Developmental Science 7:5 (2004), pp 581–598
PAPER
Blackwell Publishing, Ltd.
Learning during the newborn’s first meal: special resistance to
retroactive interference
Newborn resistance to retroactive interference
Sarah J. Ferdinand Cheslock,1 Sarah K. Sanders and Norman E. Spear
Center for Developmental Psychobiology, Department of Psychology, Binghamton University, USA
1. Sarah J. Ferdinand Cheslock is now at the Department of Psychology, Ithaca College, USA
Abstract
At their first postnatal meal, 3-hour-old rats learned an association between an odor and a sweet or bitter taste. Retention after
a long interval or after associative interference was compared to that of 1-day-old rats. Despite equivalent and negligible effect
of the long retention interval, contrary to infantile amnesia, newborns differed strikingly from 1-day-olds in susceptibility to
associative interference. When lemon odor predicted saccharin in the first episode but quinine in the second, 1-day-olds had
strong retroactive interference, but the newborn’s first memory was unaffected by the second. The results were identical when
the first memory was a lemon-quinine association and the second a lemon-saccharin association. It is uncertain whether this
special robustness of memories associated with the first postnatal meal is best understood in terms of cognitive primacy or
neurochemical and physiological consequences of the birth process.
Introduction
Nearly 100 years ago, Freud coined the term infantile
amnesia, which refers, most popularly, to the inability of
adults to recall the events of their infancy (see Pillemer
& White, 1989, for review). Most adults report that their
earliest autobiographical memory is of an event that
occurred somewhere between 2 and 4 years old (e.g.
Dudycha & Dudycha, 1933; Usher & Neisser, 1993). The
phenomenon of infantile amnesia is not unique to the
human condition; it is pervasive across every altricial
species tested to date (Spear & Riccio, 1994). Immature
animals (and humans) forget faster than their mature
counterparts, even when the level of original learning is
equivalent (e.g. Arnold & Spear, 1997; Campbell & Spear,
1972; Hartshorn et al., 1998; Spear & Rudy, 1991).
Much research on the accelerated rate of forgetting
in infancy has been done in the rat, which is born very
immature, both deaf and blind, but matures to independence from the mother in about 3 weeks and to
sexual maturity in about 2 months (Spear, 1979).
Compared to older infants and adults, infant rats have a
particularly difficult time learning trace conditioning
(Spear & Rudy, 1991). In a trace conditioning preparation, the removal of the conditioned stimulus (CS), e.g.
tone, and the introduction of the unconditioned stimulus
(US), e.g. shock, are separated by a temporal gap, usually several seconds. Theoretically, to form an association between the CS and the US, the rat has to maintain
a memory representation of the CS during the trace
interval. In this view, the difficulty infant rats have learning trace conditioning could be considered an example
of accelerated forgetting in infants across very short
retention intervals (Spear & Rudy, 1991).
Cheslock, Varlinskaya, Petrov & Spear (2000), however, found that newborn odor-taste conditioning was
unusually rapid and robust, even resistant to the influence
of a trace interval. One hour after a single experience
with lemon odor paired with a taste of milk, saccharin
or sucrose, naïve newborn rats altered their responding
to an empty surrogate nipple in the presence of the
lemon odor. The control groups responded to the empty
nipple in the same way unconditioned pups normally
would, with only brief grasps (e.g. Smotherman, Goffman, Petrov & Varlinskaya, 1997). Pups previously given
a paired presentation of lemon and milk (or saccharin or
sucrose), however, attached persistently to the empty
nipple, spending more than 50% of the test time on the
nipple. Essentially, in the presence of the lemon odor CS,
experimental subjects responded to the empty nipple as
Address for correspondence: Norman E. Spear, Center for Developmental Psychobiology, Department of Psychology, Binghamton University,
Binghamton, New York 13902-6000, USA; e-mail: nspear@binghamton.edu
© Blackwell Publishing Ltd. 2004, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA.
582
Sarah J. Ferdinand Cheslock, Sarah K. Sanders and Norman E. Spear
if it provided the palatable fluid with which the odor was
previously paired (e.g. milk; see Smotherman, Petrov &
Varlinskaya, 1997).
That newborns were capable of expressing single-trial
olfactory conditioning is not particularly surprising.
Altered responding to a nipple capitalizes on the newborn’s
ontogenetic niche and is an age-appropriate behavioral
measure (see Alberts & Gubernick, 1984; Campbell,
1967; Johanson & Terry, 1988; Oppenheim, 1981; Spear,
1984; Spear & Rudy, 1991; West & King, 1987). Furthermore, learning an odor-taste pairing is biologically significant to the neonatal rat – odor signals are critical to
its survival, guiding the neonate to the mother’s nipple,
where it will receive warmth, protection and milk
(Blass, 1990; Blass & Teicher, 1980; Singh & Hofer, 1978;
Teicher & Blass, 1976, 1977). Accordingly, novel odors
(e.g. citrus, cedar) are commonly used as successful
conditioned stimuli in infant learning experiments (e.g.
Johanson & Hall, 1982; Johanson, Hall & Polefrone,
1984; Johanson & Teicher, 1980). Similarly, stimuli that
mimic the mother, or her care, function quite effectively
as unconditioned stimuli in both operant and Pavlovian
conditioning paradigms in infant rats. For example,
pairing an odor with stroking (which mimics mother’s
licking) results in preference for that odor in 6-day-old
rats (Sullivan & Hall, 1988), and 1-day-old rats learn to
raise their heads for an infusion of milk into their mouths
(Johanson & Hall, 1979).
What was surprising about the single-trial olfactory
conditioning found by Cheslock et al. (2000) was its
resistance to challenges. For instance, 1 hour after a
paired presentation of lemon and milk, pups attached
significantly longer than controls to a surrogate nipple
providing saline, a highly aversive fluid. Normally, the
newborn strongly rejects a surrogate nipple providing
saline, which is so aversive that suckling experience with
saline attenuates responding to a water nipple an hour
later (Nizhnikov, Petrov, Varlinskaya & Spear, 2002;
Smotherman, Petrov & Varlinskaya, 1997). Cheslock
et al. (2000) also found strong conditioning even when a
trace interval as long as 60 seconds was imposed
between CS termination and US onset. Similarly, Varlinskaya, Petrov, Simonik and Smotherman (1997) found
that fetal rats tolerated a 30-second trace interval in a
conditioning paradigm that tested an association between
a surrogate nipple (conditioned stimulus) and milk
(unconditioned stimulus), as measured by conditioned
reduction in perioral cutaneous responsiveness after a 5minute retention interval. Furthermore, with pre-exposure
to the nipple CS, fetuses in Varlinskaya et al.’s study
tolerated a trace interval of 2 minutes; the authors cite
change in motor behavior following experience with
the nipple CS (e.g. increased mouthing) as a possible
© Blackwell Publishing Ltd. 2004
behavioral trace bridging the time gap between the
nipple CS and milk US. Cheslock et al. (2000) did not
measure motor changes during the trace interval between
the lemon odor CS and the milk US. For the newborns
in Cheslock et al. (2000) and the fetuses in Varlinskaya
et al. (1997), the conditioning treatment can be construed as the subjects’ first ‘meal’ of its postnatal life.
The surprising and impressive ability of newborn (odormilk) and fetal (nipple-milk) conditioning to withstand
trace intervals suggests that learning about the first meal
may be special.
The purpose of the present study was to test whether
a newborn rat’s memory for an odor-taste association
(their first postnatal ‘meal’) is different to a 1-day-old’s
memory for the same event. The difference of interest
here is the memory’s resistance to common but ordinarily powerful sources of forgetting – long intervals and
associative interference.
