Angelo Santi

Memory codes for temporal and nontemporal samples in many-to-one matching by pigeons

Pigeons were trained to match temporal (2 and 8 sec of keylight) and color (red and green) samples to vertical and horizontal comparison stimuli. In Experiment 1, samples that were associated with the same correct comparison stimulus displayed similar retention functions; and there was no significant choose-short effect following temporal samples. This finding was replicated in Phase 1 of Experiment 2 for birds maintained on the many-to-one mapping, and it was also obtained in birds that had been switched to a one-to-one mapping by changing the comparison stimuli following color samples. However, in Phase 2 of Experiment 2, when the one-to-one mapping was produced by changing the comparison stimuli following temporal samples, a significant choose-short effect was observed. In Experiment 3, intratrial interference tests gave evidence of temporal summation effects when either temporal presamples or color presamples preceded temporal targets. This occurred even though these interference tests followed delay tests that failed to reveal significant choose-short effects. The absence of significant choose-short effects in Experiment 1 and in Phase 1 of Experiment 2 indicates that temporal samples are not retrospectively and analogically coded when temporal and nontemporal samples are mapped onto the same set of comparisons The interference test results suggest that the temporal summation effect arises from nonmemorial properties of the timing system and is independent of the memory code being used

Prospective representation: The effects of varied mapping of sample stimuli to comparison stimuli and differential trial outcomes on pigeons’ working memory

The mapping of sample stimuli onto comparison stimuli and the nature of trial outcomes were factorially manipulated in a delayed matching-to-sample procedure. In the many-to-one condition (MTO), responding to a particular comparison was correct following several sample stimuli, whereas in the one-to-many condition (OTM), responding to several comparison stimuli was correct following a particular sample. Probabilities of reinforcement for correct responding to comparison stimuli were either differential (DO) or nondifferential (NDO). Four groups of pigeons were trained under four combinations of mapping and outcome conditions, MTO-DO, MTO-NDO, OTM-DO, and OTM-NDO. Testing at delay intervals of 1, 2, 4, and 8 sec revealed significant effects due to both the mapping variable and the differential outcomes variable. It was argued that the poorer performance obtained in the OTM condition was due to the differential prospective coding requirements placed on reference and working memory by this mapping. In the OTM conditions, a greater number of response codes had to be retrieved from reference memory and multiple response codes may have overburdened working memory, which has a limited capacity.

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.

Reinforcement expectancy and trial spacing effects in delayed matching-to-sample by pigeons

Four experiments assessed the role of reinforcement expectancies in the trial spacing effect obtained in delayed matching-to-sample by pigeons. In Experiment 1, a differential outcome (DO) group received reinforcement with a probability of 1.0 for correct comparison responses following one sample stimulus and a probability of 0.2 for correct comparison responses following the other sample stimulus. The nondifferential outcome (NDO) group received reinforcement with a probability of 0.6 for correct responses to either stimulus. While matching accuracy was higher for the DO group than for the NDO group, both groups showed an equivalent decline in accuracy as the intertriai interval (ITI) duration was decreased. However, within the DO group, ITI duration affected performance on low-probability-of-reinforcement trials but not on high-probability-of-reinforcement trials. In Experiment 2, delay interval (DI) duration was 5, 10, or 15 sec and accuracy was higher for the DO group than for the NDO group at all DI durations. In addition, accuracy decreased similarly on high- and low-probability-of-reinforcement trials for the DO group as DI was increased. In Experiment 3, all birds were studied under DO conditions and ITI duration was manipulated along with DI duration. At the short DI duration, decreasing ITI duration had a detrimental effect on low-probability-of-reinforcement trials but no effect on high-probability-of-reinforcement trials. At the long DI duration, decreasing ITI duration had detrimental effects on both types of trials. In Experiment 4, unsignaled ITI reinforcers disrupted accuracy when the DI was long and when the ITI was short. The applicability of scalar expectancy theory to these data is discussed.

Effects of the dopamine D2 agonist, quinpirole, on time and number processing in rats

In Experiment 1, rats were trained to discriminate discrete sound sequences that varied in time or number. On time trials, the number of sounds was held constant at 4 and the duration of the sound sequence was either 2 or 8 s. On number trials, the duration of the sound sequence was held constant at 4 s, and the number of sounds was either 2 or 8. Psychophysical functions for time and number were obtained by presenting unreinforced sequences of intermediate duration or number. In agreement with previous research, sensitivity to variation in time was greater than variation in number. Rats received injections of the specific D2 agonist, quinpirole (0.08 mg/kg), or saline. Quinpirole significantly attenuated control by both time and number, but it did not increase behavioral estimates of time or number. In Experiment 2, rats were given different dosages of quinpirole (0.02, 0.04 or 0.06 mg/kg). The steepness of the psychophysical functions for both time and number was reduced in a dose-related fashion without any evidence of an increase in the estimation of time or number. The similarity of the effect of quinpirole on both time and number processing is consistent with the idea that the same internal mechanism is used for timing and counting. However, it is not consistent with the idea that D2 dopamine agonists selectively increase the rate of the internal clock, which is assumed to underlie performance in a temporal bisection procedure. Quinpirole (at doses between 0.02 and 0.08 mg/kg) reduces sensitivity to time and number in a bisection procedure without altering the speed of the internal clock.

