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A duration matching-to-sample procedure was used to examine the influence of signal properties on temporal estimation and working memory. The results indicated that pigeons responded to durations of a light as if they were longer than equal durations of the absence of the same light, but delay performances did not differ between the two sample types. Similarly, pigeons responded to durations of a bright white light as if they were longer than equal durations of a dim red light, but again, delay performances did not differ between the two sample types. These results are discussed in terms of theoretical issues pertinent to timing, attention, and working memory.

This experiment established that for the infant rat, 10 days postpartum, a preference conditioned to an olfactory stimulus (conditioned stimulus) could be substantially-decreased-by subsequently lowering the value of the unconditioned stimulus (heat). This devaluation effect disappeared when a sufficiently long interval elapsed between the devaluation treatment and the test, despite maintained retention of the original conditioned preference over this same interval. This suggests that devaluation in infant rats does not permanently change the animal’s original representation of the unconditioned stimulus, but instead may replace it temporarily with a conflicting representation.

Two experiments examined the effects of nonreinforced flavor exposure on the strength of a conditioned taste aversion. Rats were conditioned by pairing maple flavor with LiC1. Prior to or subsequent to this pairing, some animals received nonreinforced exposure to either maple or saccharin. In separate subjects, preference for maple was tested 1 or 21 days after the last training episode. In the first experiment, the nonreinforced stimulus exposure occurred before conditioning (latent inhibition, or LI, procedure); in the second experiment, the nonreinforced exposure occurred after conditioning (extinction, or EXT, training). In both experiments, nonreinforced exposure to maple or saccharin reduced the magnitude of a conditioned maple aversion when testing occurred soon after conditioning. When testing was delayed, however, the attenuation due to nonreinforced saccharin exposure dissipated, both with the LI procedure and with EXT. In contrast, the nonreinforced exposure to maple was found to attenuate conditioned reactions at both short and long retention intervals. The release from generalized LI and spontaneous recovery following generalized EXT training are discussed in terms of retrieval processing. The possibility that the same mechanism may underlie LI and EXT is considered.

Pigeons were trained on duration matching-to-sample in which each of four combinations of signal type (red or white light) and duration (2 or 10 see) was mapped onto a different choice stimulus. Probe trials in Experiments 1 and 2 involved a successive presentation of two duration samples. In each experiment, birds tended to summate two durations when the same signal was presented twice, but not when two different signals appeared. These results contrast with those reported by Spetch and Sinha (1989), who found a summation effect with both same-signal and different-signal compounds. In Experiment 3, pigeons chose among two alternatives which were both associated with the duration of the sample but of which only one was also associated with the signal type of the sample. Pigeons systematically chose the stimulus that matched both sample duration and signal type. The implications of these findings are discussed in terms of transfer of training and coding of event duration.

Two experiments examined the performance of pigeons on symbolic-matching-to sample in which the relevant sample dimension consisted of duration. Each pigeon was trained on two problems that had the same two sample durations, 2 and 10 sec, but were different with respect to other physical properties of the samples. Durations of light and tone were used in Experiment 1; durations of two different color-location compounds were used in Experiment 2. In each experiment, a unique choice stimulus was associated with each of the four possible combinations of duration and signal type. Test sessions contained probe trials in which the choice stimuli were these appropriate for a long and a short duration of the signal type opposite to that actually presented. Pigeons in both experiments displayed asymmetrical performance deficits. Accuracy on long durations dropped to chance or below, whereas accuracy on short durations remained high. This pattern is similar to the choose-short effect that is obtained when animals are tested with long retention intervals. The implications of these results for duration memory, coding, and transfer of training are discussed.

A conditioned emotional response procedure was used to study the interactive effects of stimulus preexposure and retention interval in rats. In Experiment 1, the subjects were conditioned by presenting a light CS paired with mild footshock as the US. Half of the subjects were given nonreinforced preexposure to the CS, and the others were not. Separate preexposed and nonpreexposed groups were then tested 1,7, or 21 days after conditioning. Suppression of ongoing activity was used to assess the degree of conditioned fear. Latent inhibition was found at the 1-day retention interval; the preexposed subjects displayed less conditioned fear than did the nonpreexposed subjects. In contrast, equally strong conditioned fear was expressed by the preexposed and the nonpreexposed groups tested after the 7- and the 21-day retention intervals. These results indicate a release from latent inhibition similar to that obtained with conditioned taste aversions (Kraemer & Roberts, 1984). The results of Experiment 2 suggest that retention-interval-induced increases in sensitization, pseudoconditioning, or neophobia cannot account for the release from latent inhibition effect obtained in Experiment 1. The implications of these findings for a retrievaloriented view of latent inhibition are discussed.

