What makes a landmark effective? Sex differences in a navigation task

In Experiment 1, two groups of female rats were trained in a triangular pool to find a hidden platform whose location was defined in terms of a single a landmark, a cylinder outside the pool. For one group, the landmark had only a single pattern (i.e., it looked the same when approached from any direction), while for the other, the landmark contained four different patterns (i.e., it looked different when approached from different directions). The first group learned to swim to the platform more rapidly than the second. Experiment 2 confirmed this difference when female rats were trained in a circular pool but found that male rats learned equally rapidly (and as rapidly as females trained with the single-pattern landmark) with both landmarks. This second finding was confirmed in Experiment 3. Finally, in Experiment 4a and 4b, male and female rats were trained either with the same, single-pattern landmark on all trials or with a different landmark each day. Males learned equally rapidly (and as rapidly as females trained with the unchanged landmark) whether the landmark changed or not. We conclude that male and female rats learn rather different things about the landmark that signals the location of the platform.     

Overshadowing of geometric cues by a beacon in a spatial navigation task

In three experiments, we examined whether overshadowing of geometric cues by a discrete landmark (beacon) is due to the relative saliences of the cues. Using a virtual water maze task, human participants were required to locate a platform marked by a beacon in a distinctively shaped pool. In Experiment 1, the beacon overshadowed geometric cues in a trapezium, but not in an isosceles triangle. The longer escape latencies during acquisition in the trapezium control group with no beacon suggest that the geometric cues in the trapezium were less salient than those in the triangle. In Experiment 2, we evaluated whether generalization decrement, caused by the removal of the beacon at test, could account for overshadowing. An additional beacon was placed in an alternative corner. For the control groups, the beacons were identical; for the overshadow groups, they were visually unique. Overshadowing was again found in the trapezium. In Experiment 3, we tested whether the absence of overshadowing in the triangle was due to the geometric cues being more salient than the beacon. Following training, the beacon was relocated to a different corner. Participants approached the beacon rather than the trained platform corner, suggesting that the beacon was more salient. These results suggest that associative processes do not fully explain cue competition in the spatial domain.     

A landmark blocks searching for a hidden platform in an environment with a distinctive shape after extended pretraining

In the blocking phase of three experiments, rats had to find a submerged platform beneath a spherical landmark in one corner of a triangular pool. Prior to this treatment, they were required to find the platform relative to either a sphere above it (blocking groups) or a rod attached to it (control groups). The position of the platform changed from trial to trial for the initial training. The sphere did not restrict learning about the geometric cues provided by the triangular arena in the blocking phase when 12 sessions of initial training took place in either the triangular (Experiment 1) or a circular (Experiment 3) pool. Blocking was observed, however, after 24 sessions of initial training in either the triangular (Experiment 2) or the circular (Experiment 3) pool. Thus, blocking of geometric cues by a landmark is possible after extended initial training with the blocking cue.     

Overshadowing in the spatial domain

In Experiments 1 and 2, rats were trained in a Morris water maze to locate a hidden platform, the location of which in the circular pool was defined by four visual landmarks (A, B, C, and D), spaced at equal intervals around the edge of the pool. Control animals were trained with these four visual landmarks only. But for animals in the overshadowing groups, an auditory component, X, was added to Landmark D. Test trials, given at the end of training, consisted of placing the rat in the pool with no platform present and recording the time rats spent in the platform quadrant. In Experiment 1, the overshadowing group spent less time in the platform quadrant than controls when tested with D, but the two groups performed equally well on test trials that did not use D. We conclude that the auditory Component X overshadowed the visual Landmark D. In Experiment 2, we obtained evidence of reciprocal overshadowing, of D by X and of X by D. The results of Experiment 3 suggested that an appeal to generalization decrement might not be sufficient to explain these overshadowing effects.     

Location, location, location: development of spatiotemporal sequence learning in infancy

We investigated infants' sensitivity to spatiotemporal structure. In Experiment 1, circles appeared in a statistically defined spatial pattern. At test 11-month-olds, but not 8-month-olds, looked longer at a novel spatial sequence. Experiment 2 presented different color/shape stimuli, but only the location sequence was violated during test; 8-month-olds preferred the novel spatial structure, but 5-month-olds did not. In Experiment 3, the locations but not color/shape pairings were constant at test; 5-month-olds showed a novelty preference. Experiment 4 examined "online learning": We recorded eye movements of 8-month-olds watching a spatiotemporal sequence. Saccade latencies to predictable locations decreased. We argue that temporal order statistics involving informative spatial relations become available to infants during the first year after birth, assisted by multiple cues.     

