Memory For Event Duration Research Articles

Rats’ memory for event duration in delayed matching-to-sample with nonspatial comparison response alternatives

Previous research has suggested that using stationary and moving levers as nonspatial response alternatives can significantly enhance the speed of acquiring a temporal discrimination in rats. In Experiment 1, rats were trained to discriminate 2 and 8s of magazine light illumination by responding to either a stationary lever or a moving lever with a cue light illuminated above it. Rats learned to discriminate event durations at a high level of accuracy after 25 sessions of training. During subsequent delay tests, rats exhibited a strong choose-long bias, indicating that they were timing from the onset of the magazine light until the entry of levers into the chamber. This occurred regardless of whether intertrial intervals and delay intervals were dark or illuminated. On test trials in which the sample was omitted, rats responded as if the short sample had been presented. In Experiment 2, the rats received extensive training with dark and illuminated variable delay intervals (1-4 s). However, they continued to exhibit a tendency to time from the onset of the magazine light until entry of the levers into the chamber. Although the use of stationary/uncued and moving/cued levers as response alternatives enhanced the speed of acquisition of the event duration discrimination in rats, additional procedural modifications will be necessary to prevent rats from timing during the delay interval.

The role of “nothing” in memory for event duration in pigeons

The research reported herein is designed to test a signal detection account of thechoose-short effect, which is the tendency of birds to report (after a long delay) that a short-duration sample was presented, regardless of whether a short or long sample initiated the trial. According to the detection account, the choose-short effect arises because birds learn to selectively search memory for evidence that the long sample appeared on a given trial. This idea is tested, in part, by replacing short-sample trials with nosample trials and showing that performance is unaffected by this manipulation for birds exhibiting a choose-short effect. In addition, when no samples and long samples were associated with the same choice alternative (and the short sample was associated with the other alternative), birds were flexible enough to learn to respond on the basis of the presence versus the absence of the short sample (and, as a result, a choose-long effect was observed).

Pigeons’ memory for event duration: Differences between visual and auditory signals

In Experiment 1, pigeons were trained to discriminate short (2 sec) and long (8 sec) durations of tone by responding to red and green comparison stimuli. During delay testing, a systematic response bias to the comparison stimulus correct for the long duration occurred. Tests of responding without the tone reduced accuracy on long-sample trials but not on short-sample trials suggesting that the pigeons were attending to the tone and not simply timing the total trial duration. The pigeons were then trained to match short (2 sec) and long (8 sec) durations of light to blue/yellow comparisons. During delay testing, “choose-long errors” occurred following tone durations, but “choose-short errors” occurred following light durations. In Experiment 2, accuracy was assessed on test trials in which the tone and the light signals were simultaneously presented for the same duration or for different durations. Pigeons responded accurately to durations of light, but were unable to accurately respond to durations of tone simultaneously presented with the light. The data from Experiment 1 suggest that there are important differences between light and tone signals with respect to the events that control the termination of timing. The data from Experiment 2 indicate that pigeons cannot simultaneously time visual and auditory signals independently and without interference. Consequently, they are inconsistent with the idea that there is a single internal clock that times both tone and light durations.

Rats' memory for event duration: differential effects of delaying the discriminative choice cue as opposed to the opportunity to execute the choice response

Rats were trained to discriminate short or long durations of houselight illumination using a choice procedure. During the test phase of each trial, the left and right levers were presented with an auditory cue above one of them on (cued lever) while the other was off (uncued lever). The auditory cue was presented immediately after sample offset and the levers were inserted after the auditory cue had been presented for 2 s. For half of the rats, the correct response following the short sample was to press the cued lever, while following the long sample, it was to press the uncued lever. This was reversed for the remaining rats. Following acquisition of the discrimination, two different types of delay tests were administered. In the first set, the delay between offset of the sample and onset of the auditory cue was manipulated (Cue Delay Test). In the second set, the delay between onset of the auditory cue and entry of the levers into the chamber was manipulated (Response Delay Test). Cue Delay testing resulted in a choose-long bias at the longer delays. Response Delay testing did not result in a systematic response bias and there was little forgetting over the delay interval. These data suggest that the rats did not stop the internal clock when the nominal sample was offset, but allowed it to keep running until the auditory cue was presented. The data from the Response Delay Test indicate that either a response decision was made based on the clock reading as soon as the auditory cue was presented, or the clock reading itself was retained over the delay with no subjective shortening and little forgetting.

