The effect of filled and empty intervals on clock and memory processes in pigeons

According to the mixed memory model (Penney, Gibbon, & Meck, Journal of Experimental Psychology: Human Perception and Performance, 26, 1770–1787, 2000), different clock rates for stimuli with different nontemporal properties must be stored within a single reference memory distribution in order to detect a difference between the clock rates of the different signals. In Experiment 1, pigeons were trained in a between-subjects design to discriminate empty intervals (bound by two 1-s visual markers) and filled intervals (a continuous visual signal). The intervals were signaled by different visual stimuli, and they required responses to different sets of comparison stimuli. Empty intervals were judged as being longer than filled intervals. The difference between the point of subjective equality (PSE) for the empty intervals and the PSE for the filled intervals increased proportionally as the magnitudes of the anchor duration pairs were increased from 2 and 8 s to 4 and 16 s. In Experiment 2, the pigeons were trained to discriminate intervals signaled by the absence of houselight illumination (Group Empty) or the presence of houselight illumination (Group Filled). The psychophysical timing functions for these intervals were identical to each other. The results of Experiment 1 indicate that memory mixing is not necessary for detecting a timing difference between empty and filled intervals in pigeons. The results of Experiment 2 suggest that the nature of the stimuli that signal the empty and filled intervals impacts how pigeons judge the durations of empty and filled intervals.     

Processing of empty and filled time intervals in pigeons

Pigeons were trained initially with 2- and 8-sec empty or filled intervals as sample stimuli. Interval onset and termination was signaled by 1-sec start and stop markers. Following retention and psychophysical testing, both groups were trained with the alternative type of interval, and the tests were repeated. Group empty-first demonstrated a choose-long effect with both empty and filled intervals. Group filled-first demonstrated a weak (and nonsignificant) choose-short effect with filled intervals and a robust choose-long effect with empty intervals. Both groups tended to time the markers and to add that duration to the sample duration only on filled-interval trials. Initial training with empty intervals alters the way pigeons process temporal information on filled-interval trials, whereas initial training with filled intervals has little effect on the processing of temporal information on empty-interval trials.     

Memory for empty time intervals in pigeons

Pigeons were trained to discriminate short (2 sec) and long (8 sec) empty intervals that began each trial. In group consistent, onset of an empty interval was marked by a brief presentation of red keylight, and termination of the interval was marked by a brief presentation of green keylight. In group inconsistent, red and green served equally often as the first and second markers across trials. Testing revealed that, in group consistent, (1) birds were sensitive to the relation between marker color and marker type and (2) presentation of the second marker did not initiate timing a new interval. Testing also revealed a robust choose-long effect at delays longer than the training delay and indifference between the comparisons on no-sample trials. Both of the latter findings differ from those typically obtained when filled intervals are employed. It was concluded that pigeons process filled and empty intervals differently.     

The effect of the controllability of auditory discriminative stimuli in the performance of go/no-go discriminations by pigeons

Compared with their performance with localized (on-key) visual stimuli, pigeons are notoriously poor at performing go/no-go discriminations when keypecking for food in the presence of auditory discriminative stimuli. The difference might reflect the fact that an aversive visual onkey stimulus signaling nonreward can be escaped by looking away and not pecking, which contributes to the measure of good discriminative performance, while an auditory stimulus cannot be escaped. In Experiment 1, discriminative performance was significantly improved by providing pigeons with a response incompatible with keypecking by which they could escape a tone S+ and a tone S?. However, the pattern, frequency, and duration of escape responses were found to be insufficient to explain the improvement. In Experiment 2, it was found that the capacity to escape only S+ or only S? enhanced discriminative performance as much as the capacity to escape both. It is theorized that the Pavlovian relationship between the absence of the discriminative stimuli and the nonoccurrence of food might transfer to the instrumental relationships learned in a go/no-go discrimination. The possibility that intermittent stimuli command more attention than continuous stimuli is also considered.     

Present/absent sample matching in pigeons: Is comparison choice controlled by the sample stimulus or by differential sample responding?

In two experiments involving present/absent sample matching, we tested whether the visual stimuli or differential sample behavior served as the basis for comparison choice. In both experiments, one group (FR/DRO) was required to peck the present sample and to refrain from pecking the absent sample (as typically occurs with fixed duration present/absent samples), and the other group (FR/FR) was required to peck both samples. In Experiment 1, the samples were a black dot on a white field (present) and the white field alone (absent). In Experiment 2, the samples were a yellow hue (present) and a dark response key (absent). In both experiments, divergent retention functions were found only for the FR/DRO group. These results suggest that, in nonhedonic present/absent sample matching, it is the behavior directed toward the present sample, rather than the visual stimulus itself, that serves as the basis for comparison choice.     