Blocking in landmark-based search in honeybees

Two experiments tested blocking in landmark-based search in honeybees. Honeybees in the experimental group were trained in Phase 1 with a single landmark in a constant spatial relation to the target (sugar water). In the compound training second phase, the landmark used in Phase 1 (blocking landmark) and a new landmark (blocked landmark) were presented at constant spatial relations to the target. The blocking and blocked landmarks differed from each other in color and position, and the blocking landmark retained the same spatial relationship to the target as in Phase 1. In Experiment 1, the control group experienced only Phase 2 training with two landmarks. In Experiment 2, the control group was trained with a different landmark in a different position in Phase 1. Blocking was found in both cases.     

Blocking in autoshaped lever-pressing procedures with rats

Rats will approach and contact a lever whose insertion into the chamber signals response-independent food delivery. This “autoshaping” or “sign-tracking” phenomenon has recently attracted considerable attention as a platform for studying individual differences in impulsivity, drug sensitization, and other traits associated with vulnerability to drug addiction. Here, we examined two basic stimulus selection phenomena—blocking and overshadowing—in the autoshaped lever pressing of rats. Blocking and overshadowing were decidedly asymmetrical. Previously reinforced lever-extension conditioned stimuli (CSs) completely blocked conditioning to auditory cues (Exps. 1 and 2), and previously nonreinforced lever-extension CSs overshadowed conditioning to auditory cues. By contrast, conditioning to lever-extension CSs was not blocked by either auditory (Exp. 3) or lever-insertion (Exp. 4) cues, and was not overshadowed by auditory cues. Conditioning to a lever-insertion cue was somewhat overshadowed by the presence of another lever, especially in terms of food cup behavior displayed after lever withdrawal. We discuss several frameworks in which the apparent immunity of autoshaped lever pressing to blocking might be understood. Given evidence that different brain systems are engaged when different kinds of cues are paired with food delivery, it is worth considering the possibility that interactions among them in learning and performance may follow different rules. In particular, it is intriguing to speculate that the roles of simple cue–reinforcer contiguity, as well as of individual and aggregate reinforcer prediction errors, may differ across stimulus classes.     

Spatial integration with rats

Rats were trained to find the hidden platform in a Morris pool, whose location was defined by reference to a small number of landmarks around the circumference of the pool. In each of three experiments, an experimental group was trained on alternate trials with two different subsets of three of the available landmarks, with the two subsets sharing one landmark in common. When tested with landmarks drawn from both of their training configurations, but without the landmark common to the two sets, they had no difficulty in locating the platform. In Experiment 1, they performed at least as well as a group trained with all the available landmarks present on every trial. In Experiment 2, they performed significantly better than a group trained with two different subsets of landmarks that shared no one landmark in common.     

Taste and odor in conditioned flavor preference learning

Almond and peppermint extracts were combined with salt and citric acid as cues in conditioned flavor preference conditioning. In Experiment 1, extracts overshadowed tastes, although tastes and extracts conditioned equally well when presented in isolation. In Experiments 2 and 3, tastes and extracts were conditioned in isolation prior to conditioning of a taste/extract compound. The conditioning history of the tastes and extracts did not affect the overshadowing of taste by extract. The results of Experiment 4 showed that rats could learn to discriminate between a taste and extract presented in isolation vs. the taste/extract compound. Thus, extracts do not interfere with sensing the tastes. We suggest that a taste/extract compound produces a configural stimulus that is more characteristic of the extract than the taste.     

Play-solicitation behavior in juvenile male and female rats

Play-solicitation and social investigatory behaviors were observed in male and female juvenile rats exposed to playful and nonplayful juvenile social stimuli. A nonplayful state was induced by treatment with scopolamine HBr. In Experiment 1, the play-solicitation behavior of males exposed to nonplayful stimuli was reliably greater than that of females; social investigation did not differ by gender. In Experiment 2, males and females were exposed to nonplayful male and female stimuli. Male subjects engaged in more soliciting than did female subjects, and male social stimuli were subjected to more soliciting than were female social stimuli. Experiment 3 compared the influence of varying social deprivation intervals on play soliciting by male juveniles. Although social investigation did not vary reliably with interval of social deprivation, play soliciting increased reliably with longer intervals of isolation. In Experiment 4, play-soliciting behavior of males exposed to nonplayful males correlated positively and reliably with play fighting behavior upon exposure to normally playful males. The results support the proposal that some specific behaviors functionally provoke interactive play fighting.     