Experiment 1 of the present study compared simple
retention for lemon-saccharin pairing(s) after both short
(1-hour) and long (24-hour) retention intervals in naïve
newborns and relatively sophisticated 1-day-old rats.
Using parameters established by Experiment 1, subsequent experiments introduced a source of associative
interference. Experiments 2 and 3 tested resistance of an
original odor-taste memory to a subsequent, conflicting
odor-taste experience. Experiment 2 of the present study
used aversive conditioning (lemon odor paired with
quinine taste; Nizhnikov, Petrov & Spear, 2002) in an
attempt to interfere retroactively with appetitive conditioning (lemon odor paired with saccharin) and tested
pups both 1 hour and 24 hours after conditioning.
Experiment 3 used appetitive conditioning (lemon paired
with saccharin) to interfere retroactively with aversive
conditioning (lemon paired with quinine).
General method
Subjects
Experimental subjects were cesarean-delivered, neonatal,
Sprague Dawley rats (Taconic Company, Germantown,
New York) bred in our colony at Binghamton University.
Adult animals were housed in groups of two females and
one male in opaque, maternity tubs (45 × 23 × 20 cm),
partially filled with pine shavings. For time breeding,
vaginal smears were collected daily, between 7:30 and
9:30 , for the microscopic detection of sperm. The day
sperm was detected was considered embryonic day zero
(E0). When both females in a given cage had positive
smears (or after a maximum of 5 days), the male was
removed. For non-time breeding (for use as foster dams)
Newborn resistance to retroactive interference
vaginal smears were not taken and the male was removed
after 10 days. Females were separated 3 weeks after the
first day of pairing with the male and were checked twice
daily for births (between 7:00 and 9:00  and again
between 4:00 and 7:00 ).
Cesarean section
Near expected term (E21) pups were delivered by cesarean section. Under ether anesthesia, the pregnant female
was sacrificed by rapid cervical dislocation. A midline
incision was then cut through the abdominal wall and
the uterus was externalized. Pups were delivered in rapid
succession through individual, small incisions in the
uterus. After the delivery of the last pup, extra-embryonic
membranes were removed and pups were stimulated to
breathe by vigorous, but gentle, wiping with warm, moist
toweling. Pups were then turned on their sides and
gently compressed with balled up, moist toweling, which
helped to clear the airway of fluids. This procedure, from
cervical dislocation to the firm establishment of independent respiration in all pups, was typically completed
within 3–5 minutes. Umbilical cords were then tied and
cut, and pups were placed in a plastic container (12 × 12
× 6 cm), lined with warm, moist paper towels, maintained
at 35 degrees (± 1 degree) Celsius by a heating pad. Once
in the plastic container, subjects were occasionally
stimulated to maintain independent respiration. Only pups
that remained pink throughout the entire procedure were
used as experimental subjects. The container was then
placed in an incubator (35.0 degrees (± 0.5 degree)
Celsius; 90% humidity; Nursery Hospital Incubator,
Petiatrics, Wichita, Kansas) to simulate nest conditions.
Fostering
When necessary, experimental pups were fostered to a
dam that delivered her litter vaginally between 12 and 36
hours earlier (e.g. Grota, 1968a, 1968b). No fewer than
six and no more than 12 experimental pups were fostered
per dam. During fostering, the dam was placed in a
holding cage while all of her pups were removed from
her home cage and the experimental pups were introduced. Before returning the foster dam to her home cage,
the experimental pups were gently rolled in her soiled
shavings. All animals were housed in a temperaturecontrolled (22 degrees Celsius) vivarium maintained on
a 14 hour light/10 hour dark cycle (lights on at 7:00 )
with ad libitum access to food (Purina ‘Formulab diet’
[5008] breeding formula, Ralston-Purina, St. Louis,
Missouri) and water. Rats were maintained and treated
in accordance with guidelines established by the National
Institutes of Health (1986).
© Blackwell Publishing Ltd. 2004
583
Experimental design and data analysis
In order to conservatively compare newborns and 1-dayolds, method of delivery was made equivalent (cesareansection) and all animals were tested wearing a restraining vest (see General procedures and equipment). It was
thought important to make equivalent gestation age
(cesarean-delivered pups are born slightly immature),
but especially exposure to the catecholamine surge at
birth, which is thought to be stronger after vaginal delivery
than elective cesarean-section delivery (e.g. Vyas, Milner,
Hopkin & Falconer, 1983). Norepinephrine, the primary
catecholamine associated with the hormonal surge at
birth, has also been linked to olfactory learning in infant
rodents (Sullivan, McGaugh & Leon, 1991; Sullivan,
Wilson & Leon, 1989).
Newborn (P0) and one-day-old (P1) pups were given
odor-taste, classical conditioning and were tested for
responsiveness to an empty surrogate nipple in the presence
of the odor. All subjects were tested for responsiveness
to the empty surrogate nipple only once – either 1 hour or
24 hours after conditioning. This procedure yielded a 2
(age) × 2 (retention interval) testing factorial and generated four possible testing groups: trained and tested on
P0 (P0-1 hr), trained on P0 and tested on P1 (P0-24 hr),
trained on P1 and tested on P1 (P1-1 hr), and trained on
P1 and tested on P2 (P1-24 hr). After cesarean section,
pups to be conditioned on P1 spent 4 hours in the incubator before being fostered, to match the P0-24hr group.
To avoid confounding litter with treatment, no more
than one male and one female per litter were assigned to
the same conditioning treatment group, and all conditioning treatment groups were represented within a litter
(Holson & Pearce, 1992). Number of males and females
in each group were equated, and order of testing was
counterbalanced. Videotaped records of the surrogate
nipple test were scored (by an experimenter unaware of
the subjects’ conditioning treatment; inter-experimenter
reliability > 90%) for (a) latency to the first oral grasp
response (calculated to the nearest 1.0 seconds), (b) total
time attached to the nipple (sum of the duration of all
grasps, to the nearest 1.0 seconds), (c) mean grasp duration (total time attached/number of grasps). Comparisons
were made with between-group analysis of variance
(ANOVA) supplemented by Tukey HSD (honestly
significant differences) tests (for discussion see Howell,
2002, pp. 391 and 398–399).
General procedures and equipment
One hour after cesarean section, pups were voided and
weighed to the nearest 0.01 grams (model BP410, Sartorius, Edgewood, New York). Pups less than 5.25 grams
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Sarah J. Ferdinand Cheslock, Sarah K. Sanders and Norman E. Spear
were not used as experimental subjects. Immediately
after both conditioning and testing, each subject was
again weighed. Conditioning experiments involved exposing
the subject to lemon odor (lemon, Lorann Oils, Inc.,
Lansing, Michigan) and to 5 µL/1 second intraoral infusions of fluids (0.1% saccharin sodium or quinine sulfate,
Fisher Scientific, Fair Lawn, New Jersey) in a variety of
schedules, which are detailed within the method section
of each experiment. Fluids were delivered through a
tongue cannula, which was inserted midline of the lower
jaw, with the flanged tip resting on the dorsal surface
of the subject’s tongue – in a medial position, centered
between anterior and posterior placement (Hall &
Rosenblatt, 1977; Kehoe & Blass, 1985). The free end of
the cannula was connected to PE-50 polyethylene tubing,
which was connected to a Gilmont syringe in a rotary
micro-syringe pump, controlled by an on/off switch
(Kashinsky, Rozboril, Robinson & Smotherman, 1990).