Amphetamine and memory for event duration in rats and pigeons: Disruption of attention to temporal samples rather than changes in the speed of the internal clock

In Experiment 1, pigeons were trained to match short (2-sec) and long (8-sec) sample durations to red and green comparison stimuli and red and green samples to vertical and horizontal line comparisons. They received injections of d-amphetamine (2.0 mg/kg) or saline, and the delay interval was manipulated (0, 1, 3, and 9 sec). In Experiment 2, rats were trained to discriminate short (2-sec) or long (12-sec) durations of either houselight illumination (light group) or tone (tone group), using a choice procedure. During the test phase of each trial, the left and right levers were presented with the cuelight above one of them on (cued lever) while the other was off (uncued lever). For some of the rats, the correct response following the short sample was to press the cued lever, whereas following the long sample, it was to press the uncued lever. This was reversed for the remaining rats. The rats received injections of methamphetamine (1.5 mg/kg) or saline, and the delay interval was manipulated (0, 1, 3, and 9 sec). Memory for color samples in pigeons was not affected by amphetamine. However, for both rats and pigeons, amphetamine reduced the accuracy of event duration memory even at the 0-sec delay but did not increase the slope of the retention functions. In addition, in neither experiment was there any evidence that amphetamine produced an overestimation of perceived sample duration. Amphetamine predominantly affected temporal memory in both pigeons and rats by disrupting attention to temporal samples rather than by simply increasing the speed of the internal clock.

Memory for event duration in rats

Rats were trained to discriminate short or long durations of either houselight illumination (light group) or tone (tone group) using a choice procedure. During the test phase of each trial, the left and right levers were presented with the cuelight above one of them on (cued lever) while the other was off (uncued lever). For half of the rats, the correct response following the short sample was to press the cued lever, while following the long sample, it was to press the uncued lever. This was reversed for the remaining rats. Delay testing conducted in the context of a 10-s intertrial interval (ITI) indicated that for the light group, accuracy declined equivalently on short and long sample trials as the delay interval increased. However, for the tone group, a choose-long bias was seen at the longer delays. When the ITI was increased from 10 to 25 s, both groups displayed a choose-long bias. These data suggest that rats in both groups had the clock running during the ITI prior to presentation of the nominal sample, and that both groups used a change in lighting condition within the chamber to stop the clock. For the light group, this event was offset of the houselight sample, while for the tone group it was onset of the cuelight itself. Subsequent delay testing in the context of a 25-s ITI indicated that, for the tone group, accuracy declined equivalently on short and long sample trials as the delay interval was increased. However, for the light group, a choose-short bias was seen at the longer delay. Following the shift to a longer ITI, both tone and light groups appeared to evaluate the memory of the subjective duration of the sample as being ‘shorter’ in the context of a long ITI than in the context of a short ITI. When trained to make choices based on the duration of a preceding event, both rats and pigeons make systematic errors when the delay between the event and the opportunity to choose is varied. The nature of the systematic errors displayed are complexly determined by the temporal context within which they are exhibited (in both rats and pigeons) and by the modality of the remembered event duration (in rats).

Pigeons′ Memory for Event Duration in Choice and Successive Matching-to-Sample Tasks

Pigeons were trained to discriminate between short (2 s) and long (10 s) house-light samples within successive or choice delayed matching-to-sample tasks, and then were given four types of tests. During one type of test, the delay interval between the sample and test period was manipulated within sessions. Consistent with earlier findings, pigeons in the choice task displayed a reliable choose-short tendency at longer delays, whereas those in the successive task failed to show an analogous respond-short tendency. A second type of test entailed multiple sample presentations: the short and long samples were preceded by short or long pre-samples. Pigeons in both the choice and successive tasks showed a temporal-summation effect rather than a sample-consistency effect. In a third type of test, the intertrial interval was manipulated within sessions. Pigeons in both tasks showed a respond-long bias when the intertrial interval preceding the trial was shortened. In a final test, pigeons received no-sample tests, and tests with a 40-s sample. Pigeons in both groups showed a strong tendency to respond short on no-sample tests, and to respond long following a 40-s sample. It appears that timing processes are similar in the two tasks, but that memorial processes differ.

Temporal context effects in pigeons' memory for event duration

These experiments extend previous studies of how pigeons' memory for the duration of events is affected by the length of the delay and intertrial intervals. Pigeons were trained using a delayed matching-to-sample procedure to peck one color following a long sample and another color following a short sample. Experiments 1a and 1b tested a prediction, derived from the relative duration hypothesis, that systematic errors in response to delay manipulations should be greater in the context of a short intertrial interval than in the context of a long one. Pigeons were given delay tests in the context of either 10-s or 60-s intertrial intervals. Systematic errors in response to delay manipulations were of greater magnitude in the context of the 10-s intertrial interval, as predicted by the relative duration hypothesis. Experiments 2a and 2b examined whether stable delay and intertrial intervals during training are important for the development of a temporal reference memory. Variability in the intertrial or delay interval during training had only minor effects on acquisition and did not affect pigeons' tendency to make systematic errors in response to subsequent manipulations of these intervals. Moreover, even during acquisition, birds trained with variable delays showed a systematic increase in the tendency to make choices appropriate to the short sample as a function of the training delay, with the least bias occurring at an intermediate delay. Thus, reference memory appeared to be based on an average of the delays experienced during training.

