Intertrial interval effects in pavlovian serial feature positive discriminations

The effects of the intertrial interval (ITI) on learning and performance in Pavlovian appetitive serial feature positive (SFP) discriminations were examined in three experiments with rats. With longer ITIs, acquisition was more rapid, and there was less transfer of the feature’s behavioral control to a separately trained target cue, suggesting that longer ITIs encouraged the use of an occasion setting strategy. Behavior was also affected by discrimination-specific ITIs. Rats were trained with two SFP discriminations. The overall ITI was held constant, but the intervals between trials of one discrimination were varied by intermixing different numbers of trials from the other discrimination. Learning was more rapid when the intervals between trials of a single discrimination were longer. A sequential analyses showed that performance on a trial was impaired when it was preceded by a trial that included the same target cue but with the opposite trial outcome. The results are discussed in the frameworks of proactive interference effects and deletion-comparator processes (Cooper, Aronson, Balsam, & Gibbon, 1990.)     

Effect of intertrial interval on variable-interval discrete-trial barpressing

In seven experiments, an effect of the intertriai interval (ITI) duration on barpressing by rats was studied. A stimulus signaled a 15-sec variable-interval trial. The first response after the interval elapsed turned the stimulus off and was rewarded with food. Trials were separated by long (about 300 sec) or short (about 10 sec) ITIs. A within-subjects design established that response rate on trials after long ITIs was lower than that after short ITIs (Experiments 1 and 3–7). The effect was not cumulative (the effect of one and five consecutive short ITIs was the same). Response rate after short and long ITIs was the same when a between-subjects design was used (Experiment 2). Response rate was higher after 160-sec ITIs than after 300-sec ITIs, suggesting that the ITI duration at which all longer ITIs are treated the same (i.e., the upper limit) is greater than 160 sec (Experiment 3). When food, the trial stimulus, a novel stimulus, or a familiar stimulus never paired with food, was presented 10 sec before the next trial during some of the long ITIs, response rate on the next trial was similar to that found after 10-sec ITIs (Experiments 4–6). This similarity suggested that these events could mark the start of the ITI. However, the familiar stimulus did so only when it reliably predicted that the next trial would occur after a short interval. The effect of ITI duration on responding was apparently attributable to response latency. Response latency was greater after long ITIs, but once responding began, it was similar after long and short ITIs (Experiment 7).     

Influence of intertrial interval duration on the intertrial agreement effect in delayed matching-to-sample with pigeons

Accuracy on even-numbered trials was assessed as a function of (1) the relation between the sample on the immediately preceding trial and that on the current trial and (2) the length of the intertrial interval (ITI) that intervened between odd- and even-numbered trials. A relatively long interval intervened between pairs of trials in the clustered-dyads procedure, whereas this interval was equal to the ITI in the massed-trials procedure. Both procedures revealed an intertrial agreement effect in that accuracy was higher when the sample on the immediately preceding trial was identical rather than opposite. A decrease in the magnitude of this effect at longer ITIs was apparent only in the clustered-dyads procedure. The insensitivity of the intertrial agreement effect to variations in ITI in the massed-trials procedure may reflect floor effects and the carryover of memory from multiple prior trials that mask the true magnitude of the intertrial agreement effect at short ITIs.     

Serial discrimination reversal learning in pigeons as a function of intertrial interval and delay of reinforcement

Pigeons learned a series of reversals of a simultaneous red-green visual discrimination. Delay of reinforcement (0 vs. 2 sec) and intertrial interval (ITI; 4 vs. 40 sec) were varied across blocks of reversals. Learning was faster with 0-sec than with 2-sec delays for both ITI values and faster with 4-sec ITIs than with 40-sec ITIs for both delays. Improvement in learning across successive reversals was evident throughout the experiment, furthermore, even after more than 120 reversals. The potent effects of small differences in reinforcement delay provide evidence for associative accounts and appear to be incompatible with accounts of choice that attempt to encompass the effects of temporal parameters in terms of animals’ timing of temporal intervals.     