Lemon odor was injected into a cotton swab (0.1 cc),
which was ‘waved’ about 2 centimeters above the subject’s head, during conditioning. For the test, the cotton
tip was cut from the wand and mounted on the handle
of the surrogate nipple with an alligator clip, about
1.5 cm from the pup’s nose. Several air cleaners and fans
were used for odor clearance, making sure that no conditioning or testing surface was vibrated or in the path
of air from a fan. During both conditioning and testing,
the subject was swaddled snuggly in a vest made of soft,
light-weight, quick-drying material, adjusted for the subject’s size with a clip (Heron & Spear, in preparation;
Petrov, Varlinskaya & Spear, 2001). Once swaddled, the
subject was placed in the conditioning or testing apparatus and clipped into a stationary holder in a lateral
recumbent position, rotated partially supine, simulating
the natural position of neonatal pups when suckling
from the dam (Eilam & Smotherman, 1998). This vest
calms the hyperactivity of the 1-day-old, allowing for
presentation of the surrogate nipple. Conditioning took
place on a sheet of Plexiglas maintained at 35.0 degrees
(± 1 degree) Celsius by heating pads and illuminated by
an overhead light. Testing occurred in a transparent
glove box (63 × 50 × 25 cm). The ambient temperature
of the glove box was maintained at 28 degrees (± 1
degree) Celsius by two heating pads placed inside the
box. Two openings in the front wall of the glove box
allowed access to the subject for presentation of the
surrogate nipple. The subject was positioned on a small,
round mirror, 7.5 centimeters in diameter, maintained at
35.0 degrees (± 0.5 degree) Celsius via a temperature
controller (Model 40-90B; Frederick Haer Co.,
Brunswick, Maine). The subject was illuminated by a
fiberoptic, cool light (Model LS-150; Lights by O’Ryan,
Vancouver, Washington). The surrogate nipple was
© Blackwell Publishing Ltd. 2004
carved from a block of soft vinyl, 25 millimeters in
length and tapered to a 1-millimeter diameter at the
rounded tip. The base of the nipple was attached to a
dental probe in order to facilitate presentation by the
experimenter. Perioral stimulation applied with the
surrogate nipple was employed to promote attachment
(Petrov, Varlinskaya & Smotherman; 1997; Smotherman,
Goffman et al., 1997).
General conditioning methods
Handling of 1-day-olds was based on extensive pilot work
by Heron & Spear (in preparation), who found that
minimizing maternal separation was key in controlling
variance. Minimal disturbance of the dam was also very
important. To minimize handling of the dam, a white,
plastic divider (which was placed over the open section
of the cage top) was used to gently nudge the dam off
the litter. The divider was then positioned between the
dam and the litter, while the experimenter located the
pup. Once the pup was located, the divider was removed,
the cage top was replaced, and the divider was again
placed over the part of the cage top that was not covered
by food. Care was taken to avoid disturbing a foster dam
(retrieving and returning pups for training and/or testing)
no more than 6–8 times in total on a given day.
Briefly, pups were retrieved from the dam and immediately tongue cannulated, then placed in the incubator
for a 15-minute recovery before the beginning of conditioning. After conditioning, pups were returned to the
incubator for 2 minutes before the tongue cannula was
removed and they were returned to the dam for the
retention interval. Before testing, pups were retrieved
from the dam and immediately placed in the testing
chamber. Newborn procedures matched 1-day-old procedures, except that P0-1hr pups spent the retention
interval in the incubator with littermates. Pups tested 24
hours after newborn conditioning were fostered together
after the last subject’s cannula was removed. The first
newborn cannulation took place 2.5 hours after delivery,
keeping conditioning and testing roughly between 3 and
6 hours after delivery, the optimum time window for
nipple presentation (e.g. Smotherman, Goffman et al.,
1997).
Experiment 1: simple retention
Experiment 1 compared newborn and 1-day-old memory for lemon odor–saccharin taste pairing(s) after both
short (1-hour) and long (24-hour) intervals. As a general
rule, the younger the animal (or human), the faster
the forgetting (see Campbell & Coulter, 1976; Campbell
Newborn resistance to retroactive interference
& Spear, 1972; Hartshorn et al., 1998; Spear, Miller &
Jagielo, 1990). However, based on the unusually robust
memories that seemed to result from first-meal experiences
for newborn and fetal rats (tolerance of trace intervals –
Cheslock et al., 2000; Varlinskaya et al., 1997), memory
for an odor-taste association was expected to be more
robust to the challenge of a lengthy retention interval
in the newborn (its first postnatal ‘meal’) than in the
1-day-old, which would be contrary to the general
rule.
Odor-taste conditioning in the newborn, as measured
by change in responsiveness to an empty surrogate nipple
scented with the odor, seems to require only a single
trial; the behavioral consequences of one pairing did not
differ from that of five massed pairings (Cheslock et al.,
2000). However, as mentioned above, pilot data in the
newborn suggested substantial forgetting 24 hours after
a single pairing. (These pilot data were collected before
the advent of the restraining vest. It is possible that the
difference in 1-hour and 24-hour conditioned responding is something as uninteresting as ease of presentation
of the surrogate nipple: the 1-day-old pup is much more
active than the newborn, making it substantially more
difficult for the experimenter to keep the tip of the surrogate nipple positioned in front of the pup’s mouth.)
Similarly, pilot data collected in the 1-day-old (Heron &
Spear, in preparation) suggested that one pairing may
not be sufficient for expression of 1-day-old learning
even after only 1 hour. (These pilot data were collected,
however, before refinement of 1-day-old handling procedures.) One-day-olds showed strong conditioning after
five pairings – and after refinement of handling procedures. Nevertheless, it was predicted that both newborns
and 1-day-olds would show memory after the short and
long retention intervals with some measure of forgetting
after 24 hours.
Increasing number of pairings and/or distribution of
trials has been shown to help reduce the fast rate of
forgetting that characterizes infancy (e.g. Coulter, 1979;
Spear, 1979; Sussman & Ferguson, 1980). Therefore, to
maximize the degree of conditioning and to find parameters that yield roughly equivalent conditioned responding across age and time, Experiment 1 included a onepairing group as well as a six-pairings group, which
received three distributed trials of two pairings each.
Based on previous experiments and some pilot data, it
was expected that at the 1-hour test, newborn memory
for one or six pairings would not differ; 1-day-old
short-term retention (1 hour), however, was expected
to be better when conditioned with six pairings. After
24 hours, it was expected that retention after six
pairings might be superior in both the newborn and the
1-day-old.
© Blackwell Publishing Ltd. 2004
585
Method
A total of 144 cesarean-delivered neonatal rats, derived
from 30 pregnancies, served as subjects in Experiment 1
(n = 12, per cell – age × conditioning treatment group ×
retention interval). During conditioning, subjects were
presented the lemon odor (CS) either paired (one or six
times) or explicitly unpaired (six times) with discrete,
intraoral saccharin infusions (US). Conditioning in the
C6 group occurred in three trials of two pairings each.
All subjects received one saccharin infusion for every 30
seconds of odor exposure, and all subjects spent a total
of 21 minutes on the conditioning surface. Pups in the
UP6 group received the same amount of CS and US
exposures presented to the C6 group. The UPS group
served as the control group for both C1 and C6 in an
effort to reduce total conditioning time. P0-P0 conditioning and testing had to be conducted between about
3 and 6 hours after delivery, creating a time constraint.
Pups in the C1 and C6 groups were expected to express
memory of the lemon-saccharin pairing(s) by increasing
their attachment behavior toward the nipple compared
to pups in the UP6 group.
