Alleviation of Forgetting in Preweanhg Rats NORMAN E. SPEAR GREGORY J . SMITH State University o f New York Binghamtorz, New YorJc Three experiments tested the learning and retention of neonatal rats (7, 9, and 1 2 days of age) with a nondirectional active avoidance task, using a vibrotactile conditioned stimulus. The hypothesis was that the substantial deficit of these animals in 24-hr retention is due, at least in part, to a deficiency in memory retrieval. In Experiment I, a reactivation treatment was found to aIleviate the forgetting over the 24-hr period for 12-day olds although having somewhat lesser effect for animals 9 days of age. The reactivation treatment seemed ineffective for rats 7 days of age. Experiments I1 and 111 confirmed the reliability of the reactivation effects with 9- and 12-day olds, while adding further control conditions and providing new information concerning the ontogenesis of latent inhibition.

Following learning, a performance decrement linked to memory processing (i.e., forgetting) typically accompanies the passage of time spent away from the learning situation. A growing body of evidence suggests that such forgetting may be greater in immature than in mature organisms (e.g., Campbell & Coulter, 1976; Campbell & Spear, 1972). This enhanced forgetting by immature animals, including humans, has been termed “infantiIe amnesia.” The present experiments were designed in part to determine whether infantile amnesia has different properties from other forgetting phenomena that are more typically studied in the adult organism. In other words, is the “extra” forgetting found in immature animals attributable to a unique deficit in the processing of memories, a deficit not responsible for the forgetting found in adults? To establish such uniqueness, we need evidence that independent variables influence the forgetting of immature animals in a different manner, or with a different magnitude, than with more mature animals. The specific purpose of the present experiments was to investigate the effects of a reactivation treatment administered to animals of differing ages. Reactivation treatments have been applied frequently in studies of the alleviation of forgetting in adult animals and humans (for reviews, see Spear, 1973, 1976). In a recent series of experiments comparing the relative effectiveness of reactivation treatments in alleviating long-term forgetting over several days or weeks among rats that had learned as preweanlings (16 days of age), weanlings (21 days of age), or as young or old adults, Reprint requests should be sent to Dr. Norman E. Spear, Department of Psychology, State University of New York at Binghamton, Binghamton, New York 13901, U.S.A. Received for publication 1 April 1977 Revised for publication 9 July 1977 Developmental Psychobiology, 11( 6 ) : 5 13-529 (1978) @ 1978 by John Wiley & Sons, Inc.

0012-1630/78/0011-0513$01.00

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Spear and Parsons (1976) found reactivation treatments to be less effective the younger the animal was at the time of original learning. However, the authors recognized that additional study was necessary before a firm conclusion could be reached on this matter. The present experiments tested alleviation of forgetting by neonatal rats over relatively short periods (24 hr). A series of experiments by Misanin, Nagy, and their colleagues established that although clear instrumental learning may be found in rats and mice in the age range 7-12 days, subsequent forgetting is very rapid; under many circumstances, no evidence could be found 24 hr later that these animals had learned (Misanin, Nagy, Keiser, & Bowen, 1971; Nagy & Murphy, 1974). Tasks employed previously to study instrumental learning and retention by preweanlings have most often required simple escape or discriminated escape from electrical shock (although preparations for appetitive instrumental conditioning of neonatal rats have begun to appear, e.g., Anisel, Letz, & Burdette, 1976; Kenny & Blass, in press). The present experiments employed an avoidance task based on the nondircctional escape task devised by Misanin, Chubb, Quinn, and Schweikert (1974). These experiments followed a long series of pilot studies in which a number of control conditions were determined to be necessary to allow conclusions about retention. The inclusion of these control conditions permits study of several phenomena in preweanlings. The data to be presented, therefore, are relevant t o theoretical considerations of age-related differences in retention and forgetting of instrumental avoidance learning, alleviation of the forgetting that occurs over a 24-hr period, disruption of instrumental learning caused by prior experience with noncontingent footshock, and Iatcnt inhibition caused by prior noncontingent presentation of the conditioned stimulus.

