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Using a variation on the standard procedure of conditioned inhibition (Trials A+ and AX-), rats (Rattus norvegicus) in a circular pool were trained to find a hidden platform that was located in a specific spatial position in relation to 2 individual landmarks (Trials A [right arrow] platform and B [right arrow] platform; Experiments 1a and 1b) and to 2 configurations of landmarks (Trials ABC [right arrow] platform and FGH [right arrow] platform; Experiment 2a). The rats also underwent inhibitory trials (Experiment 1: Trials AZ [right arrow] no platform; Experiment 2a: Trials CDE [right arrow] no platform) interspersed with these excitatory trials. In both experiments, subsequent test trials without the platform showed both a summation effect and retardation of excitatory conditioning, and in Experiment 2a rats learned to avoid the CDE quadrant over the course of the experiment. Two further experiments established that these results could not be attributed to any difference in salience between the conditioned inhibitors and the control stimuli. All these results contribute to the growing body of evidence consistent with the idea that there is a general mechanism of learning that is associative in nature. Keywords: landmark learning, conditioned inhibition, rats, Morris water maze DOI: 10.1037/a0015630

According to O-R theory of instrumental learning, incongruent biconditional discriminations should be impossible to solve in a goal-directed manner because the event acting as the outcome of one response also acts as a discriminative stimulus for an opposite response. Each event should therefore be associated with two competing responses. However, Dickinson and de Wit (2003) have presented evidence that rats can learn incongruent discriminations. The present study investigated whether rats were able to engage additional processes to solve incongruent discriminations in a goal-directed manner. Experiment 1 provides evidence that rats resolve the response conflict that arises in the incongruent discrimination by differentially encoding events in their roles as discriminative stimulus and as outcome. Furthermore, Experiment 2 shows that once goal-directed control has been established the dorsomedial prefrontal cortex is not directly involved in its maintenance but rather plays a central role in conflict resolution processes. Keywords: instrumental learning, goal-directed action, response conflict, rats, prefrontal cortex

Four experiments examined the effects of varying the conditioned stimulus--unconditioned stimulus (CS-US) interval (and US density) on learning in an appetitive magazine approach task with rats. Learning was assessed with conditioned response (CR) measures, as well as measures of sensory-specific stimulus--outcome associations (Pavlovian-instrumental transfer, potentiated feeding, and US devaluation). The results from these studies indicate that there exists an inverse relation between CS-US interval and magazine approach CRs. but that sensory-specific stimulus-outcome associations are established over a wide range of relatively long, but not short. CS-US intervals. These data suggest that simple CR measures provide different information about what is learned than measures of the specific stimulus--outcome association, and that time is a more critical variable for the former than latter component of learning. Keywords: CS-US interval, S-O associations, Pavlovian-instrumental transfer, motivational processes, timing processes

Three experiments showed that 2 associatively activated stimulus representations may engage in excitatory or inhibitory learning, depending on their temporal relationship. Experiment 1a suggested that simultaneously activated stimulus representations show evidence of inhibitory learning in an acquisition test. Experiment 1b showed similar evidence of inhibition in a summation test. Experiment 2 found that activation of 2 stimulus representations in a serial compound resulted in excitatory learning between the antecedent and the subsequent (forward) and inhibitory learning between the subsequent and the antecedent (backward). The results show the dynamic influence of temporal contiguity on mediated learning. Keywords: conditioning, mediated learning, rats

Six experiments used rats to study blocking and unblocking of fear learning. An excitatory stimulus (A) blocked tear learning to a neutral stimulus (B). Unblocking of B occurred if the AB compound signaled an increase in unconditioned stimulus (US) intensity or number. Assessments of associative change during blocking showed that more was learned about B than A. Such assessments during unblocking revealed that more was learned about B than A following an increase in US intensity but not US number. These US manipulations had no differential effects on single-cue learning. The results show that variations in US intensity or number produce unblocking of fear learning, but for each there is a different profile of associative change and a potentially different mechanism. Keywords: associative change, surprise, prediction error

