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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 intertrial interval (ITI) duration was decreased. However, within the DO group, ITI duration affected performance on low-probability-of-reinforcement trials but not on high-probabilityof-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 IT! duration had a detrimental effect on low-probability-of-reinforcement trials but no effect on high-probabilityof-reinforcement trials. At the long DI duration, decreasing ITI duration had detrimental effects on both types of trials. In Experiment 4, unsignaled IT! 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.Numerous studies have shown that choice delayed matching-to-sample (DMTS) performance by pigeons is degraded by decreases in the temporal separation of trials (Grant, 1975;Hogan, Edwards, & Zentall, 1981;Maki, Moe, & Bierley, 1977). The poorer performance obtained with short intertrial intervals (ITI) was initially viewed as being due to competing memories from previous trials (Grant, 1975;Maki et al., 1977). However, this explanation has been called into question by several recent findings. Roberts (1980) found that short ITls disrupted performance on homogeneous DMTS trials as well as on standard DMTS trials. Homogeneous DMTS trials present the same sample stimulus on each trial, eliminating the possibility of competing sample memories. Although there is evidence that memories on trial n -1 do carry over the ITI and affect performance on trial n, both Roberts (1980) and Roitblat and Scopatz (1983) have shown that these are memories of the choice made on the previous trial rather than of the sample stimulus presented on the trial n -1. More importantly, neither study found any evidence that the carryover of memories between trials interacts with ITI duration, thus weakening explanations of the trial spacing effect based on proactive inhibition (PI). Finally,

Pigeons were trained to match temporal (2 and 8 sec of key light) 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-onemapping 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 chooseshort 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 ofthe memory code being used.Researchers in the area of animal cognition have developed a variety of research approaches in an attempt to determine whether the memory code for visual samples in a delayed matching-to-sample (DMTS) task is a prospective representation of the correct comparison stimulus to respond to, or a retrospective representation of sample stimulus attributes (Honig & Thompson, 1982). These approaches include (1) comparisons of simple delayed and delayed conditional discriminations (Cohen,

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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 1sec 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.Procedural variables are important in the study oftime perception. A comparison of the discrimination offilled intervals and the discrimination of empty intervals has been the focus of some human research (Abel, 1972a(Abel, , 1972bGrondin, 1993;Rammsayer & Lima, 1991), as well as ofanimal research (Mantanus, 1981). In the empty stimulus condition of the study by Mantanus, a flash of light marked the beginning ofthe interval, and presentation of the choice keys marked the end of the interval. The filled interval consisted of the continuous presentation of the light. Mantanus found that pigeons were more accurate with filled intervals than with empty intervals. The interpretation of this effect is ambiguous, because of a number of design and general test procedure problems that were recently outlined by Kraemer, Randall, and Brown (1997). In order to address these concerns, Kraemer et al. (1997) conducted a study in which pigeons were trained to discriminate either filled intervals (light present) or empty intervals (light absent). Consistent with the findings ofMantanus, they found that pigeons judged This research was supported by a grant from the Natural Sciences and Engineering Research Council of Canada (OGPOOD6378). The authors thank Marion Corrick and David Hemsworth for their technical assistance and Melanie Bucking for assisting with some of the data collection. The authors are also very appreciative ofthe substantial contribution of an a...