Conditioning of the nictitating membrane response of the rabbit (Oryctolagus cuniculus) as a function of length and degree of variation of intertrial interval

The study involved three experiments. The first, a parametric investigation of nictitating membrane conditioning with eight constant intertrial intervals (ITIs) between 5 and 120 sec, orthogonal to interstimulus intervals (ISIs) of 250 and 750 msec plus three temporal conditioning control groups, revealed that performance improved rapidly with increasing ITI but stabilized at relatively low ITI values. At 750-msec ISI, a decrement in performance was found at 60-sec ITI. Experiment II, using constant ITIs of 45–75 sec in 5-sec steps, at 750-msec ISI confirmed the trend toward a performance decrement around 60 sec, although the trend was weak and highly variable. Experiment III evaluated the differences in performance between constant and variable ITI, using three ITI values and three conditions of variation at each value. Findings were discussed in terms of differences in conditioning resulting from both length and degree of variation of ITI and some subtle effects which may emerge only when constant ITIs are used.     

The role of trial tracking in rats’ working memory

The experiments reported in the present study tested whether decreasing intertrial intervals (ITIs) intensifies the disruptive effects of increasing retention intervals (RIs) in a delayed conditional discrimination by decreasing the animal’s trial tracking accuracy (Cohen & Armstrong, 1996; Cohen & Roberts, 1996). Rats responded on a fixed ratio (FR) 1 or fixed interval (FI) 10-sec reinforcement schedule at a second light or tone stimulus, S2, when the first light or tone stimulus, S1, had signaled an FI 10-sec or FR 1 schedule, respectively. RIs between S1 and S2 were increased from 3 to 24 sec and never exceeded ITIs that were reduced from 24 to 6 sec. For some rats, the trials were separated from each other by extending the lever at S1 and retracting it at the end of S2 (ITI lever-retracted group). For other, control rats, the lever remained extended throughout the session (lever-extended group, Experiment 1) or was extended and retracted with the onset and offset of each stimulus (RI/ITI lever-retracted group, Experiment 2). The rats under all trial conditions learned to delay leverpressing on the FI 10-sec schedule. Latency to begin leverpressing on the FI 10-sec schedule declined as RIs were increased, but this effect was attenuated in the ITI lever-retracted groups in both experiments, as would be predicted by thetrial tracking hypothesis. Decreasing ITIs from 24 to 6 sec intensified the disruptive effects of increasing RIs from 3 to 6 sec in the RI/ITI lever-retracted group (Experiment 2), as would be predicted by the trial tracking hypothesis.     

A role of stimulus compounds in eliciting responses: Relatively spaced extinction following massed acquisition

One of two schedules of rewarded (R) and nonrewarded (N) trials (RNR vs. RRN) was combined factorially with intertrial interval (ITI) in acquisition (1 vs. 45 min), with extinction occurring at a 45-min ITI. The RNR schedule produced greater resistance to extinction than the RRN schedule regardless of acquisition ITI. The shift in ITI from acquisition to extinction reduced resistance to extinction slightly in the RNR group but not in the RRN group. These findings suggest that there are cues common to 1-min and 45-min ITIs. It was suggested that these ITI-associated cues enter into compound with memories to control instrumental responding.     

Response events as well as goal events as sources of animal memory

Current approaches emphasize the control exercised over behavior by various internal stimuli arising from goal events such as reward and nonreward. In contrast, certain earlier views emphasized the behavioral control exercised by internal stimuli arising from responding. The purpose of the three experiments reported here was to determine if stimuli arising both from goal events and from response events control instrumental behavior, and, if so, how much behavioral control is exercised by each. In the three experiments reported, rats received partial reinforcement in a runway either at a 24-h ITI or at both a 24-h ITI and a shorter ITI (2 min, 15 min, 30 min, or 1 h), with extinction always occurring at the shorter ITI. The results suggest that both goal-produced and response-produced internal stimuli control behavior simultaneously. Too, it was found that at ITIs as long as 30 min, response-produced cues regulate responding about as strongly as goal-produced cues.