The conditioning parameters for each group were as
follows: Pups in the C1 group were positioned on the
conditioning surface (see General methods) and left
undisturbed for 20.5 minutes. Lemon odor was presented
between minutes 20.5 and 21, and a single 5 µL intraoral
infusion of 0.1% saccharin was given at minute 21. Pups
in the C6 group were undisturbed for 8 minutes. Between
minutes 8 and 9, lemon odor was presented, and at 8.5
minutes and 9 minutes, discrete infusions of saccharin
were given. Between minutes 14 and 15, and between
minutes 20 and 21, the procedure was repeated, resulting
in a total of six pairings. Pups in the UP6 group received
intraoral infusions of saccharin at 1, 1.5, 2, 13.5, 14 and
14.5 minutes; they received lemon odor presentations
between minutes 7 and 8.5 and between minutes 19.5
and 21. Either 1 hour or 24 hours after the completion
of conditioning, pups were tested for responsiveness to
an empty surrogate nipple in the presence of the lemon
odor CS. See Figure 1 for a schematic of the conditioning protocols for Experiment 1.
Because latency to the first grasp and mean grasp
duration violated the assumption of homogeneity of
variance, those measures were subjected to a log 10
transformation before statistical analysis. A total of five
scores met the outlier criterion of greater than 2.5 standard deviations above or below the mean: one from log
10 (latency), two from log 10 (mean grasp duration) and
two from total time attached (raw data). Because leaving
outlying scores in the analysis did not influence the
outcome of the ANOVAs for total time attached or for
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Sarah J. Ferdinand Cheslock, Sarah K. Sanders and Norman E. Spear
three-way analysis of variance (2 age × 2 gender × 3
treatment groups, ANOVA) showed no body weight differences. Analysis of variance for the UP6 groups alone
showed no differences across age or retention intervals,
for any measure. The lack of differences across age
and retention interval in the UP6 groups suggests that
unconditioned responsiveness to the empty surrogate nipple
in the presence of lemon odor did not differ according
to age. These UP results mirror the findings from Petrov
et al. (2001) of no difference in unconditioned responding to an empty nipple between cesarean-delivered newborns and vaginally delivered 1- and 2-day-old pups.
Figure 1 Schematic representation of the conditioning
protocols for Experiment 1: simple forgetting
Results
mean grasp duration, statistics are reported with the
outliers remaining in the dataset for those measures.
Removing outliers did influence some results of ANOVAs for latency to the first grasp; where removal of outliers influenced results, both sets of statistical data are
reported; otherwise, outliers remained in the analysis.
For 120 of the 144 subjects, the experimenter testing
the pup was unaware of the pup’s conditioning treatment. Individual one-way analyses of variance (ANOVA)
showed no effect of whether the experimenter conducting the surrogate nipple test was aware of the conditioning treatment of the subject, for any measure, whether
outliers were removed or not. Body weight on postnatal
day 1 (P1) was compared between pups in the P0-24 hr
group and pups in the P1-1 hr group to assess whether
conditioning experience on postnatal day 0 (P0) altered
fostering success as measured by P1 body weight. A
Individual three-way analyses of variance (ANOVA, 3
conditioning treatment groups × 2 ages × 2 retention
intervals) were conducted for log 10 (latency to the first
grasp), total time attached and log 10 (mean grasp duration).
There were no significant interactions for any dependent
variable. As described in the paragraphs below, the results
of Experiment 1 showed strong and equivalent conditioning in newborns and 1-day-olds, with no evidence
of forgetting. Figure 2 presents total time attached,
and Figure 3 presents log 10 (mean grasp duration).
Because the experimenter presents the nipple to the
pup, effects of latency to the first grasp (as a function of
conditioning treatment) are not usually expected. In the
present study, however, it was important to evaluate
whether latency to grasp may have been related to age.
If latency effects were sizeable, they could influence total
time attached to the nipple. Analysis of log 10 (latency
to the first grasp), however, revealed no effects of latency
Figure 2 Newborn data are on the left and 1-day-old data are on the right. There was no effect of age on total time attached to
the surrogate nipple. There was also no evidence of forgetting; 1-hour responsiveness did not differ from 24-hour responding.
There was a sizeable effect of conditioning treatment group, with subjects in groups C1 and C6 not differing from each other but
attaching much longer than the unpaired controls (U6).
© Blackwell Publishing Ltd. 2004
Newborn resistance to retroactive interference
587
Figure 3 Newborn data are on the left and 1-day-old data are on the right. There were no effects of age on log 10 (mean grasp
duration). Both ages displayed significant conditioning at both retention intervals. Mean grasp duration was greater in the paired
groups (C1 and C6) than in the unpaired group (UP6). C1 and C6 did not differ from each other. Furthermore, mean grasp duration
was greater 1 hour after conditioning than after 24 hours.
in the present study. When the outlier was removed, the
main effect of retention interval reached significance,
with latency slightly longer after 1 hour than after
24 hours, F(1, 131) = 5.28, p < .03; mean/SEM: 1 hour =
2.07/.04; 24 hours = 1.94/.04. Although there was no
effect of age at time of conditioning or an age × retention interval interaction, a quick look at the raw means
suggests that the main effect of retention interval was
driven by the P1-24 hr group (raw means/SEM: P0-1 hr
= 138.64s/14.54; P0-24 hr = 130.50s/16.43; P1-1 hr =
152.61s/18.98; P1-24 hr = 97.31s/9.95). In other words,
2-day-olds tended to be slightly quicker in grasping than
newborns or 1-day-olds. Differences between groups,
however, were not significant and were not large enough
to influence the results of total time attached. When testing
newborn, 1- and 2-day-old pups for responsiveness to an
empty surrogate nipple 1 hour after experience with
an empty surrogate nipple, water, ethanol or saccharin,
Petrov et al. (2001) found a similar main effect of age;
newborns showed longer latencies than did one- and
two-day-olds.
Analysis of total time on the surrogate nipple showed
strong and equivalent conditioned responding in both
the newborns and the 1-day-olds and no evidence of
forgetting across time. For total time attached there was
a main effect of conditioning treatment group: F(2, 132)
= 126.61, p < .001; subjects in the paired groups (C1 and
C6) did not differ from each other and attached significantly longer than subjects in the unpaired treatment group
(UP6). Figure 2 depicts the main effect of conditioning
treatment and the lack of effects of age and retention
interval for total time attached to the surrogate nipple.
Mean grasp duration (total time attached/number of
grasps) essentially revealed that newborns and 1-day© Blackwell Publishing Ltd. 2004
olds responded equivalently at both time intervals, with
both ages displaying significant conditioning (relative to
unpaired controls). Analysis of log 10 (mean grasp duration) revealed a main effect of retention interval, F(1,
132) = 7.74, p < .007, as well as main effect of conditioning
treatment, F(2, 132) = 116.28, p < .001. Mean grasp
duration was greater in the paired groups (C1 and C6)
than in the unpaired group (UP6); C1 and C6 did not
differ from each other. Furthermore, mean grasp duration
was greater 1 hour after conditioning than after 24 hours,
implying forgetting (Figure 3). There was no retention
interval × conditioning treatment group interaction,
however. As such, there was no evidence of forgetting.
Furthermore, retention did not differ as a function of
age; there was no main effect of age nor any interactions
with age.
Taken together, the results of Experiment 1 revealed
equivalent conditioning and retention across time in
newborns and 1-day-olds. Even with spaced conditioning and the considerable challenge of a long-retention
interval, there was no evidence of an effect of number of
CS–US pairings on retention. In view of the similar
responsiveness to the surrogate nipple at test by newborns and 1-day-olds, the present procedures could then
be used to compare resistance to conflicting memory
treatments at these ages without confounding by strength
of the initially acquired memory.
