
complete 5 cumulative seconds of drinking before the onset of the CS were
eliminated from the study; on the basis of this criterion, 1 subject from
Group Short–Ext B and 1 from Group Long–Ext B were excluded.
Results and Discussion
In Figure 2, one can observe that Groups Short–Ext B and
Long–Ext A showed a stronger suppression to the target stimulus
X than did Groups Long–No Ext, Short–No Ext, Short–Ext A, and
Long–Ext B. That is, with a short retention interval, recovery from
blocking was observed only when the most recently trained block-
ing cue was extinguished (a recency effect). In contrast, with a
long retention interval, recovery from blocking occurred only
when the first-trained blocking cue was extinguished (a primacy
effect). A 2 (interval: short vs. long) 3 (extinction: A, B, or
none) analysis of variance (ANOVA) performed on pre-CS scores
showed no main effects or interactions, ps .17. A 2 (interval)
3 (extinction) ANOVA on the CS scores revealed no main effect
of interval, p .63, and no main effect for extinction, p .19.
However, the ANOVA revealed an interaction between extinction
and interval, F(2, 64) 21.57, p .01, MSE 1.86. Planned
comparisons detected a difference among the three groups in the
short condition. The group extinguished to CS B (Short–Ext B)
suppressed more than both the group extinguished to CS A (Short–
Ext A), F(1, 64) 17.34, p .01, and the no-extinction control
group (Short–No Ext), F(1, 64) 7.25, p .01. Further planned
comparisons among the three groups in the long condition, which
experienced a retention interval of 22 days, revealed that the group
extinguished to CS A (Long–Ext A) suppressed more than both the
group extinguished to CS B (Long–Ext B), F(1, 64) 23.61, p
.01, and the control group (Long–No Ext), F(1, 64) 16.31, p
.01.
These results suggest that, although the two blocking stimuli
were identically trained except for their order, they differed in their
potential to block the target CS. As anticipated, after the short
retention interval, extinction of the more recently trained blocking
stimulus (B) produced a recovery from blocking, resulting in
strong suppression to the target CS X as compared with the
no-extinction control group and the group that received extinction
to A. In fact, there was no appreciable recovery from blocking
when the first blocking stimulus (A) was extinguished. This dem-
onstrates that recency is a determinant of the effective competing
stimulus. By contrast, after a 22-day retention interval, we ob-
served the opposite results. That is, extinction of A resulted in
recovery from blocking of the target stimulus, producing strong
suppression of X, as compared with both the group that received
extinction to the second-trained blocking stimulus (B) and the
no-extinction control group. This suggests that there was a primacy
effect with respect to the effective competing stimulus after a
22-day delay. In summary, the present results demonstrate that a
shift in the temporal context yields a shift in the effective com-
peting stimulus from the most recently trained one to the first
trained one. Also, as mentioned previously, the results of this
experiment contraindicate the interpretation of weaker responding
to X as a result of blocking treatment being the consequence of less
stimulus generalization in the blocking group than in the control
group. If responding to X was augmented by generalized excitation
from the other excitatory stimuli (i.e., A and B in the blocking
groups and A, B, C, and D in the control groups of Experiment 1),
then extinction of A and B should have reduced responding to X.
Instead, responding to X was, if anything, inversely related to the
excitatory status of A or B. One could still argue that the blocking
effect observed in Experiment 1 was augmented by generalization
from C and D in the control group. However, this possibility does
not significantly affect the interpretation of our results because it is
Figure 2. Mean times to complete 5 cumulative seconds of drinking on
presentation of the target conditioned stimulus (X) in Experiment 2. See
Table 2 for treatments of the six groups. Ext A extinguished to Stimulus
A; Ext B extinguished to Stimulus B; No Ext did not experience
extinction of either stimulus. Error bars denote standard errors of the
means.
401
RECENCY-TO-PRIMACY SHIFT
clear in Experiments 2 and 3 that A and B do act as competing
stimuli for X.