Rats homed with food in a large lighted arena. Without visual cues, they used dead reckoning. When a beacon indicated the home, rats could also use the beacon. Homing did not differ in 2 groups of rats, I provided with the beacon and 1 without it; tests without the beacon gave no evidence that beacon learning overshadowed dead reckoning (Experiment 1). When the beacon was at the home for 1 group and in random locations for another, there was again no evidence of cue competition (Experiment 2). Dead reckoning experience did not block acquisition of beacon homing (Experiment 3). Beacon learning and dead reckoning do not compete for predictive value but acquire information in parallel and are used hierarchically.

Rats (Rattus norvegicus) were allowed to hide food items on an 8-arm radial maze by carrying the items from the center to boxes at the end of each arm. Retrieval tests given after rats had hidden 4 items showed that they selectively returned to the maze arms where food had been hidden (Experiments 1 and 2). When rats were allowed to hide pieces of cheese (preferred food) and pretzels (less preferred food) on different arms, they both hid and retrieved cheese before pretzels (Experiments 2-5). In Experiment 6, rats chose between arms where cheese and pretzels were hidden, with cheese degraded at one delay interval but not the other. Together, these experiments indicate memory for what and where but not when.

Rats collected nuts from a container in a large arena in four experiments testing how learning about a beacon or cue at a goal interacts with learning about other spatial cues (place learning). Place learning was quick, with little evidence of competition from the beacon (Experiments 1 and 2). Rats trained to approach a beacon regardless of its location were subsequently impaired when the well-learned beacon was removed and other spatial cues identified the location of the goal (Experiment 3). The competition between beacon and place cues reflected learned irrelevance for place cues (Experiment 4). The findings differ from those of some studies of associative interactions between cue and place learning in other paradigms.

Three experiments used a Pavlovian magazine-approach procedure in rats to explore, in 3 theoretically interesting cases, the inferences made from savings tests. In each experiment, a compound test procedure allowed the separation of differences in rate of acquiring new associative learning from differences in initial level of associative strength. Experiment 1 found that the slower acquisition after prior nonreinforced exposure (latent inhibition) reflected differences in learning rate, not initial level. By contrast, Experiments 2 and 3 found that the higher performance observed to a previously trained and extinguished stimulus, and lower performance observed to a conditioned inhibitor, both arose primarily because of differences in initial associative value. These experiments illustrate the usefulness of a novel testing procedure in sharpening the conclusions from savings procedures.

Neural manipulations were used to examine the mechanisms that underlie the acquired equivalence and distinctiveness of cues in rats. Control rats and those with excitotoxic lesions of either the hippocampus (HPC) or entorhinal cortex (EC) acquired the following conditional discrimination: In Contexts A and B, Stimulus X [right arrow] food and Stimulus Y [right arrow] no food, and in Contexts C and D, Y [right arrow] food and X[right arrow] no food. Rats then received many food pellets in A but not in C. After this treatment, control rats showed more magazine activity in B than in D--an acquired equivalence-distinctiveness effect. This effect was also evident in HPC rats but not in EC rats. These results indicate that changes in stimulus distinctiveness are dissociable from the process of conditional learning.