Experiment 2: appetitive-aversive conflicting
memories and retention interval
To further test the tenacity of the first meal experience,
Experiment 2 presented conflicting information in two
588
Sarah J. Ferdinand Cheslock, Sarah K. Sanders and Norman E. Spear
different conditioning phases. In the first, an odor (CS)
was paired with ingestion of a preferred flavor (saccharin), and in the second, the same odor was paired with
an aversive flavor (quinine). Both 3-hour-old (newborns)
and 1-day-old rats were given these conditioning phases
and both were tested for retention after 1 hour or 24
hours. The same CS was employed in both conditioning
phases, but the US to which the CS was paired was quite
different, leaving the subject with two conflicting memories about the CS. If a memory acquired about the first
meal is especially robust, newborns should be resistant
to the influence of the second conditioning experience,
even at the relatively short retention interval (1 hour),
when retroactive interference normally dominates (e.g.
Bouton & Peck, 1992; Spear, 1967, 1971). In other words,
after 1 hour, newborns were expected to respond based
on phase 1 with very little influence from phase 2, while
responding in 1-day-olds was expected to be greatly
influenced by phase 2. After a long interval (24 hours),
when proactive interference normally dominates (e.g.
Bouton & Peck, 1992; Spear, 1967, 1971), responding in
both the newborns and the 1-day-olds was expected to
be directed by phase 1 conditioning, with little influence
from phase 2.
Just as increasing the amount of training or practice
usually increases simple retention, increasing the number
of conflicting memory trials traditionally increases the
degree of interference (see Slamecka & Ceraso, 1960;
Spear, 1978). Typically, maximum retroactive interference
is found when the number of conflicting memory trials
(phase 2) somewhat exceeds original learning (phase 1).
Dramatically increasing the number of conflicting memory trials, however, does not add to the effect of increased
retroactive interference; the curve flattens (Slamecka &
Ceraso, 1960). Therefore, to maximize the potential for
retroactive interference at the short retention interval
(1 hour), there was one pairing in phase 1, but four (in
two trials of two pairings each) in phase 2.
Method
Cesarean-delivered, newborn and 1-day-old rats were
assigned to one of four conditioning groups: (1) paired
saccharin/paired quinine (PS/PQ; both phase 1 and
phase 2 paired), (2) paired saccharin/unpaired quinine
(PS/UPQ; phase 1 paired but phase 2 unpaired), (3)
unpaired saccharin / paired quinine (UPS/PQ; phase 1
unpaired but phase 2 paired), or (4) unpaired saccharin/
unpaired quinine (UPS/UPQ; both phases unpaired).
Subjects were also assigned to one of two testing groups:
1 hour (P0-1 hr and P1-1 hr) or 24 hours (P0-24 hr and
P1-24 hr). Phase 1 was the ‘source of proactive interference,’
and phase 2 was the ‘source of retroactive interference.’
© Blackwell Publishing Ltd. 2004
Either 1 hour or 24 hours after conditioning treatment,
subjects were tested for responsiveness to an empty surrogate nipple in the presence of lemon odor. One hundred and twenty-eight pups from 27 pregnancies served
as experimental subjects in Experiment 2 (n = 8 per cell
– age × conditioning group × retention interval).
There was both a source of proactive and of retroactive interference in group PS/PQ; responding at test, relative to appropriate controls, indicated whether subjects
in group PS/PQ were displaying retroactive interference,
proactive interference or neither. PS/UPQ received no
source of retroactive interference and served as the ‘retroactive interference’ control group. Because only phase
1 (appetitive) was paired in PS/UPQ, those subjects
were expected to show strong conditioned responding to
the surrogate nipple at test. If PS/PQ < PS/UPQ, retroactive interference would be indicated. UPS/PQ served
as the ‘proactive interference’ control group. UPS/PQ
subjects received no source of proactive interference as
only the aversive phase was paired. Therefore, UPS/PQ
subjects were expected to reject the surrogate nipple. If
PS/PQ > UPS/PQ, proactive interference would be indicated. UPS/UPQ completed the factorial; because none
of the stimuli were paired in this group, UPS/UPQ subjects were expected to respond to the nipple in the manner
in which a pup given no conditioning experience would
– a series of brief grasps followed by indifference (Petrov
et al., 2001). Although UPS/UPQ was not necessary to
test for retroactive and proactive interference, this group
served to test for any unexpected changes in responsiveness to the empty surrogate nipple that might result
from unpaired experiences with saccharin, quinine and
lemon, possibly resulting from backward, trace or context
conditioning.
For all groups, conditioning treatment lasted 20
minutes. The conditioning protocol for the P group (the
experimental group) was as follows: phase 1 = lemon
odor exposure from minute 7.5 to minute 8 with a 5 µL,
intraoral infusion of 0.1% saccharin at minute 8; phase
2 = lemon odor exposure from minute 13 to minute 14
and from minute 19 to minute 20 with intraoral infusions of 0.1% quinine at minute 13.5, 14, 19.5 and 20.
PS/UPQ, UP/PQ and UPS/UPQ were variations of the
PS/PQ group in which one or both of the phases were
unpaired. PS/UPQ: phase 1 = lemon odor from minute
6 to minute 6.5 with intraoral saccharin infusion at
minute 6.5; phase 2 = intraoral quinine infusions at
minute 11.5, 12, 12.5 and 13 and lemon odor exposure
from minute 18 to 20. UPS/PQ: phase 1 = intraoral saccharin infusion at 2.5 minutes and lemon odor exposure
from 7.5 to 8 minutes; phase 2 = same as PS/PQ group.
UPS/UPQ: phase 1 = intraoral saccharin infusion at
minute 1 and lemon odor exposure from minute 6 to 6.5;
Newborn resistance to retroactive interference
Figure 4 Schematic representation of the conditioning
protocol for Experiment 2: appetitive-aversive conflicting
memories.
phase 2 = same as PS/UPQ. Figure 4 provides a schematic of experimental design for Experiment 2. For all
groups, the infusion pump direction was reversed 2 minutes after the phase 1 infusion to remove any saccharin
remaining in the intraoral cannula. The tubing was then
switched from the saccharin syringe to the quinine syringe
in preparation for phase 2.
Because latency to the first grasp and mean grasp
duration violated the assumption of homogeneity of
variance, those measures were subjected to a log 10
transformation before statistical analysis. Only one score
(from log 10 [mean grasp duration]) met outlier criterion
– greater than 2.5 standard deviations above or below
the mean. Removal of that score did not alter the results
of the ANOVA, so it was left in the dataset.
Half of the subjects were tested without knowledge of
the pup’s experimental treatment and the other half
were tested with knowledge of the pup’s experimental
treatment. This procedure allowed a thorough analysis
of the effect of knowledge of the pup’s treatment group
at testing. Independent, one-way ANOVAs revealed no
effect of knowledge of conditioning treatment for any
dependent measure.
Results
The results of Experiment 2 showed that at the short
retention interval, newborns responded according to what
they had learned in phase 1 as if phase 2 had never
happened. One-day-olds, however, responded according
to what they learned in phase 2 as if phase 1 never happened. After a long retention interval, both newborns
and 1-day-olds were responding according to phase 1
© Blackwell Publishing Ltd. 2004
589
training (see Figures 5 and 6). These results supported
our predictions: expression of newborn memory for the
first meal was very resistant to the influence of subsequent experiences. One-day-old memory tested at the
short retention interval after the same experience, however, was not resistant, but instead very responsive to the
subsequent conflicting memory treatment. These results
were verified by the following statistical analysis.