Experiment I The 1st experiment tested the effect on rats 7-, 9-, or 12-days old of giving a reactivation treatment prior to a retention test. Retention was tested 24 h r after the rats had been trained on nondirectional active avoidance. For this simple task, the response requirement for an avoidance or escape was climbing out of a shallow well, in any direction. Within each age group, 2 sets of animals were equated in terms o f duration, intensity, and source of a footshock (the unconditioned stimulus, UCS) ;ind in the distributions of onset and offset times of this UCS. For 1 set, the experimental trained animals, onsct of the footshock was contingent upon their failure to emit the instrumental response within 5 sec of the onset of a signal, the conditioncd stimulus (CS; these experiments used a vibrotactile CS like that found effective for neonates by Caldwell & Wcrboff, 1962). Offset of the UCS was contingent upon emission of the instrumental response. For the other set of animals, the shock-maturation controls, responding, and the onset or offset of the UCS were not contingent. A further subdivision of each of these groups evaluated the effect of the reactivation treatment (presentation of the UCS shortly before the retention tests); half the animals were exposed to this treatment and half were not.

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Method Subjects One hundred and twenty albino rat pups of the Sprague-Dawley strain ( R a m s nowegicus) 7-, 9-, and 12-days old were subjects in this experiment. These animals were born and raised in our colony at the State University of New York, Binghamton. Each subject was born within 1 hr of the eventual time of day for training and testing. All animals were housed with their parents and littermates throughout the duration of the experiment with food and water available. They were maintained on a 16-hr light/S-hr dark cycle with lights on at 0700 hours. All training and testing took place during the 1st half of the light period.

Apparatus The apparatus was a revised prototype of the shock well designed and reported by Misanin et al. (1974). The grid floor was 26-cm wide and 27-cm long, comprised of 1-mm rods spaced 8 mm (center to center) apart. On t h s grid floor sat a 26-cm smoked Plexiglas platform in which a 10.5 x 11 .O-cm rectangle was cut out of the center to expose the shock grids below. A bottomless 10 x 10 x 9-cm smoked Plexiglas box with a hinged top was then fitted for this center square so as t o control the release of the animal simultaneous with the onset of the CS. Also located on the same shock grids, but off the platform, was an identical bottomless restraining cage that permitted delivery of an equal number of footshocks of equal intensity to animals serving as maturation shock controls.

Design The design was a 3 x 2 x 2 factorial with 3 age groups (7-, 9-, and 12-days old), 2 treatment conditions at training (experimental-trained and maturation-shock control groups), and 2 treatment conditions at testing (reactivation treatment and no reactivation treatment).

Procedure All animals from each litter were assigned randomly to an age group. Within each age group, animals were randomly assigned to a treatment condition for training (experimental-trained and maturation-shock control groups) and for reactivation (reactivation and no reactivation). The training procedure consisted of removing 2 pups from the litter at a time (1 experimental and 1 maturation-shock control) and carrying them from the room where they were housed into the experimental room in a Plexiglas holding cage. This holding cage was then placed on a heating pad to maintain the body temperature of the pups at or near that found in their littercage. To initiate a training trial, we placed both the experimental-trained and maturation-shock control animals inside their respective Plexiglas restrainers located over the shock grids. For the experimental animal, after 5 sec this Plexiglas restrainer was lifted

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off the grids and away from the platform simultaneously with the onset of the CS. The animal was then required t o climb a 2.5-cm wall to avoid the shock. The CS w;1s a vibrotactile stimulus produced by a BD-1 Line dc buzzer which was initiated 5 scc prior to the onset of a .I-mA footshock delivered by a Grason-Stadler shock generator and scrambler (model No. El064 GS). The CS and UCS remained on until the animal escaped the well. The criterion for an escape was that no more than one paw could have contact with the grid bars. If the animal received 90 sec of shock, it was gently forced up out of the well onto the platform and the escape latency was recorded as 90 sec. The maturation-shock control animal was placed inside a bottomless Plexiglas cube that rested on the grid floor. Animals in this condition received the same intensity, duration, and distribution of footshock as the experimental animal but were unable to avoid or escape the shock. llpon completion of each trial, the yreweanlings were placed into a heated holding cage for an intertrial interval of 30 sec. All animals were trained in 3 blocks of 12 trials, with each block separated from the next by 20 min. The neonates spent the duration of this 20-min period with their parents and littermates. All animals were trained to the criterion of 5 avoidances within 6 consecutive trials, or a maximum of 36 trials. An avoidance response required that no more than 1 paw be on the shock grids within the 5-sec interstimulus interval, so that at least 2 paws were on top of the platform and the 3rd paw was lifted off the grids prior to the onset of the UCS. This criterion was found t o be best suited for this short (1s interval (5 sec) in that limited motor development impaired ambulatory movement at these ages. All animals were tested 24 hr later in the same apparatus. Animals given the reactivation treatment were exposed to a .l-mA footshock for a 20-sec duration, on 2 occasions separated by 30 sec. (Twenty seconds was chosen as the duration of the reactivation footshock because in pilot studies this was the average duration of shock received by each animal on a training trial.) After the reactivation treatment, the animal was confined in the holding cage for 5 min prior to testing. The no-reactivation animals received only confinement in the holding cage for 5 min. Testing consisted of further training trials identical to those of original training. The primary response measure was latency either to avoid or to escape the shock. A score equal to or less than 5 sec was defined as an avoidance whereas one greater than 5 sec was defined as an escape. All responses were recorded to the nearest .I sec.