Effects of signaled retention intervals on pigeon short-term memory

In three delayed matching-to-sample experiments, pigeons were given distinctive stimuli that were either correlated or uncorrelated with the scheduled retention intervals. Experiment 1 employed a single-key, go/no-go matching procedure with colors as the sample and test stimuli; lines of differing orientations signaled short or long delays for one group, whereas the lines and the delays were uncorrelated for the other group. The function relating discriminative test performance to delay length was steeper in the correlated group than in the uncorrelated group. In addition, the line orientation stimuli controlled differential rates of sample responding in the correlated group, but not in the uncorrelated group. In Experiment 2, subjects extensively trained with correlated line orientations were exposed to reversed cues on probe trials. Miscuing decreased discriminative test responding at the short delay, but enhanced it at the long delay. As in the correlated group of the first experiment, rates of sample keypecking were higher in the presence of the “short” time tag than in the presence of the ”long” time tag. Experiment 3 used a three-key choice-matching procedure and a within-subjects design, and equated reinforcement rate at the short and long delays. When auditory stimuli were correlated with delay length, the function relating choice accuracy to delay was steeper than when the stimuli and the delays were uncorrelated. The consistent effects of signaled retention intervals on memory performance may be understood in terms of differential attention to the sample stimuli.     

Required pecking and refraining from pecking alter judgments of time by pigeons

There is evidence that humans' perception of time is affected by the activity in which they are engaged while they are timing. The more demanding the task, the faster time appears to pass. A similar effect has been found in pigeons. Pigeons trained to discriminate between a short-duration (2-sec) and a long-duration (10-sec) stimulus were required to peck when the stimulus was one color and to refrain from pecking when it was a different color. On probe trials of intermediate durations, the bisection point (50% choice of the stimulus associated with both long and short stimuli) for trials in which the pigeons were required to peck was almost 1 sec longer than on trials in which the pigeons were required to refrain from pecking (Zentall, Friedrich, & Clement, 2006). In the present research, we replicated this effect and determined the relation between this effect and the typical bisection point that occurs when pecking is permitted but not required. Results indicated that the typical procedure results in a bisection point that is between required pecking and refraining from pecking. Furthermore, the rate of pecking when pecking is allowed but not required also falls between the rate of pecking for the required-pecking and refrain-from-pecking conditions. This result suggests that, similar to humans, pigeons underestimate the passage of time when they are active or when attention to time-related cues has to be shared with attention to satisfying the response requirement.     

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.     

Stimulus control of behavior during the postreinforcement pause of FI schedules

Pigeons were given the opportunity to terminate certain segments of fixed intervals by pecking a control key. When 30-sec segments of negative and positive stimuli alternated across the interreinforcement interval (Experiment 1), most birds terminated a large proportion of negative segments. However, few control-key responses were made during the negative segment immediately following food presentation. Under schedules during which only one negative segment was programmed, during the first 30 sec of 1-min intervals (Experiment 2), control-key responses, when they occurred at all, were made after several seconds of the interval had elapsed. Similar findings were obtained when a peck on the control key merely changed the color on the food key (Experiment 3). These findings suggest that the post-reinforcement extinction state (Schneider, 1969) during fixed-interval schedules consists of two phases: an immediate postreinforcement inhibitory phase, followed by a second phase during which a control-key response may occur. These two phases and their associated behavior may be related to Staddon’s (1977) distinction between interim and facultative activities.     

Remembering: The role of extraneous reinforcement

In two experiments, pigeons' responding on an extraneous task was explicitly reinforced during delayed matching-to-sample trials. In Experiment 1, red or green sample stimuli were followed by retention intervals of 0.2, 1, 4, or 12 sec, during which pecks to a white center key were reinforced with 2.5-sec access to wheat according to extinction, variable-interval 30-sec, and variable-interval 15-sec schedules in different conditions. A proportion of .2, .5, .7, or .9 of subsequent red or green choice responses that matched the sample were reinforced with 3-sec access to wheat. The result was that increasing center key reinforcement, or reducing reinforcer probability, lowered overall accuracy. Initial discriminability fell, but with no change in the rate of forgetting. In Experiment 2, initial discriminability was affected by extraneous reinforcers that were contingent on center key pecking, but not by noncontingent reinforcers. A plausible conclusion is that initial discriminability decreases when reinforcers strengthen competing behaviors.     