Individual four-way ANOVA were conducted for
log 10 (latency to the first grasp), total time attached and
log 10 (mean grasp duration). There were 2 conditions
for phase 1 (paired or unpaired) × 2 conditions for phase
2 (paired or unpaired) × 2 ages (P0 or P1) × 2 retention
intervals (1 hour or 24 hours). There were no effects of
latency to the first grasp. Analysis of total time attached,
however, showed main effects of phase 1, phase 2, age
and retention interval: F(1, 112) = 1150.89, p < .001; F(1,
112) = 17.83, p < .001; F(1, 112) = 6.48, p < .02; and
F(1, 112) = 16.78, p < .001, respectively. Similarly, there
were main effects of phase 1, phase 2 and age for mean
grasp duration: F(1, 112) = 1068.99, p < .001; F(1, 112)
= 25.14, p < .001; and F(1, 112) = 35.87, p < .001. For
both total time attached and log 10 (mean grasp duration) there were also four-way-interactions of phase 1,
phase 2, age and retention interval: F(1, 112) = 27.98,
p < .001; F(1, 112) = 36.10, p < .001, respectively (see
Figures 5 and 6).
Total time attached and log 10 (mean grasp duration)
showed the same pattern of results for the main effects
of phase 1, phase 2 and age. Overall, when phase 1
(appetitive) was paired, subjects attached for much
longer, and if phase 2 (aversive) was paired, subjects
attached for slightly less time and vice versa. Newborns
attached slightly longer than 1-day-olds. Furthermore, the
main effect of retention interval for total time attached
showed that attachment was slightly greater after 24
hours than after 1 hour. The effects of interest, however,
were revealed by the four-way interactions of phase 1,
phase 2, age and retention interval (see Figures 5 and 6).
Newborns showed strong proactive interference and no
evidence of retroactive interference at either retention
interval. Both PS/PQ and PS/UPQ showed lengthy
attachments at both the 1-hour and 24-hour retention
intervals. They did not differ from each other or across
time. Phase 2 had no measurable impact on responding.
Furthermore, responsiveness in PS/PQ was significantly
greater than responsiveness in UPS/PQ after both 1
hour and 24 hours, indicating substantial proactive
interference after both the short and long-retention
intervals. Phase 1 strongly determined responding as if
Phase 2 had never occurred.
The 1-day-old subjects, however, showed a much more
conventional pattern of interference. Group PS/PQ showed
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Sarah J. Ferdinand Cheslock, Sarah K. Sanders and Norman E. Spear
Figure 5 The top panels show analyses of retroactive interference – PS/PQ compared to PS/UPQ (the retroactive interference
control group). The bottom panels show analyses of proactive interference – PS/PQ compared to UPS/PQ (the proactive interference
control group). Data for the short retention interval are on the left, long retention interval on the right. As measured by total time
attached (seconds), newborns showed strong proactive interference at both the short and long retention intervals. One-day-olds
showed strong retroactive interference at the short retention interval, but strong proactive interference at the long retention interval.
substantial retroactive interference at the 1-hour test,
but proactive interference at the 24-hour test. After 1
hour, but not 24 hours, responding in PS/UPQ was significantly greater than responding in PS/PQ, indicating
retroactive interference at the short retention interval.
For PS/PQ, experience with the aversive phase (phase 2)
significantly reduced responsiveness to the surrogate
nipple. In fact, phase 2 attenuated responding at the short
retention interval so much that PS/PQ did not differ
from UPS/PQ. After 24 hours, responding in PS/PQ
was significantly greater than UPS/PQ and did not
differ from PS/ UPQ, indicating proactive interference
after the long retention interval. Taken together,
these results show that learning about the first meal is
© Blackwell Publishing Ltd. 2004
powerfully resistant to modification, lending support
to the hypothesis that learning about the first meal
is special.
Of note is that UPS/PQ and UPS/UPQ never differed
from each other or across time for any dependent measure;
responding was consistently very low in both of these
groups. To demonstrate the lack of differences between
these two control groups, Figure 7 presents UPS /PQ and
UPS/UPQ for log 10 (mean grasp duration) in both ages
and at both retention intervals. Paired presentation of
lemon and quinine in phase 2 was not capable of reducing responding to an empty surrogate nipple beyond that
of a group which received unpaired presentation of those
stimuli. The lack of differences found between UPS/PQ
Newborn resistance to retroactive interference
591
Figure 6 The top panels show analyses of retroactive interference – PS/PQ compared to PS/UPQ (the retroactive interference
control group). The bottom panels show analyses of proactive interference – PS/PQ compared to UPS/PQ (the proactive interference
control group). Data for the short retention interval are on the left, long retention interval on the right. As measured by log 10
(mean grasp duration), newborns showed strong proactive interference at both the short and long retention intervals.
One-day-olds showed strong retroactive interference at the short retention interval, but strong proactive interference at the long
retention interval.
and UPS/UPQ in the present study most likely resulted
from a floor effect. In other words, because unconditioned
responding to an empty surrogate nipple is very low,
testing for responsiveness to the empty surrogate nipple
was probably not a sensitive enough measure to show a
conditioned reduction in responding. Testing on a more
desirable nipple, such as a nipple providing milk, would
likely have revealed a difference between UPS/PQ and
UPS/UPQ, with the former showing reduced responsiveness compared to the later. Quinine is a very aversive
stimulus for the neonate, capable of inducing strong
aversive conditioning (Nizhnikov, Petrov, Varlinskaya &
Spear, 2002).
© Blackwell Publishing Ltd. 2004
Experiment 3: aversive-appetitive conflicting
memories
The present study expected that memory for the first
meal experience would be robust against challenges to its
expression. Experiment 1, however, found no measurable
evidence that simple forgetting – the effect of a long
retention interval – differed for a newborn’s memory of
lemon-saccharin association (their first meal) and a 1day-old’s memory for the same association, contrary to
the usual finding that younger infants forget more rapidly. Experiment 2, on the other hand, revealed dramatic
differences in the effects of associative interference on
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Sarah J. Ferdinand Cheslock, Sarah K. Sanders and Norman E. Spear
Figure 7 Log 10 (mean grasp duration) for UPS / UPQ and UPS/PQ groups, showing that there was no significant difference between
those two groups at either age (newborns on left, 1-day-olds on the right) or at either retention interval (1 hour or 24 hours; on
the x-axis).
newborns and 1-day-olds. It is possible that measures of
simple forgetting are not sensitive enough to reveal differences between newborn and 1-day-old learning about
odors predicting a taste. It is also possible that the special properties of newborn learning lie, at least in part,
in its resistance to later modification and not in its durability to extra-experimental effects associated with time.
In Experiment 2, pups were first presented an appetitive association to lemon odor (saccharin), then an aversive one (quinine), and then tested responding to the
empty surrogate nipple in the presence of lemon after 1
hour or 24 hours. In most experiments testing conflicting
memories, the most recent association (quinine) directs
responding over the short term, but the original association (saccharin) regains control over responding after
a lengthy interval (e.g. 24 hours; Bouton & Peck, 1992;
Spear, 1967, 1971). The 1-day-olds responded with
exactly that pattern of behavior. In the newborns,
however, the first association (their first meal) strongly
directed responding at both intervals, as if the subsequent conflicting memory had never been acquired. The
purpose of Experiment 3 was to test whether newborn
learning of an odor-taste association would still be similarly robust against the influence of a subsequent conflicting experience if the first experience was aversive
(lemon-quinine) and the second pleasant (lemon-saccharin).
One could argue that a particularly robust first odortaste experience that was appetitive is advantageous to
the neonate’s survival, because acquiring milk from the
dam is the newborn rats’ only means of nourishment. A
robust memory of a negative odor-taste experience at the
newborn’s first meal would, however, seem to be a disadvantage if it interfered drastically with the pup’s subsequent need to obtain nutrition.
© Blackwell Publishing Ltd. 2004
Method
The conditioning and testing protocols for Experiment 3
were similar to those used in Experiment 2. As in Experiment 2, there were two conflicting phases of conditioning in Experiment 3. Opposite of Experiment 2, however,
phase 1 was aversive (lemon odor-quinine taste) and
phase 2 was appetitive (lemon odor-saccharin taste).