Increasing the amount of sunflower seeds under a baited object when it occurs either within or external to a simultaneous object discrimination increases the value transferred to the nonbaited object. Rats were trained with two object discriminations, [A.sup.1][B.sup.0] and [C.sup.1][D.sup.0], where the superscript refers to the number of sunflower seeds obtained for pushing aside the object. The amount under A was increased to six on other trials in which the [A.sup.6] or [C.sup.1] object alternated with regular [A.sup.1][B.sup.0] or [C.sup.1][D.sup.0] discrimination trials. The A and C objects on the alternating trials were presented alone (Experiment 1), with a common nonbaited object ([A.sup.6][X.sup.0] and [C.sup.1][X.sup.0]; Experiments 2 and 3), or with two different nonbaited objects ([A.sup.6][X.sup.0] and [C.sup.1][Y.sup.0]; Experiment 4). Subsequent probe tests with pairs of usually nonbaited objects (e.g., [B.sup.1][D.sup.1], [X.sup.1][Y.sup.1]) showed that the rats preferred B to D in Experiments 1-3, but not in Experiment 4, where they preferred X to Y. These preferences cannot be accounted for solely by differential inhibition, because initial training with only [A.sup.1][B.sup.0] and [C.sup.1][D.sup.0] discriminations in three of the four experiments prevented the rats from making or greatly reduced their making more nonreinforced responses within the [C.sup.1][D.sup.0] than within the [A.sup.1][B.sup.0] discrimination. Therefore, these preferences must be considered to reflect a value transfer effect. Other preference probe tests between X or Y and B or D in Experiments 2-4 revealed a possible differential inhibition effect, however, since the rats preferred a more recently introduced nonbaited object to an originally presented nonbaited object regardless of which baited object had accompanied it.

Three experiments used rats to examine the acquired equivalence or distinctiveness of cues occurring when 2 auditory and 2 visual stimuli were associated with either the same or with different reinforcing outcomes ([A.sub.1]+, [A.sub.2]+, [V.sub.1]*, and [V.sub.2]* vs. [A.sub.1]+, [A.sub.2]*, [V.sub.1]+, and [V.sub.2]*). Subsequent single modality discrimination learning (e.g., [A.sub.1]+, [A.sub.2]-) was assessed in Experiment 1, whereas in Experiment 2, visual and auditory discriminations were tested concurrently (i.e., {A.sub.1] +, [A.sub.2]-, [V.sub.1]-, [V.sub.2]*). In Experiment 3, auditory and visual discriminations ([A.sub.1] +, [A.sub.2]-, [V.sub.1]-, [V.sub.2]*) were trained and then reversed, using either the same ([A.sub.1]-, [A.sub.2] +, [V.sub.1]*, [V.sub.2]-) or different outcomes ([A.sub.1]-, [A.sub.2]*, [V.sub.1]+, [V.sub.2]-) within each stimulus modality. Discriminations were learned more rapidly in these studies when different outcomes were associated with stimuli from the same modality. These results challenge associative mediational theories of acquired equivalence and distinctiveness.

Hungry rats were trained to perform two instrumental actions, one for salt- and the other for lemon-flavored polycose solution. When they were sated on one of these two outcomes by prefeeding immediately prior to a choice extinction test, the action trained with the prefed solution was performed less than the other action. The subsequent experiments examined the role of incentive learning in this specific satiety-induced outcome revaluation effect. The second experiment demonstrated that the experience of consuming a flavored polycose solution to satiety enabled the state induced by polycose consumption to control the devaluation of the flavored outcome. By contrast, the third study found that, although devaluing the prefed outcome, specific-satiety treatments could induce a relative inflation in the incentive value of other food outcomes. The final two studies demonstrated an increased outcome-devaluation effect in instrumental performance when these devaluation and revaluation effects were combined. Taken together, these studies demonstrate that specific satiety treatments produce changes in outcome value that depend upon incentive learning.

Rats' ability to discriminate between familiar conspecifics was investigated in a series of three experiments using a go/no-go discrimination procedure. Results of the first experiment revealed that rats are able to recognize conspecifics as individuals. The second experiment showed that rats had more difficulty in acquiring a reversed discrimination than a novel discrimination. Results of the last experiment indicated that rats can recognize individuals using only their olfactory signatures.