     

Trial spacing effects in pavlovian conditioning: A role for local context

Two conditioned lick-suppression experiments with rats were conducted in order to replicate and extend findings by Ewing, Larew, and Wagner (1985). Ewing et al. observed that excitatory responding to a CS paired with a footshock US was attenuated when the ITIs thatpreceded each CS-US trial were short (60 sec) relative to when they were long (600 sec). This effect was isolated in the influence of the preceding ITI because the preceding ITI was consistently short for one CS and consistently long for a different CS, while the following ITIs were equally often short and long for both CSs. Ewing et al. interpreted this finding in the framework of Wagner’s (1981) SOP model. Experiment 1 replicated this trial-spacing effect and demonstrated a similar effect under conditions in which thefollowing ITI was consistently short for one CS and consistently long for a different CS, while the durations of preceding ITIs were equally often short and long for both CSs. Experiment 2 revealed that the detrimental effect of a short preceding or a short following ITI could be alleviated by extinguishing the conditioning context after CS-US training. The latter observation indicates that the trial-spacing effect is not mediated by a failure of a CS trained with a short ITI to enter into excitatory associations with the US, a conclusion that is not wholly consistent with the SOP model. Finally, we suggest that short pretrial and short posttrial ITIs may enhance the excitatory value of local context cues that modulate responding to a CS.     

Pigeons’ memory for event duration: Intertrial interval and delay effects

The effects of within-session variations in the intertriai interval (ITI) and delay on pigeons’ memory for event duration were studied in delayed symbolic matching-to-sample tasks. Pigeons were trained to peck one color following a long (8 sec) sample and another color following a short (2 sec) sample. In the first three experiments, the baseline conditions included a 10-sec delay (retention interval) and a 45-sec ITI. During testing, the delay was varied from 0 to 20 sec, and the ITI that preceded the trial was varied from 5 to 90 sec. When the ITI and delay were manipulated separately (Experiments 1 and 2), the pigeons displayed a choose-short tendency when the delay was longer than 10 sec or when the ITI was longer than 45 sec, and a choose-long tendency when either the delay or the ITI was shorter than these baseline values. These effects occurred whether the sample was food access or light. When the ITI and delay were manipulated together, the pigeons showed a large choose-long error tendency when the short delay was tested together with a short ITI, and no systematic error tendency when the short delay was tested together with a longer ITI. A very large choose-short error tendency emerged on trials with a long delay and a long ITI; a reduced choose-short tendency was present when the long delay was presented together with a short ITI. In Experiment 4, the baseline conditions were a 0-sec delay and a 45-sec ITI. In this case variations in the ITI had a smaller and unidirectional effect: the pigeons showed a choose-long error tendency when the ITI was decreased, but no effect of ITI increases. Two hypotheses were proposed and discussed: (1) that pigeons judge sample durations relative to a background time composed of the ITI and delay, and (2) that the delay and ITI effects might arise from a combination of subjective shortening and proactive effects of samples from previous trials.     

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 effects of illumination on the acquisition of delayed matching-to-sample