Experiment 3
Experiment 2 demonstrated that after a 22-day retention inter-
val, there was a shift from recency to primacy in the identity of the
effective competing stimulus. The time delay that was introduced
by the retention interval can be viewed as a shift in the temporal
context. Bouton (1993) suggested that as time passes after training,
the context provided by internal and sometimes external cues
constituting the background also changes. Thus, the passage of
time presumably is accompanied by a gradual change in context.
Consequently, a long interval can be viewed as a temporal context
change in the same manner as a physical context change is viewed,
and both treatments can produce a recency-to-primacy shift with
respect to simple excitation (Bouton, 1993). To extend our finding
of a recency-to-primacy shift in cue competition over a long
retention interval, in Experiment 3, we evaluated the effect of a
physical context shift by varying the context of extinction relative
to that of testing (see Table 3). That is, we tested whether a change
in physical context between training and testing would produce a
shift in the effectiveness of two competing stimuli after sequential
training. As in Experiment 1, we sequentially trained two compet-
ing stimuli using single-phase blocking and tested with a short
retention interval; however, the physical context was manipulated.
When the subjects were trained, extinguished on A or B, and then
tested on X all in the same context (Context 1), we expected to
observe recovery from blocking only when we extinguished the
second-trained competing stimulus. Such a result with a short
retention interval would partially replicate Experiment 2 and indi-
cate that without a context shift, the most effective competing
stimulus is B. We also anticipated that, if we trained the subjects
in Context 1 and then extinguished and tested them in a different
context (Context 2), we would observe a shift to primacy, such that
the first-trained competing stimulus (A) would be more effective,
thereby producing a recovery from blocking in the group that was
extinguished on the first blocking stimulus. Additionally, to make
Experiment 3 parallel to Experiment 2, we had the physical context
shift in Experiment 3 occur just prior to extinction treatment.
Method
Subjects and Apparatus
The subjects were 36 male (264 –378 g) and 36 female (190–254 g)
Sprague–Dawley, experimentally naive, young adult water-restricted rats
(n 12 per group) bred in our colony. The apparatus, stimuli, and Phases
1 and 2 of treatment were identical to those used in Experiment 2. The
subjects that were trained and tested in different contexts were either
trained in an R chamber and extinguished and then tested in a V chamber
or vice versa.
Procedure
Acclimation. On Days 1 and 2, all subjects were exposed to both
contexts. Within each group, for half the subjects, Context 1 was an R
chamber and for the other half it was a V chamber. On Day 1, all subjects
were exposed to Context 1 (the training context) for 30 min and then
Context 2 for 30 min, both with lick tubes present. On Day 2, subjects were
first exposed to Context 2 for 30 min and then Context 1 for 30 min. For
each subject, these sessions were separated by 180 min. No nominal stimuli
were presented. At the end of these sessions, the lick tubes were removed.
Blocking by A (Phase 1) and B (Phase 2). On Days 3–8, Phases 1 and
2 were conducted in the same manner as they were in Experiment 2.
Training occurred in Context 1 for all subjects.
Extinction (Phase 3). On Days 9–12, subjects in Groups Same–Ext A
and Different–Ext A were exposed daily to 100 nonreinforced presenta-
tions of Stimulus A, one every 36 s in the daily 60-min sessions. Subjects
in Groups Same–Ext B and Different–Ext B were exposed daily to 100
nonreinforced presentations of Stimulus B, one every 36 s in the daily
60-min sessions. Groups Same–Ext A and Same–Ext B received these
extinction trials in Context 1, whereas Groups Different–Ext A and
Different–Ext B received them in Context 2. Groups Same–No Ext and
Different–No Ext were merely exposed to Contexts 1 and 2, respectively,
for 60 min. At the end of this session, the lick tubes were restored to the
apparatus.