Cesarean-delivered newborn and 1-day-old rats were
assigned to one of three conditioning treatments: paired
quinine/paired saccharin (PQ/PS), unpaired quinine/
paired saccharin (UPQ/PS), or paired quinine/unpaired
saccharin (PQ/UPS). UPQ/PS subjects did not receive
a source of proactive interference as only phase 2 (the
appetitive phase) was paired. Therefore UPQ/PS subjects were expected to show sustained attachment to the
nipple at test and served as the proactive interference
control group. If PQ/PS < UPQ/PS, proactive interference would be indicated. PQ/UPS served as the retroactive interference control group. PQ/UPS did not receive
a source of retroactive interference as only phase 1 (the
aversive phase) was paired. Accordingly, PQ/UPS subjects were not expected to show sustained attachment at
test. If PQ/PS > PQ/UPS, retroactive interference would
be indicated. The group given both phases unpaired
(UPS /UPQ) in Experiment 2 was not included in Experiment 3 (the equivalent would have been UPQ/UPS). In
Experiment 2, UPS/UPQ did not differ from UPS/PQ,
indicating that the empty surrogate nipple test is not
sensitive enough to show reduced responding in a group
given paired experience with lemon odor and quinine
taste, compared with a group given experience with quinine unpaired with lemon. Thirty-six cesarean-delivered
rat pups, derived from seven pregnancies, served as
Newborn resistance to retroactive interference
experimental subjects in Experiment 3 (n = 6 per cell;
age × conditioning treatment).
All subjects remained in the conditioning apparatus
for a total of 20 minutes. For pups in the PQ/PS group,
phase 1 consisted of exposure to lemon odor from minute
7.5 to 8, with a 5 µL infusion of 0.1% quinine administered at minute 8. Phase 2 consisted of lemon odor
exposure between minute 13 and 14 and again
between minute 19 and 20, with intraoral infusions of
0.1% saccharin occurring at minute 13.5, 14, 19.5 and
20. For UPQ /PS, phase 1 consisted of an intraoral
quinine infusion at 2.5 minutes and lemon odor exposure from 7.5 minutes to 8 minutes; phase 2 was identical
to that in the PQ/PS group. For PQ/UPS, phase 1
consisted of lemon odor exposure from 6 minutes to 6.5
minutes, with an infusion of quinine at 6.5 minutes;
phase 2 consisted of saccharin infusions at 11.5, 12, 12.5,
and 13 minutes and lemon odor exposure between 18
and 20 minutes.
There were no effects of age at the 24-hour test in
Experiment 2; both newborns and 1-day-olds showed
strong proactive interference. Therefore, pups in Experiment 3 were tested for responsiveness to an empty surrogate nipple in the presence of the lemon odor CS only
after 1 hour, to assess whether the differential responsiveness at the short retention interval found in Experiment 2 (proactive interference in newborns but retroactive
interference in 1-day-olds) would hold when the aversive
phase was first. Figure 8 provides a schematic of the
conditioning protocols for Experiment 3.
Latency to the first grasp and mean grasp duration
violated the assumption of homogeneity of variance.
Therefore, those measures were subjected to a log 10
593
Figure 8 Schematic representation of conditioning protocols
for Experiment 3: aversive-appetitive conflicting memories.
transformation before statistical analysis. No scores met
criterion for outliers.
Results
As in Experiment 2, analysis of total time attached and
log 10 (mean grasp duration) both revealed substantial
retroactive interference in 1-day-olds and proactive
interference in newborns (Figures 9 and 10, respectively). There were no effects of latency to the first grasp.
The results generally mirrored those of Experiment 2 in
revealing that the newborn’s first meal was represented
by an especially robust memory that was more resistant
Figure 9 The analysis of retroactive interference is shown on the left (PQ/PS compared to the retroactive interference control
group, PQ/UPS). The analysis of proactive interference is shown on the right (PQ/PS compared to the proactive interference control
group, UPQ/PS). As measured by total time attached (seconds) newborns showed strong proactive interference, while 1-day-olds
showed strong retroactive interference.
© Blackwell Publishing Ltd. 2004
594
Sarah J. Ferdinand Cheslock, Sarah K. Sanders and Norman E. Spear
Figure 10 The analysis of retroactive interference is shown on the left (PQ/PS compared to the retroactive interference control
group, PQ / UPS). The analysis of proactive interference is shown on the right (PQ/PS compared to the proactive interference control
group, UPQ / PS). As measured by log 10 (mean grasp duration) newborns showed strong proactive interference, while 1-day-olds
showed strong retroactive interference.
to retroactive interference than a similar memory
acquired by 1-day-old pups.
For both total time attached and log 10 (mean grasp
duration) there were main effects of conditioning treatment
and age as well as an interaction of the two (a) total
time attached: F(2, 30) = 227.02, p < .001; F(1, 30) =
129.40, p < .001; and F(2, 30) = 85.81, p < .001, respectively, (b) mean grasp duration: F(2, 30) = 318.39, p <
.001; F(1, 30) = 78.13, p < .001; and F(2, 30) = 97.48, p
< .001, respectively (Figures 9 and 10). The pattern of
effects was the same for total time attached and for
mean grasp duration. Overall, 1-day-olds spent more
time attached and had longer mean grasp duration than
did newborns. Subjects in the UPQ/PS group spent
more time attached and had longer mean grasp duration
than subjects in the PQ/PS group. In turn, PQ/PS subjects spent more time attached and had longer mean
grasp duration than did PQ/UPS subjects. PS/UPQ >
PQ/PS > PQ/UPS.
The two-way interactions showed strong proactive
interference in newborns and strong retroactive interference in the 1-day-olds. For the newborns, UPQ/PS
spent significantly longer time attached and longer
mean grasp duration than did subjects in both PQ/PS
and PQ /UPS, which did not differ from each other. The
measures used, in other words, detected absolutely no
effect of the retroactively conflicting memory treatment.
This contrasts strikingly with effects on the 1-day-olds.
At this age, subjects in PQ/PS and subjects in the UPQ/
PS did not differ from each other and showed significantly longer time attached/longer mean grasp duration
than did PQ/UPS subjects. For the 1-day-old, in short,
experience with a retroactively conflicting memory
treatment influenced subsequent responding so strongly
© Blackwell Publishing Ltd. 2004
that original learning experience was completely
undetectable.
General discussion
Experiment 1 of the present study compared newborn
learning and memory for an odor-taste association after
a short (1-hour) and a long (24-hour) retention interval.
The prediction was that if there is something special
about memory for the first postnatal meal, newborn
memory for lemon–saccharin association(s) might be
more durable than 1-day-old memory for the same
experience. Overall, the results of Experiment 1 showed
strong simple retention in both newborns and 1-dayolds, even after 24 hours, which is impressive given the
rapid forgetting that normally characterizes infancy
(e.g. Campbell & Coulter, 1976; Campbell & Spear, 1972;
Hartshorn et al., 1998; Spear, Miller & Jagielo, 1990). There
was no effect of age on retention – not even the expected
interaction between age and retention interval – and, in
fact, no evidence of forgetting. The magnitude of conditioned responding after both intervals was substantial.
For example, after 1 hour, paired subjects (collapsed
across age) showed an average of more than 7 minutes
(in a 10-minute test) attached to the nipple in the presence of lemon odor, with unpaired controls showing less
than 2.5 minutes attached.
Experiment 2 revealed the special nature of the newborn pup’s memory for its first postnatal meal by introducing conflicting memories. The first learning experience
in Experiment 2, phase 1, was appetitive (saccharin),
and the second, phase 2, was aversive (quinine). It was
predicted that newborn learning would be highly resistant
Newborn resistance to retroactive interference
to the influence of a subsequently acquired memory. The
1-day-olds were expected to respond in the more
conventional manner, responding at test based on the
last thing learned after a short retention interval and
responding at test based on the first thing learned after
a long interval (e.g. Bouton & Peck, 1992; Spear, 1971).
