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...

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.

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