Acclimation. On Days 13 and 14, each subject was exposed to its test
context to stabilize its baseline drinking rate after training. Times to
complete the first 5 cumulative seconds of drinking were recorded. On Day
13, each subject was exposed to Context 1 for 30 min with lick tubes
present and then Context 2 for 30 min. On Day 14, each subject was first
exposed to Context 2 for 30 min and then Context 1 for 30 min. No
nominal stimuli were presented.
Test. On Day 15, subjects were tested for suppression to Stimulus X in
an 11-min test session. Subjects in the same condition were tested in
Context 1, and subjects in the different condition were tested in Context 2
Table 3
Design of Experiment 3
Group Phase 1, 3 Days Phase 2, 3 Days Phase 3, extinction Test
Same–Ext A (12A/3AX)
1
(12B/3BX)
1
(400 A)
1
X
1
Same–Ext B (12A/3AX)
1
(12B/3BX)
1
(400 B)
1
X
1
Same–No Ext (12A/3AX)
1
(12B/3BX)
1
(Context)
1
X
1
Different–Ext A (12A/3AX)
1
(12B/3BX)
1
(400 A)
2
X
2
Different–Ext B (12A/3AX)
1
(12B/3BX)
1
(400 B)
2
X
2
Different–No Ext (12A/3AX)
1
(12B/3BX)
1
(Context)
2
X
2
Note. A and B competing stimuli; X target cue; ⫹⫽footshock; Same trained and tested in the same
context; Different trained in Context 1 and tested in Context 2; Ext extinction. Numbers next to the stimuli
indicate the total number of trials in that phase. Subscripts indicate different contexts. Slashes indicate
interspersed trials.
402
LIPATOVA, WHEELER, VADILLO, AND MILLER
(testing for all groups occurred in the same context as Phase 3 treatment
and reshaping). Other than this, testing was conducted as in the prior
experiments. As in the previous experiments, animals taking more than
60 s to complete their initial 5 cumulative seconds of drinking were
eliminated from the study. On the basis of this criterion, 1 subject from
Group Same–Ext B was excluded from the analyses.
Results and Discussion
The results of Experiment 3 indicated a recency-to-primacy shift
in cue competition as a consequence of a change in physical
context from training to extinction and testing (see Figure 3). A 2
(context: same vs. different) 3 (extinction: A, B, or none)
ANOVA of the pre-CS scores to complete 5 cumulative seconds of
drinking did not show any main effects or an interaction, all ps
.24. A 2 (context) 3 (extinction) ANOVA on CS scores revealed
a marginally significant main effect of context, p .05, and a
robust main effect for extinction, p .01. More important, the
ANOVA detected an interaction between extinction and context,
F(2, 65) 41.37, p .01, MSE 2.2. Planned comparisons
revealed differences among the three groups in the condition in
which subjects were trained and tested in the same context. Group
Same–Ext B exhibited greater suppression than did both Group
Same–Ext A, F(1, 65) 44.19, p .01, and Group Same–No Ext,
F(1, 65) 25.85, p .01. Additional planned comparisons
detected differences among the three groups in the condition in
which subjects were trained in Context 1 but tested in Context 2.
The group that was extinguished to CS A showed greater suppres-
sion than both the group extinguished to CS B, F(1, 65) 36.82,
p .01, and the No Ext control group, F(1, 65) 40.39, p .01.
In summary, the subjects in the condition in which they were
trained and tested in the same context showed strong responding to
Target Stimulus X (recovery from blocking) only when the
second-trained (most recent) competing stimulus was extin-
guished. This indicates that the most recent competing stimulus
was the most effective one. However, when the subjects were
trained in one context and extinguished and tested in a different
context, the first-trained competing stimulus was the more effec-
tive competing stimulus. With this context shift, only the subjects
in the group that was extinguished to the first blocking stimulus
showed recovery from blocking, suggesting that primacy is a
critical determinant of the effectiveness of a competing stimulus
after a switch in physical context. Consistent with our expecta-
tions, we observed that after a change in physical context, there
was an attenuation of the effectiveness of the most recently trained
competing stimulus and an enhancement of the effectiveness of the
first-trained competing stimulus.