Neonatal rat pups exhibit a complex constellation of behaviors in response to a variety of salient stimuli such as the odor of milk or maternal saliva, stroking with a soft brush, electrical brain stimulation, and intraoral infusions of milk. Although psychobiologists have used the term 'behavioral activation' to refer to such behavioral displays, the exact nature of 'behavioral activation' and its underlying neural substrates have yet to be elucidated. This study was undertaken to characterize 'behavioral activation' quantitatively to describe and define this apparently global pattern of response in terms of possible underlying components. Principal components analysis suggested that 'behavioral activation' may be comprised of separable ingestive, exploratory, and locomotor behavioral 'assemblies.'

Rats trained in one context to use stimuli arising from food deprivation as discriminative signals for shock were tested in other contexts to assess the basis of conditioned responding (i.e., freezing or behavioral immobility). In Experiment 1, discriminative control by 24-h food-deprivation cues failed to promote transfer responding in a test context that had no association with shock. This indicated that food deprivation cues had little direct excitatory power. However, transfer of behavioral control by 24-h food-deprivation cues was obtained in a context paired with shock only when the rats were 19 h water deprived. This finding agrees with the idea that food-deprivation cues become conditioned modulators of the capacity of external stimuli to activate their association with an unconditioned stimulus. In Experiment 2, rats trained to use 24-h food-deprivation cues as signals for shock exhibited significantly greater transfer performance when the transfer context had undergone partial extinction relative to when the transfer context had undergone only simple excitatory training. This finding with deprivation cues and transfer contexts (1) paralleled earlier results obtained with discrete (auditory and visual) conditioned modulators and transfer targets, and (2) posed difficulties for associative summation and generalization interpretations of transfer performance.

If freezing underlies barpress conditioned suppression, then it seems odd that auditory cues paired with shock evoke more freezing than do visual cues, yet evoke similar suppression. Bevins and Ayres (1992) found that auditory and visual cues also evoked similar withdrawal from the bar and dipper areas and suggested that such withdrawal could explain the similar suppression. Seeking to understand that withdrawal, we found evidence in the present study that it was due either to adventitious punishment or to place-aversion learning. The cue for shock seemed to set the occasion for such learning. For example, we found that, as training progressed, rats' tendency to leave the bar area during the cue first increased, then decreased, then increased again, reflecting, presumably, shock occurrence first inside, then outside, then inside the bar area again. Despite these changes in the rats' location, barpress suppression remained stable, implying that leaving the bar area, though sufficient for barpress suppression, is unnecessary.

Rats lived continuously in an operant chamber in which they were able to press a bar to obtain food on a chained FR50:CRF schedule that allowed them control of both the size and frequency of individual meals. Independent groups of animals were scheduled to receive 12, 24, 48, or 96 electric shocks per day, which were given randomly in time and independent of the subjects' behavior. Rats could avoid shock by remaining in a safe area of the chamber, but they were always at risk while barpressing. The introduction of shock resulted in a number of changes in feeding patterns. In rats exposed to a possible 12 shocks/day, meal size increased whereas meal frequency changed very little. At 24 shocks/day, meal frequency decreased whereas meal size increased such that net intake remained stable relative to a preshock baseline period. As shock density was increased to 48 or 96 shocks/day, total intake was suppressed. At 96 shocks/day, both meal frequency and meal size decreased dramatically. Shock-related changes were also observed in rates of operant responding and in the amount of time the animals engaged in feeding-related behavior. All of the animals were able to achieve a greater than 50% reduction in the total number of shocks received relative to equivalent random samples of their position in the apparatus taken during baseline. These results support the position that the nature of defensive changes in feeding behavior that are seen when an aversive stimulus is introduced into a simulated foraging situation varies as a function of risk.