Pigeons' memory for event duration: Differences between choice and successive matching tasks

Two experiments examined memory for event duration in pigeons using choice and successive matching tasks. In the choice task, two stimuli were presented following the sample, and the correct choice depended upon the duration of the preceding sample. In the successive task, only one of two stimuli was presented following the sample, and whether responding was or was not reinforced depended upon the duration of the preceding sample. In the first experiment, a successive matching task was employed and accuracy decreased at an equivalent rate as a function of delay on short-sample and long-sample trials, regardless of whether the events which differed in duration were samples of houselight or food. The second experiment, which employed as samples different durations of houselight, replicated this result both in naive subjects and in subjects previously trained in a choice matching task. In addition, it was found that naive subjects which were trained and tested in a choice matching task demonstrated a more rapid decrease in accuracy on long-sample trials than on short-sample trials as a function of delay; a result which has been obtained in several previous studies and has been referred to as the “choose-short” effect. It was also found, however, that the choose-short effect did not occur in the choice matching task if subjects had been trained and tested previously in the successive matching task. It was concluded that pigeons employ different coding strategies in matching to sample duration in the choice and successive tasks.

Proactive effects in pigeons' memory for event duration: Evidence for analogical retention.

An intrairial proactive interference design was used to examine the nature of pigeons' memory for duration in a delayed matching task. Short (2 s) or long (10 s) target samples were preceded on test trials by a short or long presample. The durations were consistent on some trials (shortshort or long-long) and inconsistent on others (short-long or long-short). Contrary to predictions based on prospective or categorical coding, accuracy was not related to duration consistency. Instead, accuracy was reduced on short-short and long-short trials and somewhat enhanced on short-long and long-long trials, suggesting that the pigeons summed across the durations. This occurred even with a 10-s interstimulus interval (Experiment 1) and even when the presample and target sample were physically distinct (Experiment 2). These results suggest that pigeons remember event durations in an analogical and retrospective fashion.

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.

Proactive effects in pigeons' memory for event duration: Evidence for analogical retention.

An intrairial proactive interference design was used to examine the nature of pigeons' memory for duration in a delayed matching task. Short (2 s) or long (10 s) target samples were preceded on test trials by a short or long presample. The durations were consistent on some trials (shortshort or long-long) and inconsistent on others (short-long or long-short). Contrary to predictions based on prospective or categorical coding, accuracy was not related to duration consistency. Instead, accuracy was reduced on short-short and long-short trials and somewhat enhanced on short-long and long-long trials, suggesting that the pigeons "summed across" the durations. This occurred even with a 10-s interstimulus interval (Experiment 1) and even when the presample and target sample were physically distinct (Experiment 2). These results suggest that pigeons remember event durations in an analogical and retrospective fashion.

Systematic errors in pigeons’ memory for event duration: Interaction between training and test delay

Pigeons trained to choose different stimuli following short- and long-duration signals make disproportionately more “short” choices (i.e., “choose-short errors”) following an increase in the retention interval and more “choose-long errors” following a decrease in this delay. The present experiment provided a systematic investigation of how these selective errors depend on the relationship between the training delay and the test delay. Pigeons were first trained with a 0-sec delay between the signal (2- or 8-sec food presentations) and the choice stimuli (red- and blue-lit keys). On subsequent test trials with 5- and 10-sec delays, choose-short errors predominated. Next, the birds were trained with a constant 10-sec delay and then tested with shorter or longer delays on some trials. The birds now responded accurately and without selective errors at the 10-sec training delay, but made choose-long errors at shorter delays and choose-short errors at longer delays. Finally, the birds were trained with a constant 20-sec delay and then tested with shorter and longer delays. Choose-long errors again appeared at shorter test delays, choose-short errors at longer test delays, and no differential errors at the 20-sec training delay. The selectivity of these errors generally increased with the absolute difference between the training and test delay. Theoretical implications of these results are discussed.

Subjective shortening: A model of pigeons' memory for event duration.

Subjective shortening: A model of pigeons' memory for event duration.

Subjective shortening: A model of pigeons' memory for event duration.

Subjective shortening: A model of pigeons' memory for event duration.