Noncontingent and contingent no-choice intervals and concurrent performance

Pigeons responded to changeover-key concurrent variable-interval variable-interval reinforcement schedules while there were intervals during which the changeover key was inoperative (no-choice intervals). In Experiment 1, a multiple schedule on the changeover key signaled choice and no-choice intervals. All subjects showed near-perfect discrimination during initial discrimination training and rapid reacquisition of discrimination following contingency reversals. In Experiment 2, the onset of no-choice intervals was unsignaled and contingent on interchangeover time. The temporal distribution of changeover-key responses conformed to the temporal distribution of choice intervals. The results of both experiments suggest that changeover responding is modifiable as a function of its immediate consequences. The results of Experiment 2, in particular, suggest that time or some correlate of time since the last changeover response can determine subsequent changeover behavior.     

Pigeons’ memory for empty time intervals marked by visual or auditory stimuli

In Experiment 1, pigeons were trained to discriminate the duration (2 or 8 sec) of an empty interval separated by two 1325-Hz tone markers by responding to red and green comparison stimuli. During delay testing, a choose-short bias occurred at 1 sec, but a robust choose-long bias occurred at 9 sec. Responding in the absence of tone markers indicated that the pigeons were attending to the markers and not simply timing the total trial duration. The birds were then trained to match short (2-sec) or long (8-sec) empty intervals marked by light to blue/yellow comparisons. For both visual and auditory markers, delay testing produced a choose-short bias at 1 sec and a choose-long bias at 9 sec. In Experiment 2, the pigeons were shifted from a fixed to variable intertrial intervals (ITI) within sessions. On trials with tone markers, the duration of both the empty interval and the preceding ITI affected choice responding. On trials with light markers, only the duration of the empty interval influenced choice responding. Subsequent delay testing in the context of variable ITIs replicated the memory biases previously obtained. In Experiment 3, performance was assessed at various delay intervals on trials in which either the first or the second marker was omitted. The data from these omission tests indicated that the first marker initiated timing but that the second marker sometimes initiated the timing of a new interval. Explanations of these effects in terms of the internal clock model of timing are discussed, and a simple quantitative model of the delay interval data is tested.     

Proactive interference, recency, and associative strength: Comparisons of black-capped chickadees and dark-eyed juncos

Black-capped chickadees (Parus atricapillus) and dark-eyed juncos (Junco hyemalis) were required to match to the last item from a series of up to three stimuli differing in both location and color. When rewarded for pecking the target stimulus during the study phase of each series, black-capped chickadees demonstrated proactive interference (PI) from stimuli presented prior to the target, whereas juncos did not. When they made an error, chickadees were more likely than were juncos to choose a distractor from the study series rather than a novel stimulus. When reward was no longer associated with presentation of the final target sample in a series, juncos also suffered PI. These results indicate that chickadees and juncos differ in the degree to which the recency of stimuli and the associative strength of stimuli control correct matching.     

On the role of trial outcomes in delayed discriminations

Delayed simple discriminations are typically retained more accurately over longer delays by pigeons than are delayed conditional discriminations (e.g., Honig & Wasserman, 1981). In two experiments, we investigated the extent to which trial outcomes contribute to this difference by comparing performances when all trials ended with food reinforcement versus when only half of the trials did. Experiment 1 showed that when food was presented on all trials, contingent upon either pecking or not pecking the test stimulus, levels of retention and rates of forgetting were comparable for these two tasks. By contrast, Experiment 2 showed better retention of delayed simple than delayed conditional discriminations when half of the trials ended with food and the other half in extinction. Furthermore, delayed simple discriminations were retained more accurately with food versus no-food outcomes than with food at the end of every trial, whereas the reverse was true for delayed conditional discriminations. These findings indicate that retention differences between these tasks are another instance of the differential outcomes effect.     