Experiment 1 investigated the behavior of rats trained to leverpress on a concurrent variable ratio (VR) 30 VR-30 schedule with a brief, 500-msec, light occurring at the midpoint of the ratio on one of the levers. Higher response rates were recorded on the lever associated with this stimulus, a finding that paralleled the effect produced by inserting primary reinforcement at the midpoint (i.e., by training on a concurrent VR-30 VR-15 schedule). Similar results were found in Experiment 2 using a concurrent VR-20 VR-20 schedule with a 2-sec visual stimulus presented midway through one of the components. In addition, a brief stimulus inserted midway through the VR-20 component of a concurrent VR-20 VR-10 schedule retarded the development of a difference in response rates between the components relative to a VR-20 VR-10 group lacking the signal. In Experiment 3, multiple VR VR schedules were used. Again, the response rate was higher in the component that had the added stimulus or, for a second group of subjects, on the component with the smaller response requirement. Probe-choice trials revealed a preference for the component that generated the higher rate in both groups. Presenting a stimulus partway through a ratio appears to reduce the effect on response rate and choice of a large ratio value.

We investigated the effect of associating unique contextual cues with an interpolated learning task on retroactive interference in long-term memory. Rats were originally trained in a two-bar operant chamber with an auditory conditional discrimination stimulus. During interpolated learning, which occurred in either the original or a new context, some rats were trained on a probability learning task that did not include the auditory stimuli present during original learning. Subsequent retraining on the original conditional discrimination task in the original context showed that (1) significant retroactive interference occurs in rats, and (2) the presence of unique contextual cues during interpolated learning significantly reduces this interference. These results extend the conditions under which the susceptibility to retroactive interference can be altered by contextual cues.

Long-Evans rats were reinforced for generating variable sequences of four left (L) and right (R) leverpress responses. If the current sequence, for example, LLRR, differed from each of the preceding five sequences, then a food pellet was provided. Otherwise, there was a brief time-out. After the rats were generating a variety of sequences, one sequence was concurrently reinforced whenever it occurred, that is, an 'always-reinforced' contingency was superimposed. The frequency of that sequence increased significantly from the variable baseline (Experiment 1). The 'difficulty' of the always-reinforced sequence influenced the extent to which its frequency increased - 'easy' sequences increased greatly, but 'difficult' ones increased not at all (Experiment 2, and a replication with pigeons in Experiment 3). However, when reinforcement for baseline variability was systematically decreased while a difficult sequence was always reinforced, that sequence slowly emerged and attained relatively high levels (Experiments 4 and 5). Thus, a technique for training even highly improbable, or difficult-to-learn, behaviors is provided by concurrent reinforcement of variation and selection.

A four-arm radial maze containing 10 feeders in each arm (patch) was used to study patch sampling in rats. In each of three experiments, rats foraged for 30 sessions. On each session, two randomly chosen patches were baited with food an dthe remaining two patches were empty. In Experiment 1, the number of baited feeders in baited patches (b) was varied from 1-10 over five groups of subjects. Mean visits to empty patches was an inverse function of b, as predicted by an optimal foraging model. In Experiments 2 and 3, rats' ability to discriminate between baited and empty patches was examined when food in baited patches was placed in fixed locations, either in clumps (Experiment 2) or distributed throughout the patch (Experiment 3). Rats in fixed-food-location conditions reliably visited fewer feeders in empty patches than did rats in randomly changing control groups. Examination of within-patch foraging patterns indicated that rats in fixed-food-location groups selectively sampled potentially baited locations and abandoned the patch if food was not found. It is suggested that processes of patch discrimination were responsible for these effects.

Ten rats were trained in a temporal generalization tas (the peak procedure) with variations in the time of reinforcement, the intertrial interval, and the mean and variance of the duration of nonfood trials. There were three types of asymmetry in the temporal generalization gradients: positive skew, secondary rise, and positive asymptote. Asymmetrical gradients can occur as a result of asymmetrical sources of variance, multiplicative combinations of symmetrical sources of variance, and effects of anticipation of the end of a trial and the conditions of the next trial. Ten additional rats were trained with a single time of reinforcement, a limited time of reinforcement availability, long and fixed durations of nonfood trials, and a nonresponse requirement for ending a trial. These conditions markedly reduced all asymmetrial sources of variance and led to very symmetrical gradients. These results demonstrate that none of the asymmetrical sources of variance necessarily has a substantial influence on observed temporal generalization gradients.