Illumination effects during steady-state performance of discrimination tasks in animals have been well documented, whereas research on illumination effects during acquisition has been largely ignored. Exceptions to this rule are Wasserman’s (1973) autoshaping experiments and Maki’s (1979) successive discrimination experiment. The present experiment investigated the effects of illumination changes on acquisition of a conditional discrimination—delayed matching-to-sample (DMTS). Pigeons were used in a between-groups design which factorially varied house-light illumination, on or off, during the presentations of DMTS stimuli, the delay interval, and the intertrial interval (ITI). DMTS performance over five blocks of sessions was the dependent variable. The major result was the three-way interaction of sessions, the intertrial interval, and the DMTS stimuli. Constant illumination resulted in the highest discrimination ratios over the last four blocks of sessions. A constant dark condition did not differ from a condition with dark ITIs and illuminated stimulus presentations or from a condition with illuminated ITIs and dark stimulus presentations. The proffered explanation of these data emphasizes the disruptive effects of stimulus changes and Wasserman’s (1973) cue localization hypothesis. The loci of the stimulus change and cue localization effect are suggested to be either at the beginning of a trial or at the end of a trial. A pretrial account emphasizes the role of stimulus changes on the encoding of the sample stimulus, and a posttrial account emphasizes the role of stimulus changes during consolidation processing.     

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.     

Effects of stimulus dimension and of trial and intertrial illumination on acquisition of a match/mismatch task by pigeons

Birds were trained on a higher order conditional discrimination task, one that required birds to match samples and comparisons on some trials and to mismatch on other trials. Which task component was in effect was indicated by the level of chamber illumination (houselight-on and houselight-off instructions). Acquisition of the components of a color (red and green) match/mismatch task in the first half of the experiment was not differentially affected by the level of illumination associated with each task component, by houselight changes per se, or by the level of illumination during the intertriai interval (ITI). However, when shapes (plus and circle) were used to train the task in the second half of the experiment, performance on the houselight-cued task component exceeded performance on the dark-cued task component, and ITI illumination facilitated performance on both task components. These results suggest that attention to shape stimuli, but not to colors, may vary systematically as a function of chamber illumination level.     

Effects of intertrial interval and exteroceptive feedback duration on discriminative avoidance acquisition in the gerbil

A series of studies of shuttlebox-avoidance learning in the gerbil evaluated the efficacy of an exteroceptive feedback stimulus (FS). Experiment 1 assessed the relative effectiveness of a FS at 30- and 90-sec intertriai intervals (ITIs), and found that the FS and warning signal termination contingencies were additive sources of avoidance reinforcement; i.e., they produced “supernormal acquisition” at the short ITI, but not at the 90-sec ITI. The effectiveness of a FS at the 30-sec ITI was further explored in Experiments 2 and 3, in which FS duration was varied in delayed and trace avoidance conditioning, respectively. In both studies, a FS facilitated acquisition but FS duration was not a critical determinant of performance. These results were interpreted in terms of an expectancy account of the informational value of a FS, and the problem of experimentally distinguishing between cognitive and inhibition-of-fear accounts of avoidance learning was discussed.     

Asymmetry in the discrimination of quantity by rats: The role of the intertrial interval

In three experiments, rats were trained to discriminate between 20 and five (Exps. 1 and 2), or between 40 and five (Exp. 3), black squares. The squares were randomly distributed in the center of a white background and displayed on a computer screen. For one group, the patterns containing the higher quantity of squares signaled the delivery of sucrose (+), whilst patterns with the lower quantity of squares did not (–). For the second group, sucrose was signaled by the lower, but not by the higher, quantity of squares. In Experiment 1, the intertrial interval (ITI) was a white screen, and the 20+/5– discrimination was acquired more readily than the 5+/20– discrimination. For Experiment 2, the ITI was made up of 80 black squares on a white background. In this instance, the 5+/20– discrimination was acquired more successfully than the 20+/5– discrimination. In Experiment 3, two groups were trained with a 40+/5– discrimination, and two with a 5+/40– discrimination. For one group from each of these pairs, the training trials were separated by a white ITI, and the 40+/5– discrimination was acquired more readily than the 5+/40– discrimination. For the remaining two groups, the training trials were not separated by an ITI, and the two groups acquired the task at approximately the same rate. The results indicate that the cues present during the ITI play a role in the asymmetrical acquisition of magnitude discriminations based on quantity.     