Reversal of spatial discrimination learning in a water maze by previously undernourished rats

The development of the hippocampus in rats may be vulnerable to undernutrition during the fetal and suckling periods. Hence the behavioral effects of early growth restriction may resemble those of hippocampal lesions. This suggestion was investigated by testing previously undernourished rats for reversal learning, an ability badly affected in hippocampectomized rats. Developing rats were undernourished by feeding their mothers a restricted quantity of a good quality diet during pregnancy and lactation. All rats were fed ad lib from weaning. Reversal of spatial discrimination learning was tested in adult animals using a water T-maze. Previously undernourished rats learned the initial spatial discrimination more quickly than controls. However, there was no effect of early treatment on serial reversal learning in Experiment I or on performance of a single reversal after prolonged initial training in Experiment II.     

Influences of social learning on mate-choice decisions

Evidence from both field and laboratory is consistent with the hypothesis that animals can acquire mate preferences by observing the mating behavior of others. It is difficult, however, to distinguish social learning about mates from a host of other social effects on mating that do not produce changes in preferences. Examples are drawn from laboratory studies on mate choice in female and male Japanese quail that illustrate ways in which social cues influence mating decisions. Quail of both sexes use social cues to modify their mate choices, but the sexes use the information to serve different purposes. Female quail gain preferences for males seen mating with other females, whereas males avoid females that they had observed mating with other males. This sex difference in social learning provides an example of how costs and benefits of sexual behavior can shape decision-making processes. Implications of the influence of social learning on sexual selection are briefly discussed.     

No evidence for overshadowing or facilitation of spatial pattern learning by visual cues

Two experiments were conducted to examine the effects of redundant and relevant visual cues on spatial pattern learning. Rats searched for hidden food items on the tops of poles that formed a square (Experiment 1) or a checkerboard (Experiment 2) pattern. The experimental groups were trained with visual cues that specified the locations of the baited poles. All groups were tested without visual cues so that any overshadowing or facilitation of spatial pattern learning by visual cues could be detected. Spatial choices were controlled by the spatial pattern and by the visual cues in both experiments. However, there was no evidence of overshadowing or facilitation of spatial pattern learning by visual cues in either experiment. The results are consistent with the idea that the representation of the spatial pattern that guides choices is not controlled by the same learning processes as those that produce associations between visual cues and food locations.     

Covariation in conditioned response strength between stimuli trained in compound

Conditioned lick suppression by water-deprived rats was used to elaborate on recent evidence that the attenuated conditioned response elicited by an overshadowed stimulus may be enhanced by extinction of the overshadowing stimulus with which it had been trained in simultaneous compound. Using a modified serial stimulus arrangement in which a light coexisted with the last half of a tone that terminated with footshock, it was found in Experiment 1 that the tone overshadowed the light. Extinction of the tone-shock association resulted in a virtually complete recovery of the response to the overshadowed light. Using this serial overshadowing procedure, the possibility that the strength of a conditioned response to an element trained in compound covaries as a function of the strength of the response to the other element was tested in Experiment 2. Following overshadowing training similar to that of Experiment 1, independent reinforcement of the overshadowed light, that is, associative inflation, was found to have no deleterious effect on the response to the overshadowing tone. This suggests that the effects of postconditioning extinction and inflation of one element do not have symmetrical effects upon responding to the other element. The results of Experiment 2 were replicated in Experiment 3 using a simultaneous compound stimulus as opposed to the serial compound of the previous studies. These results are discussed in terms of various associative and cognitive models of learning and performance.     

Two-choice,observational learning and reversal in the rat: S-S versus S-R effects

In Experiment 1, male rats were trained to press both bars in a two-choice apparatus and were then given observational training of a go/no-go discrimination in which the observed operation of two inaccessible, dissimilar bars by a hidden experimenter constituted S+ and S?. After discrimination was established, individual rats were permitted access to the two bars. Six of the seven rats consistently pressed the S+ bar on 10 test trials, but failed to reverse bar preference after observational training was reversed. In Experiment 2, nine naive males received the same observational training as in Experiment 1, but without any pretraining to press either bar. All rats pressed the S+ bar on initial test and did so consistently throughout the 10 trials. Six of these rats received reversal training of the go/no-go discrimination after the 10 test trials. As in Experiment 1, all rats failed to press the new S+ bar. However, five of six rats in another group, which received reversal trainingprior to any test trials, did reverse and press the new S+ bar. In Experiment 3, controls for possible confounding effects of overtraining trials were conducted. These manipulations had no effect; the rats tested before reversal still failed to press the S+ bar, and the rats reversed before testing all reversed or pressed the most recent S+ bar. That is, S-R learning predominated over S-S learning if active, though unreinforced, responding to a particular bar intervened. In contrast, however, a cognitive (S-S) interpretation of directed response learning was supported by the results of Experiment 4, in which the rats that learned the go/no-go discrimination without responding (only by auditory and light cues) failed to press the S+ bar consistently.     