General Discussion
Previous studies have shown that in situations involving sequen-
tial training, a recency-to-primacy shift in simple conditioned
responding occurs after a shift of either the temporal or the
physical context between training and testing. In the present series
of experiments, we investigated the recency-to-primacy shift with
regard to cue competition. In Experiment 1, we demonstrated that
cue competition effects could be obtained in a single-phase block-
ing paradigm, given training of the target stimulus with two
different sequentially trained competing stimuli. In Experiments 2
and 3, we evaluated the relative effectiveness of these two sequen-
tially trained competing stimuli. In Experiment 2, we examined the
effect of interposing long or short retention intervals between
training and testing and observed that when subjects were tested
shortly after training, extinction of the most recently experienced
competing stimulus was effective in reducing blocking, but ex-
tinction of the first-trained competing stimulus was not effective.
This phenomenon of recency determining cue competition after
sequential training of the target cue with two different competing
stimuli has been observed previously (Grahame et al., 1992). We
also observed in Experiment 2 that after a long retention interval,
extinction of the first-trained competing stimulus was effective in
reducing blocking, whereas extinction of the most recently trained
competing cue was not effective. This result suggests that the
effectiveness of the competing stimuli was determined by primacy
after a long retention interval. In Experiment 3, we observed the
same sort of recency-to-primacy shift after a switch of the physical
context between training and testing.
One model that speaks to part of the present results is Dickinson
and Burke’s (1996) revision of SOP (Wagner, 1981). SOP-R was
specifically designed to explain retrospective revaluation effects
such as the recovery from blocking observed in the present series
of experiments. According to SOP-R, on AX–US trials, the mental
representations of Stimulus A, Stimulus X, and the US are acti-
vated into the A1 state. All the nodes that are simultaneously active
in A1 come to be linked by binary excitatory associations. This
means that the organisms learn an A–US association, an X–US
association, and also an A–X within-compound association (along
with US–A, US–X, and X–A associations). Thus, in the case of
extinction of A after AX–US compound training, when Stimulus A
is presented in the absence of X and the US, the representation of
A is activated into A1. Moreover, although Stimulus X and the US
are not actually present, A is able to activate nodes of both stimuli
into A2 on the basis of the A–US association and the A–X
within-compound association. The simultaneous activation of X
and the US in A2 caused by exposure to A alone presumably
strengthens the excitatory association between X and the US. This
Figure 3. Mean times to complete 5 cumulative seconds of drinking on
presentation of the target conditioned stimulus (X) in Experiment 3. See
Table 3 for treatments of the six groups. Ext A extinguished to Stimulus
A; Ext B extinguished to Stimulus B; No Ext did not experience
extinction of either stimulus; Same trained and tested in the same
context; Different trained and tested in different contexts. Error bars
denote standard errors of the means.
403
RECENCY-TO-PRIMACY SHIFT
should result in an increase in conditioned responding to X that
was in fact observed at test after A was extinguished in Experi-
ments 2 and 3.
SOP-R in its original form, however, remains unable to com-
pletely account for the results of the present experiments. After
AX–US and BX–US training, either presentation of A alone or
presentation of B alone should result in an increase in the condi-
tioned response to X. Although extinction of A or B might not
yield the same amount of retrospective revaluation (e.g., the
within-compound association should be strongest between the
elements of the most recently trained compound, and this should
make extinction of B especially apt to support retrospective reval-
uation, i.e., a recency effect), this consequence should not change
because of other factors such as retention interval or the physical
context of testing. In other words, no recency-to-primacy shift is
expected to affect the relative influences of the two competing
stimuli.