Ontogenetic differences in processing light-tone compounds were discovered in preweanling (17-day-old) and adult (60–80-day-old) rats. Suppression of general activity was used as an index of the magnitude of conditioned fear following a single training session in which a CS+ was paired with mild footshock. In Experiment 1, rats were trained on discriminations in which the CS− consisted of a light and the CS+ was either a tone alone (simple discrimination) or a light-tone compound (simultaneous feature-positive discrimination). Adults and preweanlings given each type of discrimination were then tested for fear of the CS− and a target stimulus (tone alone or light-tone compound). Adults in all groups displayed greater fear of the target than of the CS−. Preweanlings, however, discriminated the CS− from the target only when the target was the same as the original CS+. Experiment 2 revealed that age-related differences in conventional stimulus generalization is not a likely explanation for the pattern of results found in Experiment 1. Experiment 3 revealed age-related differences in expressed fear of a serial feature-positive discrimination; adults, but not preweanlings, showed greater fear of the compound than of the CS−. Alternative interpretations of the results from these experiments are discussed, and the general conclusion is that adults appear more inclined to process elements of a compound stimulus selectively, whereas preweanlings seem more likely to process the compound unselectively, with roughly equivalent processing of each element.

Separate groups of pigeons were trained to perform symbolic delayed matching to sample with auditory and visual sample stimuli. For animals in the auditory group, ambient tones that varied in frequency served as sample stimuli; for animals in the visual group, ambient red and green lights served as sample stimuli. In both cases, the sample stimuli were mapped onto the yellow and blue comparison stimuli presented on left and right pecking keys. In Experiments 1 and 2, it was found that visual and auditory delayed matching were affected in the same ways by several temporal variables: delay, length of exposure to the sample stimulus, and intertrial interval. In Experiments 3, 4A, and 4B, a houselight presented during the delay interval strongly interfered with retention in both visual and auditory groups, but white noise presented during the delay had little effect in either group. These results seem to be more in line with a prospective memory model, in which visual and auditory sample stimuli are coded into the same instructional memories, than with a model based on concepts of retrospective memory and modality specificity.

Weanling rats were tested for retention of an aversion to a novel flavor (chocolate milk) that had been conditioned as a single-element conditioned stimulus (CS) or in compound with a novel ambient odor (banana). The presence of the ambient odor during conditioning had no effect on flavor aversion shortly thereafter, confirming previous results. The flavor aversion observed 21 days after conditioning, however, was significantly stronger for pups conditioned with the single-element CS than for those given the flavor-odor compound as the CS. This retention effect was due to a surprisingincrease in the conditioned aversion observed 21 days after conditioning with the single-element CS. A second experiment confirmed this paradoxical increase in retention of the aversion to chocolate milk. This experiment also verified that no such increase occurred in retention of the conditioned aversion to a different flavor (saccharin), whether the initial aversion was strong or weak. The results may be explained in terms of generalized latent inhibition from consumption of mother’s milk.

Based on several recent demonstrations of a directed forgetting effect in pigeons, three experiments were carried out in an attempt to demonstrate directed forgetting in three squirrel monkeys. During initial training with a delayed matching-to-sample procedure, retention tests were always given for sample stimuli followed by remember cues (R-cues) and were always omitted for sample stimuli followed by forget cues (F-cues). Retention of F-cued items was tested on probe trials after initial training. The first two experiments examined the effects of R- and F-cues on memory for slide-projected pictures, with different pictures used on each trial of a session. In Experiment 1, a complex design was used in which one or two sample pictures were presented on each trial; when two pictures were presented, both could be R-cued or F-cued, or one could be R-cued and the other F-cued. A simpler design was used in Experiment 2, with only single pictures presented as sample stimuli and half the trials within a session R-cued and the other half F-cued. In both of these experiments, no differential retention of R- and F-cued stimuli was found, even at a retention interval as long as 16 sec. In Experiment 3, a series of studies was performed to test for directed forgetting when only two sample stimuli were used repeatedly throughout training and testing. With two pictures as sample stimuli, clear evidence of directed forgetting was found in Experiment 3b. It is suggested that the directed forgetting effect may arise only when a small set of sample stimuli is used.

Pigeons were trained to symbolically match comparison stimuli to either visual sample stimuli presented on a center key or to spatial sample stimuli presented on side keys. Tests were carried out in which visual and spatial cues were simultaneously presented in compound and short-term memory was probed for either visual or spatial information. Symmetrical interference with the matching of visual and spatial components of compounds was found when the visual and spatial cues were presented on separate keys. However, when visual and spatial cues were superimposed on the same side key, no interference was observed relative to element control tests. Discussion of these findings focuses on accounts in terms of limited processing capacity, coding decrement, and receptor orientation mechanisms.

