Pigeons' memory for empty time intervals marked by visual or auditory stimuli

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 b ias occurred at 1 sec, but a robust choose-long bias occurred at 9 sec. Res ponding in the absence of tone markers indicated that the pigeons were atte nding to the markers and not simply timing the total trial duration The bir ds 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 m arkers, delay testing produced a choose-short bias at 1 sec and a choose-lo ng 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 m arkers, the duration of both the empty interval and the preceding ITI affec ted choice responding. On trials with light markers, only the duration of t he empty interval influenced choice responding. Subsequent delay testing in the context of variable ITIs replicated the memory biases previously obtai ned. In Experiment 3, performance was assessed at various delay intervals o n trials in which either the first or the second marker was omitted. The da ta from these omission-tests indicated that the first marker initiated timi ng but that the second marker sometimes initiated the timing of a new inter val. Explanations of these effects in terms of the internal clock model of timing me discussed, and a simple quantitative model of the delay interval data is tested.