Although SOP-R cannot account for the effect of a recency-to-
primacy shift, it can be adapted to allow for a more complete
interpretation of the present results. Assuming that a CS (such as
X) has been sequentially associated with two other CSs (e.g., as a
result of AX–US trials during Phase 1 and BX–US trials during
Phase 2), the potential of the A–X and B–X within-compound
associations to mediate retrospective revaluation could change
with time or physical context. Initially, the most recently trained
competing stimulus would be assumed to be more effective than
the first-trained one. But after extensive time has passed or a
physical context change, this situation might be expected to be
altered, with the first-trained competing stimulus becoming the
more effective in producing retrospective revaluation. In this case,
the resulting primacy effect would be a consequence of proactive
interference with associations to B by associations to A. In the
framework of SOP-R, this would mean that immediately after
sequential training with the compounds AX and BX, Stimulus B
should be more effective than Stimulus A in activating the repre-
sentation of X into A2. But with the passage of time or with a
switch to a context other than that of training, the relative poten-
tials of A and B to activate X into A2 are reversed, making
Stimulus A more effective than Stimulus B in activating X into A2.
We can only speculate how this process might take place. How-
ever, a plausible assumption is that the first-learned association
involving a specific element (in this case, X) is the strongest one
but is initially subject to retroactive interference by the most
recently acquired association. It appears that when extinction and
testing occur directly after training and lack any change in physical
context, the subjects revalue the associative status of X primarily
on the basis of the most recent information (i.e., the B–X associ-
ation). This recency effect suggests that the B–X association
retroactively interferes with the A–X association. Presumably,
recency effects result from the similarity of the spatiotemporal
context of recent training to the spatiotemporal context of testing,
thereby allowing the latter context to serve as an occasion setter
(e.g., Holland, 1992; Miller & Oberling, 1998), which favors
retrievability of the B–X association at the expense of retrievabil-
ity of the A–X association (e.g., Bjork, 2001). However, once the
similarity between training and testing is weakened by a long
retention interval or a switch in physical context, potential retro-
spective revaluation depends more on the first-trained competing
stimulus, thereby allowing the A–X association to proactively
interfere with expression of the B–X association.
An alternative perspective on the present data is provided by the
comparator hypothesis (Miller & Matzel, 1988). Without any
additional assumptions, the comparator hypothesis anticipates that
posttraining extinction of a competing stimulus after blocking
treatment will produce recovery from blocking. But, like SOP-R,
it does not speak to why extinction of A or B should differentially
impact responding to X or why this differential effect changes with
retention interval or test context. However, one could modify it by
applying known principles of interference theory to memories of
training episodes instead of to dyadic associations as is more
commonly done. The results of the present experiments demon-
strate that the rules of the comparator hypothesis are subject to
variations, such as the effects of recency and primacy. For the
comparator hypothesis to account for the present results, one might
assume that there is interference between the memories of different
episodes of training. It can be inferred from the present observa-
tions that when two comparator stimuli are sequentially trained
with a target stimulus, the effective comparator stimulus is depen-
dent on the temporal and physical contexts of testing. If neither the
temporal nor the physical context is shifted between training and
testing, the memories of the most recent training with X (Phase 2
in Experiments 2 and 3) seem to retroactively interfere with the
retrieval or expression of memories from the first phase of training.
That is why, in Experiments 2 and 3, without a temporal or
physical context shift, posttraining extinction of the first-trained
competing stimulus (A) had no appreciable effect on conditioned
responding to CS X. That is, only the most recent memories of X
(in which X was paired with B) were expressed when X was tested.
In contrast, after a physical or temporal context shift, memories of
the most recent phase of training involving X were not expressed
when X was tested, as evidenced by posttraining extinction of B
not recovering responding to X. Instead, only extinction of A
recovered responding to X. This suggests that the interference
between memories based on the different phases of training was
effectively reversed from retroactive to proactive with a physical
or temporal context shift. It is important to note that most of the
published research concerning the comparator hypothesis in rela-
tion to interference has dealt with interference between associa-
tions (e.g., Miller & Escobar, 2002). However, in this case, it must
be assumed that interference is not occurring between associations
but between the memory complexes created during each phase of
training. Although this concept of interference is not widely used
in the associative learning literature, it is prevalent in other areas
of study, such as serial list learning (e.g., Neath, 1993).