A symbolic delayed matching procedure may be used to study memory for stimulus duration in pigeons. Short and long presentations of a light sample stimulus are mapped onto the choke of visually differentiated comparison keys. When delay is varied in such a symbolic delayed matching procedure, pigeons show increasing preference for the short-sample key as the delay becomes longer (choose-short effect), even after a long sample stimulus has been presented. Two theoretical explanations of the choose-short effect are suggested. A subjective shortening model holds that the choose-short effect arises from progressive shortening of the memory of stimulus duration as the delay proceeds. An alternative coding model suggests that the choose-short effect arises from stimulus generalization after an initial response instruction to peck the long-sample key has been forgotten. These two models were tested by training pigeons to peck a third comparison key after no sample stimulus had been presented. Shifts in key preferences over delays ranging from 0 to 21 sec clearly supported the coding model.

Performance during simultaneous matching-to-sample was assessed in pigeons presented with element and compound visual samples. In Experiment 1, pigeons were trained with a symbolic matching procedure, in which different pairs of colored comparison cues presented on side keys were mapped onto a bright or dim houselight as one pair of sample stimuli and onto vertical and horizontal lines on the center key as a second pair of sample stimuli. They were then tested with houselight-line compound samples. It was found that matching accuracy for lines was significantly diminished with compound samples relative to element samples. Conversely, house-light intensities were matched as well with compound samples as with element samples. In Experiment 2, a similar effect was found with pigeons that had been trained to match only line samples. In Experiment 3, it was discovered that sample duration had no influence on the matching deficit found with lines following compound samples in birds either trained or not trained to match houselight intensities. These results, taken in combination with recent findings from experiments with auditory-visual compounds, suggest a restricted processing account of pigeon processing of simultaneously presented stimuli from different sources.

The effects of flavor preexposure and retention interval were assessed in 6- and 12-day-old rats. Conditioned aversions to a flavor appeared at both ages. The conditioning of the younger pups was unaffected by conditioned stimulus (CS) preexposure and was not evident after a 10-day retention interval. For the 12-day-old rats, preexposure to either the flavor CS or a different flavor attenuated aversion strength when the rats were tested soon after conditioning. Other 12-day-old rats that were tested 10 days after conditioning also expressed substantial aversions, but with a retention interval of this length, the aversions were equivalent for animals preexposed to the CS and those not preexposed before conditioning. This loss of the CS-preexposure effect over a long interval, which has also been observed in adult rats, identifies the locus of this effect as postacquisition and perhaps at the stage of memory retrieval.

In Experiment 1, short-term memory for lists of visual stimuli was studied in four squirrel monkeys (Saimiri sciureus). A delayed-matching procedure was used in which a subject was presented with lists containing one, three, or six stimulus patterns, and memory for serial positions was probed by requiring the subject to choose between a list item and a nonlist item. The rate of item presentation was varied, as was the delay between the final item on a list and the retention test. In Experiment 2, the same procedures were used to compare recognition memory in four monkeys and four humans. Although differences in the levels and shapes of the serial-position curves appeared between species, both monkeys and humans showed primacy and recency effects. The presentation time of stimuli had a negligible effect on performance in both monkeys and humans, whereas delay significantly affected human retention but not monkey retention.

The influence of cue type and cue configuration on radial-maze performance in rats was ex­ amined in two experiments. In the first experiment, it was found that rats provided with both salient intramaze and extramaze cues acquired the task faster than rats given only one set of cues. No difference in acquisition was found between a group trained with intramaze cues alone anda group trained with extramaze cues alone. In a cue-preference test, it was found that groups that had been trained with extramaze cues, intramaze cues, or both sets of cues relied on extra­ maze cues to avoid visited arms when given both types of cues concurrently. When all groups were transferred to intramaze-cue-alone trials, only the group that had been originally trained with extramaze cues alone showed any disruption. Also, during the second half of the intramaze­ cue-alone trials, the arrangement of these cues was randomly changed on each trial. This dis­ ruption in cue configuration did not deleteriously affect performance in any of the three groups; all remained above chance performance, although the performance of the group originally trained with extramaze cues alone was inferior to that of the other two groups. In Experiment 2, groups of rats were trained on daily alternating trials under intramaze-cue-alone and extramase-cue­ alone conditions. For one group, the configuration of intramaze cues was altered randomly on each trial; the other group had intramaze cues always presented in the same configuration over trials. It was found that acquisition was more rapid on intramaze trials in the group given static configurations. Also, acquisition of the extramaze task was faster than the intramaze task in the group given variable intramaze cue configurations. No difference was found between the intramaze and extramaze conditions in the group given static intramaze cue configurations. These data suggest that a static cue configuration may influence radial maze performance, but is not a necessary condition for such performance.