Potentiation and overshadowing between landmarks and environmental geometric cues

Rats were required in three experiments to find one of two submerged platforms that were situated in the same pair of diagonally opposite corners of a rectangular grey swimming pool. The experimental groups were trained with landmarks, comprising A4 cards attached to the walls, located in the corners containing the platforms. For the control groups, the landmarks were situated in the corners containing the platforms for half of the trials, and in the other corners for the remaining trials. Learning about the positions of the platforms with reference to the shape of the pool was overshadowed in the experimental groups when the landmarks were white, and enhanced when the landmarks were black. A fourth experiment assessed whether geometric cues influenced the control acquired by the landmarks. As in the previous experiments, the presence of the geometric cues overshadowed learning about the landmarks when they were white, but enhanced learning when the landmarks were black.     

Perceptual learning transfer in an appetitive Pavlovian task

In two experiments, rats were given intermixed or blocked preexposure to two similar compound stimuli, AX and BX. Following preexposure, conditioning trials took place in which AX (Experiment 1) or a novel compound stimulus NX (Experiment 2) was paired with a food-unconditioned stimulus in an appetitive Pavlovian preparation. Animals that were given alternated preexposure showed lower generalization from AX to BX (Experiment 1) and from NX to a new compound, ZX (Experiment 2), than animals that were given blocked preexposure, a perceptual learning and a perceptual learning transfer effect, respectively.     

Overshadowing and stimulus intensity

Two experiments on conditioned suppression in rats examined overshadowing between visual and auditory components of a compound conditioned stimulus. In the first experiment, when one component was markedly more salient than the other, the more salient overshadowed the less salient, but the latter, although acquiring significant associative strength, did not overshadow the former. When the two components were of approximately equal salience, each overshadowed the other. In the second experiment, reciprocal overshadowing was again observed between two equally salient stimuli, but only when their absolute intensities were relatively low. The failure to observe reciprocal overshadowing under all conditions raises problems for those theories of stimulus selection which assume that stimuli compete for some strictly limited resource. It was suggested, instead, that overshadowing might occur when animals fail to learn to attend to, or actually learn to ignore, stimuli that are not uniquely successful predictors of reinforcement.     

Landmark use by pigeons in a touch-screen spatial search task

Pigeons obtained food by pecking at an unmarked target location on a video screen equipped with a touch-sensitive frame. The target area was located near the top edge of the screen in Experiment 1 and near the left edge of the screen in Experiment 2. On baseline trials, a graphic landmark was located below and left of the target (Experiment 1) or below and right of the target (Experiment 2). In both experiments, baseline search distributions showed a single peak and were roughly symmetrical about the target area in both horizontal and vertical dimensions. On occasional test trials, the landmark was shifted horizontally, vertically, or diagonally by 1.5 cm or 3 cm. In both experiments, landmark shifts in the dimension parallel to the nearest edge produced systematic shifts in the peak place of search. Landmark shifts in the dimension perpendicular to the nearest edge produced inconsistent (Experiment 1) or relatively small (Experiment 2) shifts in peak place. The magnitude of the behavioral shift was always less than the magnitude of the landmark shift and was not consistently greater when the landmark was shifted by 3 cm than when it was shifted by 1.5 cm. These results demonstrated that pigeons can accurately locate an unmarked target area in a two-dimensional vertical arena and that their use of landmarks for spatial localization is similar in several respects to that found in open-field spatial search tasks.     

The relationship between shock magnitude and passive avoidance learning

Three experiments examined the relationship between shock magnitude and the rate of acquisition of a passive avoidance response. Experiment 1 indicated that the use of a relatively large magnitude of shock can disrupt learning to remain on a platform in the center of an open field to avoid shock. The inferior learning of the group trained with high shock was replicated in Experiment 2, which also demonstrated that avoidance learning can occur rapidly with this level of shock if the platform is located in the corner of the apparatus. To explain this, it was proposed that thigmotactic behavior is responsible for the disruption in avoidance behavior when training is conducted in the center with high-magnitude shock. Finally, Experiment 3, essentially a replication of Experiment 1 except that the platform was placed in the corner of the test compartment, demonstrated a direct relationship between shock magnitude and passive avoidance learning. The results are seen as being consistent with accounts which maintain that avoidance learning can be influenced by the occurrence of species-specific defense reactions.