Notably, there is a distinction between the two accounts offered
above. According to our modification of the comparator hypoth-
esis, any interference effects must occur between different training
episodes that involve X. Interference does not occur between
specific associations. If interference did occur between the A–X
and B–X associations, extinction of one of these associations
would presumably eliminate the interference when X is subse-
quently tested. In contrast, our extension of SOP-R is compatible
with a mechanism involving interference between specific associ-
ations. For example, when A is extinguished and there is a short
retention interval as in Experiment 2, the B–X association may
interfere with the expression of the A–X association, which would
404
LIPATOVA, WHEELER, VADILLO, AND MILLER
prevent revaluation of X. This distinction does allow the two
proposed accounts to make at least one differential prediction.
According to our version of SOP-R, any recency-to-primacy shift
(i.e., B to A) should not affect responding to X if a temporal or
physical context shift occurs after extinction of A or B. However,
the proposed modification of the comparator hypothesis suggests
that any recency-to-primacy shift will be effective regardless of
whether the context shift occurs before or after extinction of the
competing stimuli. These predictions could be tested experimen-
tally, but such an experiment would involve changing the physical
or temporal context after the extinction of A or B, which could
result in spontaneous recovery or renewal with respect to A or B
(i.e., poor expression of Phase 3 extinction).
The effects of recency-to-primacy shifts have been broadly
discussed. Recency-to-primacy shifts are ordinarily manifest as
increasing retrieval for the first items on a list as a direct function
of delay in time. Bjork (2001) proposed that this shift might be
adaptive with respect to the skills and knowledge one needs to
access in real-world situations. The immediate future is best pre-
dicted by the immediate past, which makes the recency effect
useful. But with changes in time or place, the most recent relevant
past event is not as apt to foreshadow the immediate future.
Consequently, it is functional for recency effects to wane. Func-
tionally, recency phenomena should give way to equal weighting
of all prior events, not necessarily a primacy effect. To account for
this, we must assume a privileged status of first-learned events.
Empirically, here and in many other situations, we see evidence
suggesting that the first learned memory is the strongest (all other
things being equal), and thus any changes in either physical or
temporal context will favor a shift away from transitory recency
effects to the original information learned. Such recency-to-
primacy shifts are evident among various procedures such as free
recall from a list of words; probe (memory-search) procedures;
competing habits; paired associate tasks; and, in the clinical treat-
ment of fears, the determination of which behaviors are elicited by
fear-inducing stimuli. These examples all suggest that the retriev-
ability of a specific association from a sequentially acquired set of
associations is subject to a recency-to-primacy shift. The present
research indicates that in the same way, the retrievability of an
association to a competing stimulus is subject to the same shift.
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Received July 25, 2005
Revision received March 1, 2006
Accepted March 3, 2006
406
LIPATOVA, WHEELER, VADILLO, AND MILLER
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This chapter describes the potential explanatory power of a specific response rule and its implications for models of acquisition. This response rule is called the “comparator hypothesis.” It was originally inspired by Rescorla's contingency theory. Rescorla noted that if the number and frequency of conditioned stimulus–unconditioned stimulus (CS–US) pairings are held constant, unsignaled presentations of the US during training attenuate conditioned responding. This observation complemented the long recognized fact that the delivery of nonreinforced presentations of the CS during training also attenuates conditioned responding. The symmetry of the two findings prompted Rescorla to propose that during training, subjects inferred both the probability of the US in the presence of the CS and the probability of the US in the absence of the CS and they then established a CS–US association based upon a comparison of these quantities. The comparator hypothesis is a qualitative response rule, which, in principle, can complement any model of acquisition.