Adult rats were injected with lithium chloride (LiCl) after consumption of a novel flavor (chocolate milk) that either was or was not presented together with a novel ambient odor (banana) as a compound conditioned stimulus (CS). In Experiment 1, the adults’ consumption of the flavor 24 h after conditioning was compared with that of weanling rats given the same conditioning treatment on Postnatal Day 21. The results confirmed previous indications that the reduction in aversion observed for adults conditioned with the compound CS (overshadowing) was weak or nonexistent in weanlings. After a longer retention interval (21 days), there was no evidence of overshadowing in adults despite maintained retention of the basic conditioned aversion. In Experiment 2 this decrease in overshadowing after a long retention interval was replicated with adult animals and extended to a different method of testing. The form of the effect was the same as in Experiment 1: The decrease in overshadowing occurred over the retention interval without loss in retention of the basic taste aversion; the decrease in overshadowing was a consequence of anincrease in the flavor aversion displayed by animals conditioned with the compound CS. The impaired flavor aversion (i.e., the overshadowing) observed shortly after conditioning apparently was due to factors associated with memory retrieval, rather than to reduced attentional or associative strength.

Gaffan (1983) argues that there is only weak evidence for Roberts and Kraemer's (1981) contention that the primacy effect may be a general characteristic of primate memory. He considers our conclusion in the larger context of his own research and points out that he has reported several failures to find a primacy effect in list memory experiments with monkeys. Our position clearly needs clarification. We considered our data and those of Sands and Wright (1980a, 1980b) in the still broader context of comparative research. We found it notable that researchers in different laboratories, using different species of monkeys, different stimuli, and different procedures, had been able to produce reliable primacy effects in monkeys. At the time our paper was written, no primacy effects had been found for retention of once-presented lists in pigeons (MacPhail, 1980; Shimp, 1976; Shimp & Moffitt, 1974), rats (Roberts & Smythe, 1979), or a dolphin (Thompson & Herman, 1977). Our intention, then, was to indicate that primacy effects were reliably demonstrable in human and nonhuman primates but had not been demonstrated in nonprimate species. It was not our intention to imply that primacy effects had been found in every list memory experiment done with monkeys, any more than we would claim that every list memory experiment with humans shows a primacy effect. Gaffan's claim that a primacy effect may be found in monkeys only under certain experimental conditions should be acknowledged, but it does not detract from the fact that production of a primacy effect now can be generalized from human to nonhuman primates. Given that a primacy effect now has been demonstrated under certain conditions with monkeys, we may entertain theoretical accounts of this effect. Gaffan (1983) advances the hypothesis that monkeys in the Roberts and Kraemer (1981) and Sands and Wright (1980a, 1980b) experiments attended more to the initial item of a list than to the following items and that the primacy effects they observed were a consequence of better memory for more highly attended items. The initial stimulus was attended to more strongly because a response was required to introduce this stimulus but not the remainder of the list. In his experiments, monkeys had to respond to produce each list item, and no primacy effects were found. It is suggested that item presentation contingent on a subject's response maintains attention and high retention for all items on a list. We too have considered the possible importance of attentional processes in the primacy effects seen in our monkeys, and we continue to do so. However, there are some problems with the position advocated by Gaffan (1983). Gaffan says nothing about the recencyeffect. Both Roberts and Kraemer (1981) and Sands and Wright (1980a, 1980b) found an increase in retention for items at the end of a list. Why should retention improve for terminal items (often to a level higher than that for the initial item) if all items beyond the first One are poorly attended? In the Roberts and Kraemer experiments, list lengths of three and six items were randomly intermixed, with twice as many six-item lists as three-item lists. Animals could not predict whether the third item presented in a list would be in the middle of a six-item list or at the end of a three-item list. Yet, the same item was poorly retained when tested as Position 3 in a sixitem list and better retained when tested as the final item of a three-item list. If this item was poorly attended to in both cases, why was it better remembered in the three-item list than in the six-itemlist? Finally, let us consider research with nonprimate species. Shimp (1976) required pigeons to peck a key designating the correct position of each item in a three-item list, and Roberts and Smythe (1979) required rats to enter each of the alleys that made up a seven-item list. Since animals responded to each item of the list in both cases, Gaffan's response-dependent attentional hypothesis suggests that consistently high retention should be found at all serial positions. Instead, marked declines in performance were seen from the ends of the lists to the beginnings. Although attentional processes certainly may be important in memory experiments, it seems unlikely that response-generated attention can account for the variety of serial position effects seen in monkeys and other animals. There seems to be substantial evidence that the position of items in a list exerts strong effects on retention when the presentation of items is both response dependent and response independent.