Pigeons exhibit higher accuracy for chosen memory tests than for forced memory tests in duration matching-to-sample

Following training to match 2- and 8-sec durations of feederlight to red and green comparisons with a 0-sec baseline delay, pigeons were allowed to choose to take a memory test or to escape the memory test. The effects of sample omission, increases in retention interval, and variation in trial spacing on selection of the escape option and accuracy were studied. During initial testing, escaping the test did not increase as the task became more difficult, and there was no difference in accuracy between chosen and forced memory tests. However, with extended training, accuracy for chosen tests was significantly greater than for forced tests. In addition, two pigeons exhibited higher accuracy on chosen tests than on forced tests at the short retention interval and greater escape rates at the long retention interval. These results have not been obtained in previous studies with pigeons when the choice to take the test or to escape the test is given before test stimuli are presented. It appears that task-specific methodological factors may determine whether a particular species will exhibit the two behavioral effects that were initially proposed as potentially indicative of metacognition.

The dopamine D2 agonist quinpirole disrupts attention to temporal signals without selectively altering the speed of the internal clock

Three groups of rats were trained to discriminate between 2 sec and 8 sec of darkness by responding to either the left or the right lever. Following acquisition of this temporal discrimination, psychophysical functions were obtained by presenting unreinforced signals of intermediate duration. Two groups of rats were trained with saline and subsequently tested with the specific D2 dopamine agonist quinpirole (0.08 mg/kg). One of these groups was naive to the drug prior to testing (DN), whereas the other had exposure to the drug but not during training sessions (DE). A third group (DT) was trained under quinpirole and tested with saline. The temporal discrimination was acquired rapidly and equiv-alently in Groups DN and DE. However, rats in Group DT were severely impaired in acquiring the discrimination. During psychophysical testing, quinpirole disrupted the accuracy of temporal discrimination equivalently in Groups DN and DE. Both the Weber fraction (WF) and the difference limen (DL) increased significantly in Groups DN and DE; however, the point of subjective equality (PSE) was not affected. In Group DT, the shift to saline during psychophysical testing did not result in any changes to the PSE, DL, or WF. These findings are not consistent with the hypothesis that the speed of the internal clock is selectively affected by D2 dopaminergic manipulations. Prior exposure to the drug does not appear to be a critical variable in the failure to observe a selective adjustment of the internal clock. The D2 agonist quinpirole appears to affect the accuracy of temporal discriminations generally, without altering the speed of the internal clock.

Memory for event duration in rats

Abstract 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).

The perception of empty and filled time intervals by pigeons

Pigeons were trained in a within-subjects design to discriminate durations of a filled interval (2 s and 8 s of light) and durations of an empty interval (2 s and 8 s bound by two 500-ms light markers). Filled intervals required a response to one set of comparisons (e.g., blue vs. yellow), whereas empty intervals required a response to a different set of comparisons (e.g., red vs. green). Psychophysical testing indicated that empty intervals were judged to be longer than equivalent durations of a filled interval. This finding was replicated when the anchor durations used during training were changed to 1 s and 4 s, or 4 s and 16 s. The difference between the point of subjective equality (PSE) for the empty intervals and the PSE for filled intervals increased as the magnitude of the anchor duration pairs increased. In addition, the difference limens (DL) for empty intervals were smaller than those for filled intervals, and they also increased as the magnitude of anchor duration pairs increased. An analysis of the Weber fractions (WF; i.e., DL/PSE) provided evidence for superimposition of the empty and filled timing functions across the different sets of anchor durations. These results suggest that the accumulation of subjective time was greater for empty intervals than for filled intervals. Within the framework of scalar timing theory, this difference in timing appeared to be the result of a clock rate difference rather than a switch latency difference.

Differences in matching-to-sample performance with element and compound sample stimuli in pigeons

Abstract This series of experiments examined zero-delay matching-to-sample performance in pigeons with element and compound sample stimuli. In Experiment 1, compound sample stimuli were consistently followed by compound comparison stimuli and matching accuracy during testing was equivalent to element sample-element comparison trials on the color dimension. In Experiment 2, element comparisons suddenly introduced following compound samples produced a decrement on the line dimension only. Subsequent testing at various sample durations revealed higher matching accuracy following element samples than following compound samples on both the color and line-tilt dimensions. Experiment 3 replicated the results of Experiments 1 and 2 and also demonstrated that the superiority of element over compound matching performance remains constant over the sample durations tested. In Experiment 4 testing at sample durations up to 30 sec produced an overall decrement in matching performance, but again the element vs compound matching difference remained constant. The stimulus-generalization decrement hypothesis provided a better explanation of these results than either the information-overload hypothesis or the rule hypothesis.