Results

Acquisition Attainment of the avoidance criterion was more rapid the older the animal. Analysis of valiance verified this difference among 7-, 9-, and 12-day-old animals ( F = 54.68, df= 2/57, p < .OOl) and also established that subgroups given different reactivation treatments prior to the retention test did not differ in their acquisition performance ( F < 1.0; see Fig. 1). The relationship between age and number of trials required to attain the acquisition criterion is not a clear indication of the relationship between age and rate of avoidance learning, because the potential contribution of

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Fig. 1. The relationship between age and number of trials required to attain the avoidance criterion during the initial training of animals later given the reactivation treatment compared to those not given the reactivation treatment.

age-related differences in sensitization to the shock or to the CS has not been extracted.

Retention: Avoidance Behavior An overall analysis of variance tested the effect of the following variables: Prior training on this avoidance task (trained vs maturation-shock control), Presence of Reactivation Treatment just prior t o the test (reactivation vs no reactivation), and Age during original training 24 hr earlier (7, 9, or 12 days of age). A significant interaction

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Fig. 2. The number of trials to attain avoidance criterion for 7-, 9-, and 12-day olds following a 24-hr retention interval.

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bctween Age and Type of Prior Training ( F = 4.77, df= 2/108, p < . O l ) indicated that the more rapid acquisition of avoidance learning by animals previously trained on this task (compared with maturation-shock controls) was more evident the older the animals. Tliis indicates that the older animals had either better learning 24 hr earlier or better retention. Note that the inclusion of the maturation-shock contrclls in this analysis precludes contamination by elementary sensitization or psuedo-conditioning effects. The effectiveness of the reactivation treatment in alleviating forgetting is illustrated by the significant interaction between Presence of Reactivation 'Treatment and Prior Training ( F = 7.85, df= 1/108, p < .Ol). Presence of the Reactivation Treatment enhanced the difference between previously trained and maturation-shock control animals in rate of attaining the avoidance criterion during the retention test. The 3-way interaction did not attain statistical significance ( F = 1.01, df= :2/108). In other words, the interaction between Presence of Reactivation Treatment and Prior Training did not depend upon Age (see Fig. 2). Analyses of variance for each age group taken separately indicated the following: ( I ) Among 7-day-old animals, neither the main effects nor the interactions involviug presence of reactivation treatment and type of prior training were significant; (3) among 9-day-old animals, more rapid relearning of avoidance responding occurred for those in the experimental-trained condition than for the maturation-shock controls (F = 6.87, d ~ f =I / 3 6 , p < .05). This effect was statistically significant among subjects given the reactivation treatment ( F =9.14, df= 1/36, p < .01) but not arnong nonreactivated subjects. The critical interaction between Presence of Reactivation Treatment and Prior Training did not, however, attain statistical significance (F = 2.74, d.[ = 1 / 3 6 ) ; (3) among 12-day-old animals, the latter critical interaction was statistically significant ( F = 6.39, d f = 1/28, I-, < .05): the overall advantage of experimental-trained animals in comparison t o maturation-shock controls ( F = 8.13, df = 1/28, p .< . O l ) was manifested only among animals given the reactivation treatment prior to the retention test ( F = 18.17, df= 1/28, p < .OOl). Among these 12-day-old animals, the reactivation treatment not only enhanced rate of avoidance relearning among previously trained animals, but also retarded avoidance learning among the maturation-shock control animals previously given only noncontingent footshocks ( F = 5.73, d f = 1 /28, p < .OS).