Many researchers have used the controlled procedures of operant methodology to test the predictions of foraging theory on the assumption that foraging is operant behavior and operant behavior is foraging. This experiment tested the assumption that leverpressing is functionally equivalent to search effort. Four rats were exposed to two situations: a naturalistic foraging situation (a 'patch') and a standard operant chamber. Each condition represented a different prey density on the patch and a corresponding change in the probability of reinforcement in the chamber. The average number of presses per pellet was linearly related to the density of pellets on the patch. This result indicates that leverpresses were functionally equivalent to search effort.

In three experiments, the specificity of action of occasion setters was examined, using a discrete-trial leverpress procedure. Rats' acquisition of an XA +, A -, XB -, B + discrimination, in which a single feature cue (X) signaled the reinfocement of responding in the presence of one target cue (A) and the nonreinforcement of responding during another target (B), was no more difficult than acquisition of XA +, A -, X -, B + or X +, A -, XB -, B + discriminations, in which X signaled only one target-response-reinforcement contingency. In transfer tests, X did not modulate responding controlled by other cues that were untrained or consistently reinforced, either when the elements of the compounds were presented serially or when they were presented simultaneously in training and testing. However, after serial, but not after simultaneous, compound training, X facilitated responding controlled by a cue that was trained and then extinguished. Implications for the nature of occasion setting and configural learning are discussed.

Two experiments investigated the effectiveness of multiple (five) sessions of signaled escapable-shock pretraining in preventing (immunizing against) the shock-escape impairment produced by an equal number of sessions of signaled inescapable shock. In Experiment 1, rats were exposed to 50 pairings per session of a white-noise stimulus with escapable shock during the immunization phase. Subsequently, they were exposed to 50 pairings per session of a different (houselight) stimulus with inescapable shock. Shock-escape performance in a shuttlebox test with constant illumination revealed no evidence of immunization relative to the performance of rats given five prior sessions of light-signalled inescapable shock only. Experiment 2 was identical in all respects to Experiment 1, except that both the escapable- and the inescapable-shock phases for animals in the immunization treatment group involved the same stimulus (houselight) as a shock signal. Under these circumstances, the prior escapable-shock training significantly reduced the shuttlebox escape deficit engendered by chronic exposure to signaled inescapable shock; performance in the shuttlebox was not reliably different from that of rats exposed to signaled escapable shock alone. These findings suggest that, under chronic conditions, the development of stimulus control using Pavlovian conditioning procedures may serve to modulate the normally prophylactic influence on later shock-escape acquisition of serial exposure to escapable and inescapable shocks.

Two experiments were carried out to study vertical jumping avoidance learning in rats. In particular, we examined the effects of the duration of a feedback stimulus and of the interval between the end of the feedback stimulus and the start of the next trial on acquisition and extinction of avoidance. In Experiment 1, the duration of feedback was manipulated while intertrial interval (feedback plus no-feedback) was held constant. Animals with feedback lasting more than 1 sec needed fewer trials to reach the acquisition criteria than did animals with no feedback or with 1-sec feedback. No differences were observed in extinction. In Experiment 2, the durations of both feedback and no-feedback were manipulated. Animals without feedback needed more trials to reach the acquisition criterion than did animals with feedback, but the performance of the feedback animals did not differ as a function of feedback duration, no-feedback duration, or total intertrial interval. Again, no differences were observed in extinction. These results indicate that the presentation of feedback improves the acquisition of vertical jumping avoidance, but that this effect is independent of the temporal characteristics of feedback.