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College students rated the causal efficacy of Elements X, A, and B of food compounds AX and BX in producing the allergic reaction of a hypothetical patient. The results of a 16-day allergy test were presented to subjects in a serial, trial-by-trial manner. The response format used was a running estimate, in which subjects were asked to rate all of the three foods after each of the 16 trials. Ratings of distinctive Elements A and B diverged and ratings of common Element X decreased as the difference in the correlation of AX and BX with the occurrence and nonoccurrence of the allergic reaction increased. These human causal judgments closely correspond with stimulus selection effects observed in the conditioned responses of animals in associative learning studies. The experiment also directly demonstrated the fact that significant changes in the causal ratings of a stimulus occur on trials in which the cue is not presented. Associative theories such as that of Rescorla and Wagner (1972) predict changes in associative strength only for those stimulus elements that are presented on a particular trial. A modification of the Rescorla-Wagner model is described that correctly predicts immediate changes in the associative strengths of all relevant cues on each trial—whether presented or not.
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In this article I review research and theory on the "interference paradigms" in Pavlovian learning. In these situations (e.g., extinction, counterconditioning, and latent inhibition), a conditioned stimulus (CS) is associated with different unconditioned stimuli (USs) or outcomes in different phases of the experiment; retroactive interference, proactive interference, or both are often observed. In all of the paradigms, contextual stimuli influence performance, and when information is available, so does the passage of time. Memories of both phases are retained, and performance may depend on which is retrieved. Despite the similarity of the paradigms, conditioning theories tend to explain them with separate mechanisms. They also do not provide an adequate account of the context's role, fail to predict the effects of time, and overemphasize the role of learning or storage deficits. By accepting 4 propositions about animal memory (i.e., contextual stimuli guide retrieval, time is a context, different memories are differentially dependent on context, and interference occurs at performance output), a memory retrieval framework can provide an integrated account of context, time, and performance in the various paradigms.
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36 female Wistar or Long-Evans rats were tested in a conditioned fear paradigm to determine whether the extinction of fear of light activates the expression of fear of noise. There was less fear of noise if it had been compounded with the light when paired with shock than if the noise alone had been paired with shock. However, a high level of fear of noise was found in Ss that subsequently underwent extinction of the fear conditioned to light. This finding suggests that overshadowing is not due just to the noise not being associated with shock; it suggests that overshadowing is due in part to a failure of the noise–shock association to be expressed in behavior. (13 ref) (PsycINFO Database Record (c) 2012 APA, all rights reserved)
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Five experiments examined the recency–primacy shift in which memory for early list items improves and memory for later items becomes worse as the delay between study and test increases. Experiment 1 replicated the shift in a recognition task in which the physical form of the study and test items differed, ruling out an explanation that invokes visual memory. Experiment 2 observed the change when only 1 serial position was tested, eliminating an explanation based on changing strategies or proactive interference. Experiment 3 showed a similar change from recency to primacy when the to-be-remembered stimuli were auditory. Experiments 4 and 5 demonstrated that the same recency–primacy trade-off occurs for words in a sentence. Although it is possible to offer piecemeal explanations for each experiment, the dimensional distinctiveness model accounts for the results in each of the 5 experiments in exactly the same way. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
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In three experiments with rats, we demonstrated that a conditioned response that is learned and extinguished in one context (Context A) can be renewed when the conditioned stimulus (CS) is tested in a second context (Context B). In Experiments 1 and 3, the effect was observed in conditioned suppression; in Experiment 2, it was produced in appetitive conditioning. The result occurs when Contexts A and B are equally familiar, equally associated with reinforcement, or equally associated with both reinforcement and nonreinforcement. The results extend the range of conditions known to produce the renewal effect, and they are consistent with the view that retrieval of extinction depends more on the context than does retrieval of conditioning.