Retention: Latencies Latency scores, especially for the 1st test trial on which few avoidances occurred, may be expected to reflect primarily retention of the escape response. In accord with the results of escape-learning tests by Misanin et al. (1974) and with those of previous studies in OUI laboratory, 24-hr retention was exhibited among 7-day-old animals on the 1st test trial; lower latencies were found among experimental-trained animals than for the maturation-shock controls ( F = 17.19, df= 1/44, p < .001). This effect, however, was not found among either 9-day olds ( F = 2.03, df= I /36) or 12-day olds ( F = 3.72, df= 1/28). As test trials progressed and number of avoidances increased, especially among the older animals, the influence of type or prior training lessened among 7-day-old animals but tended to increase among 9- and 12-day-old animals. For each age, an analysis of variance was conducted with 2 between-subject factors (Prior Experience and Presence of Reactivation Treatment) and 1 within-subject variable (Trials 1-5). For these

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Fig. 3. Mean response latencies (in sec) across the 1st 5 trials of the retention test. Dotted lines represent experimental-trained animals and solid lines represent maturation-shock controls. Black circles indicate nonreactivated experimental, open circles indicate reactivated experimental.

analyses, Prior Training had no significant effects among 7-day-old animals, but among 9- and 12-day-old animals latencies were shorter for animals previously trained on this task than for maturation-shock controls (F = 6.86, df = 1/36, p < .05, and F = 26.03, df = 1/28, p < .001, for 9- and 12-day-old animals, respectively). No other effects attained statistical significances in terms of this analysis.

Discussion This experiment establishes that neonatal forgetting of avoidance learning over a 24-hr period may be alleviated by reactivation treatment in a manner similar t o that observed for older animals and longer retention intervals (e.g., Spear & Parsons, 1976). The influence of reactivation appears to be greater the older the animal. Unequivocal reactivation effects were found among animals trained when 12 days of age, borderline effects were observed among animals trained at 9 days of age, and no evidence for reactivation effects could be found among animals trained when 7 days of age. However, because these age groups also differed markedly in rate of learning and probably degree of learning as well, the effect of age per se is difficult to determine. The effect of the reactivation treatment cannot be attributed to a simple influence

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on performance unrelated to the processing of acquired memories. The reactivation treatment enhanced rate of avoidance learning only among previously trained animals, thereby indicating that the influence of reactivation was upon processing of the previously acquired memory. The absence of an effect of the reactivation treatment on latencies during the initial trials of testing indicates, further, that this treatment had 110 effect on general activity or reactivity to footshock. Rather, the influence of the reactivation treatment seems manifested through an influence on memory processing, probably memory retrieval, that serves to enhance reattainment of the avoidance criterion.

Experiment I 1 That the reactivation treatment tended to impair avoidance learning amlmg animals previously exposed to noncontingent footshocks is an interesting findmg of potential importance. Very little is known about the ontogeny of learning associated with noncontingent events. Evidence of negative transfer from exposure t o noncontingent events can, under the proper circumstances, provide evidence of memory processing equal in value to that shown by positive transfer from experience with contingent events. Perhaps the capacity to learn about noncontingent events develops ontogenetically at different rates from the capacity to learn about contingent events. In Experiment I1 we tested only neonates that were 9 days of age during original training (or control treatment). In addition to the noncontingent events provided animals in the maturation-shock control condition, presentation of only the CS (without a following UCS) also provided an opportunity for learning about noncontingent events. Under a variety of circumstances, presentation of only the C S has been shown to impair subsequent conditioning among adult animals, an effect sometimes termed “latent inhibition” (e.g., Lubow, 1973; Carlton & Vogel, 1967). Thus, a 2nd purpose of Experiment I1 was to test 9-day-old rats for evidence of latent inhibition. As a 3rd purpose of Experiment 11, we replicated Experiment I with the inclusion of control conditions to evaluate the possibly aversive influence of the CS (a vibration). As an additional control condition, we tested in Experiment L1 littermate controls not treated at all until the retention test.

