Procedural determinants of coding processes in pigeons' memory for duration

The option to make a choice appears to determine the asymmetrical, 'respond-short' effects in pigeons' retention functions for duration. The respond-short effect occurs in choice tasks, but not in delayed-matching-to-sample tasks. Experiments with hybrid tasks eliminated several possible explanations for the difference in retention. Temporal parameters of the stimulus schedule, timing of the fixed interval, relative amount of time exposed to the sample and test stimuli, or simultaneous versus successive exposure to test stimuli did not appear to be critical features.