Information processing in seven-month-old infants as a function of risk status

7-month-old full-terms and high-risk preterms (less than 1,500 grams at birth) were compared on problems of visual recognition memory and tactual-to-visual cross-modal transfer. On the visual problems, preterm infants showed significantly less differential attentiveness to novelty than full-terms. They also required longer exposure times during visual familiarization, primarily because of longer pauses between fixations. Preterms and full-terms exhibited different patterns of looking, as indicated by the duration of fixation and the frequency in shifts of gaze. On the cross-modal problems, preterms and full-terms both exhibited similar and significant preferences for familiar rather than novel stimuli, a direction of preference which suggests that these problems were relatively difficult for both groups. For the preterms, novelty and exposure-time scores were found to be related to several medical risk factors. Novelty preferences were compromised in preterms who had suffered RDS postnatally, particularly those who had required prolonged mechanical ventilation. In general, high-risk preterms exhibited deficits in visual recognition memory and in the ability to recruit, sustain, and shift attention.     

Short-term memory for visual and auditory stimuli in pigeons

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.     

Some determinants of response summation

Response summation in pigeons was examined in four experiments. In Experiment 1, summation was not found with a compound of two visual stimuli on a television screen after they had individually been used for instrumental conditioning. In this experiment, the training and test trials were separated by an interval during which the television screen was dark. Summation was found in Experiment 2 for which the television screen was permanently white during the interval between trials and in the region that was not occupied by the experimental stimuli. These results were replicated using a within-subject design (Experiment 3) and autoshaping (Experiment 4). Experiment 2 also revealed summation with compounds of auditory and visual stimuli, but not with compounds of two auditory stimuli or two diffuse lights. The results can be explained by a variety of theories of learning, if they take account of generalization between the stimuli.     

Analysis of a trial-spacing effect with relatively long intertrial intervals

In three experiments with rat subjects, we examined the effects of trial spacing in appetitive conditioning. Previous research in this preparation suggests that self-generated priming of the conditional stimulus (CS) and/or unconditional stimulus (US) in short-term memory is a cause of the trial-spacing effect that occurs with intertrial intervals (ITIs) of less than 240 sec. Experiment 1 nonetheless showed that a trial-spacing effect still occurs when ITIs are increased beyond 240 sec, and that the effect of ITI over 60–1,920 sec on conditioned responding is best described as a linear function. In Experiment 2, some subjects were removed from the context during the ITIs, preventing extinction of the context. Removal abolished the advantage of the long ITI, suggesting the importance of exposure to the context during the long ITI. Experiment 3 still produced a trial-spacing effect in a within-subjects design that controlled for the level of context conditioning and reinforcement rate in the absence of the CS. Overall, the results are most consistent with the idea that adding time to the ITI above 240 sec facilitates conditioning by extinguishing context-CS associations—and possibly context-US associations—that otherwise interfere with CS-US learning through retrieval-generated priming (see, e.g., Wagner, 1981).     

The effect of sample and comparison ratio schedules on delayed matching to sample in the pigeon

In Experiment 1, three food-deprived pigeons received trials that began with red or green illumination of the center pecking key. Two or four pecks on this sample key turned it off and initiated a 0- to 10-sec delay. Following the delay, the two outer comparison keys were illuminated, one with red and one with green light. In one condition, a single peck on either of these keys turned the other key off and produced either grain reinforcement (if the comparison that was pecked matched the preceding sample) or the intertrial interval (if it did not match). In other conditions, 3 or 15 additional pecks were required to produce reinforcement or the intertrial interval. The frequency of pecking the matching comparison stimulus (matching accuracy) decreased as the delay increased, increased as the sample ratio was increased, and decreased as the comparison ratio was increased. The results of Experiment 2 suggested that higher comparison ratios adversely affect matching accuracy primarily by delaying reinforcement for choosing the correct comparison. The results of Experiment 3, in which delay of reinforcement for choosing the matching comparison was manipulated, confirmed that delayed reinforcement decreases matching accuracy.     

CS-US temporal relations in blocking

In four trace-conditioning experiments with rats, the influence on the blocking of differences between the blocking cue-unconditioned stimulus (US) and the blocked cue-US trace intervals was explored. Experiment 1 demonstrated blocking despite the blocked cue’s having a shorter trace interval than the blocking cue in both elemental (Phase 1) and compound (Phase 2) training. In Experiment 2, blocking was attenuated when the blocked cue had a longer trace interval than did the blocking cue in both elemental and compound training. In Experiments 3 and 4, the trace intervals of the two cues during compound training were matched (i.e., unlike in Experiments 1 and 2, neither had temporal priority). Blocking was attenuated when the blocking cue trace interval in the elemental phase was shorter (Experiment 3) or longer (Experiment 4) than the compound cue trace during compound training. The findings indicate that subjects encode interstimulus intervals, and they further suggest that cue competition is greatest when the competing cues have the same temporal information as the US.