The results of Experiment 2 supported these predictions:
after a short retention interval, 1-day-old rats were dramatically influenced by phase 2 experience, responding
as if phase 1 had never happened, whereas newborns
responded as if phase 2 had never happened. The first
learning experience, for the newborn, seems to overpower the influence of a subsequent conflicting experience.
After 24 hours, both the newborns and the 1-day-olds
were responding in accord with the phase 1 experience.
Because the 1-day-olds responded according to phase
2 after 1 hour and phase 1 after 24 hours, it is clear that
1-day-olds learned about both phases. Newborns, however, responded according to the first phase after both 1
hour and 24 hours, never showing any influence of the
second learning experience. Therefore, it is possible that
the newborns never learned about phase 2. One can
interpret the results as a failure of retrieval at test based
on interference from the first phase of conditioning
(proactive interference) or as a failure of original learning
– they never learned phase 2. It is clear that newborns
are capable of forming very strong memories about
lemon odor paired with quinine taste, as was presented
in phase 2 of Experiment 2 (Nizhnikov, Petrov & Spear,
2002; Experiment 3 of present study). Newborns are also
quite capable of learning a second phase of conditioning
shortly after a first: newborns showed both strong
sensory preconditioning and strong second-order conditioning at a 1 hour test (Cheslock, Varlinskaya, High &
Spear, 2003). Sensory preconditioning and second order
conditioning involve two phases of conditioning; in the
above mentioned study, one phase involved the temporal
pairing of two odors (lemon and banana) and the other
phase involved the temporal pairing of one of the odors
with an intraoral infusion of milk. We are inclined,
therefore, to interpret the present results as a failure of
expression of phase 2 memory as opposed to a failure of
learning. Regardless, experience with the first phase was
strong enough to override (learning of, or expression of)
the second phase.
The purpose of Experiment 3 was to confirm whether
the age-related dichotomy found after short retention
intervals in Experiment 2 (proactive interference in newborns but retroactive interference in 1-day-olds) would
also apply if the first experience was negative and the
second was positive, the reverse of Experiment 2. In
Experiment 3, the CS in phase 1 was paired with an
aversive substance (quinine), and in phase 2 the same CS
© Blackwell Publishing Ltd. 2004
595
was paired with an appetitive substance (saccharin).
One-day-olds again responded after the short interval in
the fashion typical for older rats, with strong retroactive
interference, but newborns showed strong proactive
interference. One-day-olds, in other words, behaved according to what they learned in phase 2, as one would predict
from the propensity of older rats to respond based on
the last thing learned after a short retention interval (e.g.
Bouton & Peck, 1992). Bucking convention again, the
newborns responded according to what they learned in
phase 1, completely ignoring phase 2. In other words, if
the first learning experience was negative, the newborn
rejected the nipple, despite subsequent positive experiences.
Because in the paired groups of Experiments 2 and 3
of the present study lemon odor was paired with both a
positive (saccharin) and a negative (quinine) consequence,
one might expect behavior toward an empty surrogate
nipple in the presence of lemon odor to be somewhat
conflicted, in the classic approach–avoidance sense (e.g.
Dollard et al., 1939; Dollard & Miller, 1950; Gentry &
Dunlap, 1942; Hearst, 1963, 1967). Pups in that group,
one might predict, would approach the nipple over and
over, grasping and disengaging, conflicted between a
motivation to accept the nipple based on the positive
pairing and to reject the nipple based on the negative
pairing. There was no evidence of conflict from the
measures applied in Experiments 2 and 3, however. The
newborns in the experimental group (paired), which
received conflicting information, were so unaffected by
the second (conflicting) phase of conditioning that they
never differed from the control group that did not receive
the second phase. Conversely, the 1-day-old experimental subjects were so responsive to the second phase that
they did not differ from the group that never received the
first phase of conditioning. Pups in all groups were very
clear about their responsiveness toward the nipple; pups
that accepted the nipple did so without much vacillation,
and pups that rejected the nipple showed very unstable
and ‘agitated’ testing behavior. Pups in the group given
only the appetitive conditioning phase (aversive phase
unpaired) always showed very few grasps and a long total
time attached. The group that only received the aversive
conditioning phase (appetitive phase unpaired) showed
several grasps and a short time attached. The paired
pups in Experiments 2 and 3, then, seem to be less ‘conflicted’ than expected and more absolute in their responsiveness, either to phase 1 or to phase 2.
Interference effects are often greater in younger animals
or humans (e.g. Kail, 2002; Smith & Spear, 1981; Spear,
Gordon & Chiszar, 1972). One might predict, then, that
expression of the original CS–US association would be
more likely to be altered by interference techniques in
the newborn than in the 1-day-old. However, as the
596
Sarah J. Ferdinand Cheslock, Sarah K. Sanders and Norman E. Spear
newborn’s first (or one of the very first) explicit learning experiences as well as its first meal, newborn
memory for the original odor-taste association was highly
resistant to modulation by a subsequent, conflicting
learning experience. The first learning experience,
whether appetitive or aversive, directed the newborns’
responsiveness toward the nipple. The 1-day-old, already
experienced with odor-food pairings occurring in the
nest, responded to the conflicting memory treatment in
the conventional fashion of older animals – displaying
retroactive interference at the short retention interval
and proactive interference at the long retention interval.
Why learning about the first meal may be special is
uncertain. It is possible, and perhaps likely, that newborn
rats (in this case, born by cesarean section and housed
with littermates in an incubator) have already begun to
accumulate postnatal associations (e.g. between the
smell of the dam’s fur and the taste of amniotic fluid just
after delivery). Nevertheless, cesarean-delivered newborn
rats have an extremely limited repertoire of postnatal
experiences, and as such, are in a state of virtual primacy
for postnatal learning. Primacy, the first event in a given
context, has traditionally been associated with promotion of memory strength (e.g. Kennet, McGuire, Willis
& Schaie, 2000; Wright, 1994). Primacy, then, may have
contributed to the remarkable robustness of newborn
and fetal conditioning against the challenge of trace
intervals. The results of the present study, showing that
memory for an odor-taste association differs so dramatically between newborns (naïve to suckling experience)
and relatively sophisticated 1-day-olds, encourages a
direct test of the primacy hypothesis, perhaps comparing
naïve newborns to experienced newborns. We very much
hope to conduct such an investigation in the future.
Given the importance of odor signals in guiding the
newborn rat to the mother’s nipple (Teicher & Blass, 1977),
odor-taste pairings may enjoy a certain level of ‘biological
preparedness’ (Garcia & Koelling, 1966; Seligman, 1971)
in the neonate. The newborn may be specially prepared to
learn odor-taste associations, similar to the way an adult
is prepared to learn an association between taste and illness. It is also possible that newborn learning may be
special because of the physiological consequences of birth,
including a surge of norepinephrine (e.g. Kudo, 1989;
Lagercrantz & Slotkin, 1986), a hormone that has been associated with olfactory learning in infant rats (e.g. Sullivan,
McGaugh & Leon, 1991; Sullivan, Wilson & Leon, 1989).
Acknowledgements
The research presented in this article was supported by
grants from the National Institute of Mental Health
© Blackwell Publishing Ltd. 2004
(R01MH35219) and the National Institute of Alcohol
Abuse and Alcoholism (RO1AA13098). We express our
appreciation to Teri Tanenhaus for assistance with the
manuscript and to Wayne Kashinsky and Lloyd Rozboril
for equipment design and manufacture.
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Received: 6 November 2003
Accepted: 29 January 2004