Method

Subjects Forty-eight albino pups of the Sprague-Dawley strain, 9 days old, were obtained from our colony at the State University New York-Binghamton. These animals were born within 1 hr of the eventual hour of training. Thus, all 9-day-old animals were within 1 hr of 9 days of age at the time of training. All animals were housed with their parents and littermates through the duration of the experiment, with constantly available food and water, on a 16-hr light/8-hr dark cycle with lights on at 0700 hours. All training took place during the first half of the light cycle.

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Apparatus The apparatus was the same as described in Experiment I.

Design The design was a 2 x 2 factorial, plus 2 control groups. As in Experiment I, the major variables were Training Condition (experimental-trained vs maturation-shock control) and Reactivation Treatment (reactivation vs no reactivation). One additional group was exposed to the CS but not the UCS during training (CS-only group). A littermate maturation control condition was also included in which the animals were given no specific treatment prior t o the “retention” test. The data were analyzed by using the error term of a 1-way analysis of variance to make planned comparisons.

Procedure

All animals from each litter were randomly assigned to 1 of the 6 treatment conditions. The training and testing procedure was identical to that used in Experiment I except for the following control groups. For the CS-only group the UCS was not presented during initial training, although it did occur during the retention test. The littermate maturation control group was not removed from the litter on Day 1 (of testing) and was given its initial avoidance training on Day 2. Neither the CS-only group nor the littermate maturation control group received reactivation treatments prior to testing. Results Acquisition The mean number of trials required to attain the avoidance criterion (5 avoidances among 6 consecutive trials) was 32 for animals exposed to the full set of contingencies involving the CS, USC, and avoidance response and later given reactivation treatments; also, the mean was 32 for animals given the same training but not given the reactivation treatment later. Thus, the groups performed equally prior to the reactivation treatment. None of the CS-only animals attained the avoidance criterion within the maximum 36 trials allotted whereas 7 of the 16 animals given the avoidance contingencies did attain this criterion. However, these 3 groups did not differ significantly in terms of any response measure during acquisition.

Retention Test: Relearning of Avoidance The interaction between Prior Training (experimental-trained vs maturation-shock controlled) and Reactivation Treatment (reactivation vs no reactivation) was statistically significant (F = 13.82, df= 1/28, p < .OOl). Planned comparisons indicated that among the experimental-trained animals, the reactivation treatment significantly decreased the number of trials required to attain the avoidance response ( F = 16.08,

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’“I

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NRC

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EXPERIMENTAL CONDITION

P i g 4 . The mean number of trials to attain the avoidance criterion of 5 avoidances in 6 consecutive trials, 24 hr after oriQnal training. These animals were trained originally at 9 days of age. 10: 10-day-old mate; RE: reactivated experimental; RC: reactivated maturation-!;hock control; NRE: nonreactivated experimental; NRC: nonreactivated maturation-shock control; CS: CS only.

c / f = I /42, p < .001), but among the maturation-shock controls, presence 01‘ the reactivation treatment had no significant effect (F = 1.03, df’= 1/42). (See Fig. 4.)

R e t e n t i o n : L a ten cies Latencies provided little new information beyond that obtained in the 1st experiment. On the 1st test trial rats that previously had practiced the escape response, whether reactivated prior to testing or not, had uniformly shorter latencies than those that did not have the previous practice ( F = 11.78, d f = 1/28, p < .OOl). No other differences were significant. In view of the several important differences found in terms of trials to relearn the avoidance task, these latency measures primarily reflect escape learning.

Effects of Prior Contingeiicies on Avoidance Behavior Animals given prior exposure to only the CS were significantly facilitated in rate of avoidance learning relative to both littermates given no treatment prior to testing

( F = 4.86, d f = 1/42, p < .05) and littermates in the experimental-trained condition that were not given the reactivation treatment ( F = 5.32, df= 1/42, p < .05). Together, these results indicate that latent facilitation rather than latent inhibition occurred among these 9-day-old animals as a consequence of prior exposure to only the CS. Similarly, prior exposure to only the footshock tended to improve avoidance learning as shown by the test performance of the nonreactivated maturation-shock controls in

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comparison to the previously untreated littermates. Animals in the maturation-shock control condition that did not receive the reactivation treatment required a mean of 23 trials to attain the avoidance. This was not significantly different from the number required by the previously nontreated animals (26.5 trials) nor that of the maturationshock control animals that were given the reactivation treatment (28.5 trials). These results indicate that prior exposure only to the CS resulted in positive transfer whereas prior exposure only to the UCS yielded neither positive nor negative transfer.