Ten rats pressed levers for food reinforcers delivered by multiple schedules. Behavioral contrast was measured using a within-session procedure that presented the baseline and contrast schedules within single sessions. The absolute sizes of both positive and negative contrast increased and then decreased as components lengthened. Negative induction occurred when components were very short. These results question theories that predict that the size of contrast will vary inversely with component duration. They support theories that attribute positive and negative contrast to similar theoretical mechanisms. A comparison of the present results with those of past studies indicates that keypecking by pigeons and leverpressing by rats change as different functions of component duration. Treadlepressing by pigeons and leverpressing by rats change as similar functions. These findings challenge general process theories that argue that all response obey the same behavioral laws.

Higher order occasion setting with serially presented stimuli was investigated in an appetitively motivated, discrete-trial operant study with rats. Reinforcement of barpressing during an occasuin-setting light (a discriminative stimulus) was contingent on immediately preceding second-order occasion setters (i.e., a click train or a buzzer served as a conditional discriminative stimulus). Moreover, the meanings of the clicks and buzzer were themselves indicated by a third-order occasion setter that preceded them (i.e., a white noise acted as a second-order conditional discriminative stimulus). Subjects responded more frequently and has shorter latencies to the first response in the presence of the light on trials during which barpressing was reinforced than on trials during which barpressing was not reinforced. The likelihood that the subjects solved the problem by responding to unique compound stimuli was minimized by the insertion of a 5-sec gap between the different controlling stimuli presented on each trial. Thus, these subjects appear to have a mastered a second-order conditional discrimination, which is equivalent to third-order occasion setting if the discriminative stimulus (light is viewed as a first-order occasion setter. Although the subjects learned to respond appropriately to each of the compound stimuli, differences in responding to specific stimuli were consistent with a higher order feature-positive effect. Some implications of higher order occasion setting are discussed, including the issue of independence between the different levels of occasion setting signaled by a single stimulus.

In three experiments, water-deprived rats were preexposed to a novel saccharin solution. The neophobic response to this flavor was then assessed in a choice test involving saccharin and water, administered either immediately or 24 h after preexposure. Subjects displayed a significantly greater preference for saccharin at the 24-h test than at the immediate test (Experiments 2 and 3). This 'incubation' effect was elliminated if the subjects were more water-deprived at the delayed test than at the immediate test (Experiment 1), and enhanced if the amount of saccharin consumed during preexposure was increased (Experiment 3). Possible ways in which current theories of habituation might be amended in order to accomodate this finding are discussed.

Two lick suppression experiments with rats were conducted in order to determine the nature of the temporal information that is encoded as a result of Pavlovian conditioned inhibition training (conditioned stimulus [CS] A [right arrow] unconditioned stimulus [US]/AX [right arrow] noUS). After inhibition training, the conditioned inhibitor (X) was paired with the US in order to measure inhibition, as assessed through retarded behavioral control by CS X. Three temporal relationships were manipulated: the A--US interval, the X--A interval of inhibition training, and the X--US interval of the retardation test pairings. Retardation was greatest when the X--US temporal relationship matched the time at which the US was expected (but not delivered) on the X--A inhibition training trials. Thus, the present experiments provide evidence with retardation tests that, during conditioned inhibition training, subjects encode the temporal location of the omitted US relative to the inhibitory CS. These findings complement those of previous studies using summation tests of conditioned inhibition (Barnet & Miller, 1996; Denniston, Blaisdell, & Miller, 1998; Denniston, Cole, & Miller, 1998).

Unlike older animals, weanling-age rats do not seek water to drink when they are dehydrated, despite the fact that a physiological sensitivity to dehydration is present very soon after birth. We demonstrate here that the appetitive behaviors needed to approach and obtain water become linked to dehydration only as a result of specific postnatal learning experience. Preventing early experience with dehydration retards the developmental emergence of dehydration-induced, water-oriented behavior in young rats. But a single pairing of water with dehydration can establish an appetitive response. These findings reveal a critical rote of early learning in the development of goal-oriented behavior. Such a learning process is potentially characteristic of other behavioral systems, from the most basic appetites to complex motives.