Massed and spaced training build up different components of long-term habituation in the crabChasmagnathus

The crabChasmagnathus granulatus reacts to a shadow passing overhead with an escape response that habituates after 30 trials and for 5 days at least. The effect of a wide range of different intertrial intervals (ITIs) (0, 9, 27, 45, 81, 135, and 171 sec) on theChasmagnathus long-term habituation (LTH) was evaluated at 24 h. Memory retention was estimated separately at two phases of a six-trial testing session: at first trial (the initial testing phase) and at the subsequent block of five trials (the retraining phase). A training of 30 trials with an ITI equal to or longer than 27 sec induced LTH at both testing phases, however, with a 0- or a 9-sec ITI, training wholly failed to build up LTH. When the number of trials was increased, a massed training (ITI=0 or 9 sec) induced LTH at retraining but not at initial testing. Thus, massed training produces LTH only at retraining, whereas spaced training (ITI ≥ 27 sec) produces LTH at both initial phase and retraining. An ITI shift from training to testing diminished or abolished retention at retraining regardless of the direction of the shift, thus suggesting that crabs acquire a memory of the trial-spacing at training. According to these results, it is postulated that LTH consists of two memory components: one produced by spaced training and expressed at both initial testing and retraining, and one yielded by massed training and expressed only at retraining. The possibility that the two components of LTH were differentially affected by cycloxemide and context shift is discussed.     

Temporal discrimination using different feature-target intervals in classical conditioning of the rabbit’s nictitating membrane response

In a typical conditional discrimination, a target stimulus (X) is reinforced during one feature cue (A→X+), but not during another feature cue (B→X−). The present experiments used only a single “feature” cue (a 66-sec tone). On half of the trials, the target stimulus (a 400-msec light) was paired with the reinforcer when the feature-target interval was one duration (e.g., 5 sec). On the remaining trials, the interval was different (e.g., 45 sec), and the target stimulus was presented without the reinforcer. All the animals acquired this temporal discrimination, and subsequent testing with other feature-target intervals yielded generalization-like gradients. These results provide solid evidence that each portion of a feature cue is encoded in a distinctive fashion. Had temporal encoding not occurred, the feature cue would have been just as ambiguous a predictor of the reinforcer as was the target stimulus, and discrimination would not have been possible. The integration of real-time temporal encoding mechanisms into models of conditional discrimination is discussed.     

Extinction of stimulus elements decreases the rate of conditional discrimination learning

Pigeons were trained on a successive discrete-trial conditional discrimination, in which either of two colors appeared on the center key and either of two forms appeared on the two side keys. Some combinations of color and form (S+ trials) were followed by food; other combinations (S-trials) were followed by no food. The colors on the center key appeared throughout the intertrial interval (ITI) preceding the trial period in which both the color and the form compound were presented or the color appeared only during the trial period in compound with the forms. Presentation of the colors during the ITI substantially degraded acquisition of two separate types of conditional discrimination. The results show that the temporal correlation of the eolors-with food, independent of their association with the form cues, played a substantial role in determining their availability for acquisition of the conditional discrimination.     

Outcome-selective effects of intertrial reinforcement in a Pavlovian appetitive conditioning paradigm with rats

The outcome-selective effects of presenting intertrial unconditioned stimuli (USs) in a rat appetitive conditioning paradigm were examined in two experiments. In both experiments, two stimuli were paired with different outcomes, while one of these outcomes was also presented in the intertrial interval (A+, B*, +). Two measures of learning, stimulus-elicited magazine approach and Pavlovian-to-instrumental transfer, were used to examine these effects. The presence of freely occurring outcomes in the intertrial interval (ITI) was observed to interfere more with the learning of a new association (Experiment 1) and to degrade more an already established association (Experiment 2) when the conditioned stimulus had been paired with the same outcome as that occurring in the ITI. An outcome-selective effect of ITI USs distinguishes among accounts of contingency based on general performance, attentional, and motivational mechanisms from those based on more specific associative mechanisms. Overall, the data highlight the importance of specific encoding processes in the analysis of associative learning.