Discussion This replication confirmed the findings of Experiment I with respect to the 9-day-old animals. An analysis of variance including Experiment I vs Experiment I1 as factors together with Prior Training (avoidance training vs maturation-shock controls) and Reactivation Treatment (presented or not presented) indicated no main effect of Experiment nor any involvement of this factor in any interactions (F< 1.O; df = 1/64). The critical interaction between Prior Training and Reactivation Treatment was statistically significant, however (F = 12.87, df = 1/64, p < .OOl). We, therefore, may conclude that with 9-day-old preweanlings, the forgetting found after a 24-hr interval may be reduced by presenting a reactivation treatment prior to the retention tests. Facilitation of avoidance learning by prior exposure only t o the CS may represent an ontogenetic phenomenon of considerable interest for those interested in latent inhibition, or it may represent a phenomenon of limited generality. The latter arises from the possibility that the particular CS employed in this experiment, the vibrating buzzer, may have had unique effects. The vibrating buzzer may be similar tactually to the UCS (footshock); also, it may have slight, intrinsically aversive properties. Although this CS did not appear aversive in that the animals in the CS-only condition did not escape from it rapidly, the evidence for dismissing this possibility is not as strong as one would like. To determine whether the present latent facilitation effects have the generality we expect or are restricted to conditioned stimuli having characteristics like the vibrating buzzer, we need further experiments of this type with a variety of different conditioned stimuli. Nevertheless, the present reactivation effects with 9-day-old animals, defined as they are in terms of an interaction between type of prior training and presence of reactivation treatment, seem unlikely to be due entirely to the particular type of CS employed here.

Experiment I11 The purpose of this experiment was to replicate Experiment I with 12-day-old animals and to add further control conditions. Unlike Experiment 11, an important aspect of procedure differed from that of Experiment I. In particular, shock intensity was increased in order to analyze better the effects of noncontingent events. As in Experiment 11, a purpose of this experiment was to evaluate the effects of prior exposures t o only the CS or to only the UCS. In preliminary testing of these conditions with 12-day-old animals, we saw that these conditions affected the rats’ performance equally. To determine whether these effects might diverge with more intensive treatments, we doubled the footshock intensity in the present experiment.

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Method Subjects Forty-eight albino rat pups of the Sprague-Dawley strain, 12 days of age, were obtained from the colony of the State University of New York-Binghamton. These subjects were withn 1 hr of 12 days of age at the time of training. All animals were housed with their parents and littermates throughout the duration of thc. experiment, with food and water constantly available, on a 16-hr light/8-hr dark cycle with the lights on at 0700 hours. All training and testing took place during the 1st half o f the light cycle.

Apparatus The animals were trained and tested in the same apparatus as in Expcrinients I and 11.

Design and Procedure The design was identical to that of Experiment 11, a 2 x 2 factorial with 2 additional control groups. Also, as in Experiment 11, the data were analyzed by planned comparisons using the error term derived from a I-way analysis of variance. The training and testing procedure was identical to that of Experiments I and 11, except that the intensity of footshock was a .2 mA (instead of .1 mA).

Results Acquisition The consequences of presenting 12-day-old animals with only the US were like those found with 9-day-old animals: very few “avoidances” occurred, suggesting that avoidances in the experimental-trained conditions were not due to the (possibly aversive) characteristics of the CS. Of the 8 animals in this condition only I attained the avoidance criterion (response with a latency of 5 sec or less on 5 of 6 trials) within the 36-trial limit. In contrast, all animals in the experimental-trained groups achieved this criterion within 36 trials. The mean number of trials required w.as 15.35 for experimental-trained subjects that later received the reactivation treatment and 14.25 for the experimental-trained subjects that did not; this difference was not statistically significant. These mean rates of acquiring the avoidance criterion were significantly more rapid among these experimental-trained animals than for the animals given only the CS (mean trials to criterion for the latter was 33.75; F = 29.92, df= 1/21, p < .OOl).

Re ten ti on A 2 x 2 analysis of variance with planned comparisons indicated that, overall, the experimental-trained animals attained the avoidance criterion more rapidly than the maturation-shock controls ( F = 11.32, df= 1/28, p < .Ol). (See Fig. 5.) T?ie locus of this difference was among subjects given the reactivation treatment. For thcse animals,

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Fig. 5. The mean number of trials to attain the avoidance criterion 24 hr after original training. These animals were trained originally at 12 days of age. 13: 13-day-old littermate. (See Fig. 4.)

the experimental-trained group acquired the avoidance criterion more rapidly than the maturation-shock controls ( F = 12.1 5 , d f = 1/28, p < .Ol), but among animals not given the reactivation treatment, the experimental-trained group did not differ significantly from the maturation-shock controls. Only the reactivated-experimental group acquired the avoidance criterion more rapidly during testing than the untreated littermates (F = 4.84, d f = 1 /42, p < .05). The best evidence that the reactivation treatment affected memory processing would be found in a significant interaction indicating greater benefit of reactivation among animals previously trained (experimental-trained) than among those not previously trained (shock-maturation controls). Although this interaction term did not quite attain stztistical significance (F = 2.45, df= 1/28, p < .lo), a ceiling effect on measurement may have mitigated the appearance of such an effect. The ceiling effect was in part a consequence of having increased the footshock intensity from that of Experiment I. This served to increase the rate at which the avoidance criterion was acquired. The mean performance among experimental-trained animals given the reactivation treatment in Experiment 111 was about as good as could be measured with the present techniques. (Compare Figs. 4 and 5.) We conclude that the basic reactivation effect was present in the present experiment, replicating the results of Experiment I for 12-day-old animals. Evidence for latent inhibition-impaired test performance due to prior exposure to only the CS-may be seen in terms of several planned comparisons. Those animals previously presented with only the CS required significantly more trials to attain the avoidance criterion than animals in several other conditions: littermates that had received no treatment prior to testing ( F = 10.74, df= 1/42, p < .Ol); nonreactivated experimental-trained animals ( F = 21.92, d f = 1/42, p < .001). In contrast t o the negative transfer produced by prior presentations of only the CS, prior presentations of only the UCS had no significantly deleterious effect on avoidance learning.

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General Discussion These experiments have shown that forgetting ordinarily found in neonates over a 24-hr interval may be alleviated if the retention test is preceded by a reactivation treatment. Among rats trained when either 9 or 12 days of age, this effect was reliable and quite sizeable in absolute terms, but it did not occur among animals trained when 7 days of age. The reactivation treatment was exposure to the previous UCS 5 min prior to the retention test. Such a treatment might be expected to alter the avoidance behavior of neonates for a variety of relatively trivial reasons, but the application of several control conditions precluded interpretation on these grounds. For example, one might expect that exposure to footshock just prior to a test of retention would alter activity or reactivity levels and thus contaminate avoidance performance with effects unrelated to memory processing. Although this may be so, such effects would not alter the present conclusions because these are based on the difference between the reactivation effects found for animals that previously received the avoidance contingencies and those that did not. If activity or reactivity measures alone are influenced by the reactivation treatment, we would expect that the behavior of shock-maturation controls should be altered in the same way as the behavior of the experimental-trained conditions. But this did not occur. Whatever the ultimate interpretation of reactivation effects, it must take into account that they occur only among animals that previously have acquired the memory being tested. The question of the relation between ontogenetic development and the influence of a reactivation treatment must remain unsettled, although the present data certainly are suggestive. Figure 6 presents an index of 24-hr retention found among rats 7, 9, and 12 days of age when exposed, or not exposed, to the reactivation treatment. This

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/

fw k5

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Fig. 6 . Index of retention of the contingencies of avoidance training, obtained b y subtr'icting the mean number of trials to reach the avoidance criterion (experimental-traincd animals niintts thc rnatum tionshock controls).

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index is a relatively pure measure of retention: it is the difference between the test scores of subjects previously presented with the avoidance contingencies and those not (though otherwise treated the same) or, in other words, the differences between the scores of the experimental-trained and maturation-shock-control conditions. This figure, based on the data derived in Experiment I, indicates a progressive increase in the effect of the reactivation treatment with increasing age of the animal. We must remain cautious in forming conclusions from these data about the relationship between age and the influence of reactivation. The difference in the reactivation effect for 9- and 12-day-old animals is particularly uncertain. The data of Experiment 11, which replicated precisely the previous procedures for the 9-day-old animals, obtained a slightly larger reactivation effect (difference between the reactivated and nonreactivated conditions) for this index of retention. Experiment 111, which included an important change in procedure for the 12-day-old animals, indicated a somewhat smaller difference than before. Animals trained when 7 days of age gave no indication that the reactivation treatment enhanced retention scores. However, the last is difficult to interpret because so little learning occurred among the 7-day-old animals in terms of acquisition of the avoidance criterion. A final point regarding interpretation of the present reactivation effects concerns the extraordinarily poor retention shown by nonreactivated subjects after the 24-hr interval. Animals trained with the avoidance contingencies 24 hr earlier attained the avoidance criterion no more rapidly than the shock-maturation controls. This occurred for animals trained when either 9 or 12 days of age. One may argue that rats that had attained the avoidance criterion during original training were not, in fact, exhibiting avoidance behavior but, instead, were responding in a generally activated, anticipatory manner due to sensitization from previous footshocks. If so, the significant retention by animals presented the reactivation treatment might be attributed to their greater footshockinduced sensitization during the retention test. However, t h s explanation is not sufficient because the reactivation treatment benefitted test performance only among those animals previously exposed to the avoidance contingencies; it did not affect the animals given an equal distribution and frequency of footshocks in the absence of the avoidance contingencies. The lack of evidence for retention 24 hr after training is not uncommon with neonatal rodents. hlisanin (J. R. Misanin, personal communication, 1976) also has found n o measurable retention among neonates in a task similar to that of the present experiments, and Misanin, Nagy, and their colleagues have reported a similar absence of retention, with either simple-escape or discriminatedescape behavior, among neonates of certain ages (e.g., Misanin, Nagy, Keiser, & Bowen, 1971; Nagy & Murphy, 1974). Clear evidence of latent inhibition-negative transfer from prior exposures to only the CS-was obtained in rats 12 days of age but not for 9-day-old rats. In fact, the younger animals provided some indication of the opposite effect, latent facilitation. This relationship is summarized in Figure 7. We must be cautious in interpreting age-related differences from these data because tests with the 12-day-old animals (Experiment 111) involved a higher footshock than those with the 9-day-old animals (Experiment 11). On the other hand, among subjects previously trained with the avoidance contingencies, test scores by these 9- and 12-day-old subjects did not show nearly this great a difference. Although we recognize the speculative nature of any discussion of the implications of greater age-related differences in the effects of a

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Fig. 7. The mean difference score in terms of the number of trials required to attain thc avoidance criterion on the retention tcst (CS-only animals minus the littermate control).

noncontingency (CS only; no UCS, n o response-contingencies) compared to a positive contingency (CS always followed by UCS, plus escape and avoidance contingencies), we note that such a result would be consistent with the notion expressed by Levitsky, Goldberger, and Massaro (1977) that the advantage of greater cognitive abilities (In their case, normally nourished vs malnourished animals) becomes more apparent when the elements to be learned are less structured by the demand characteristics of the task. Finally, we wish to deemphasize the absence of an impairment to test performance following exposure to only the UCS. In other tests with preweanlings in our laboratory, we have obtained negative transfer following exposure to only the UCS (which may or may not prove to be instances of “learned helplessness”). With the procedures of the present experiments, however, some conditioning may actually have occurred among animals presented only the UCS. The maturation-shock controls, because of their physical proximity during training to the yoked experimental-trained animal that received the C S , may have detected the CS to some extent and hence were exposed to the CS-UCS contingency. For our purposes, thus does not damage the control value of the shock-maturation condition; CS-UCS conditioning in these animals would only strengthen our conclusions, which assume that the shock-maturation group received only the UCS. This condition does not, however, provide a good test of the influence of UCS-only on transfer to escape learning or avoidance learning.

Notes Preparation of this article was supported by grants from the National Science Foundation (13MS 74-24194) to N.E.S. and the Graduate Research Fund, State University of New York at Binghamton to the G.J.S. The technical assistance of Norman G . Richter and the assistance of Dawn Gilman in running Experiment I are gratefully acknowledged.

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