The choose-short effect in pigeon memory for stimulus duration: Subjective shortening versus coding models

A symbolic delayed matching procedure may be used to study memory for stimulus duration in pigeons. Short and long presentations of a light sample stimulus are mapped onto the choke of visually differentiated comparison keys. When delay is varied in such a symbolic delayed matching procedure, pigeons show increasing preference for the short-sample key as the delay becomes longer (choose-short effect), even after a long sample stimulus has been presented. Two theoretical explanations of the choose-short effect are suggested. A subjective shortening model holds that the choose-short effect arises from progressive shortening of the memory of stimulus duration as the delay proceeds. An alternative coding model suggests that the choose-short effect arises from stimulus generalization after an initial response instruction to peck the long-sample key has been forgotten. These two models were tested by training pigeons to peck a third comparison key after no sample stimulus had been presented. Shifts in key preferences over delays ranging from 0 to 21 sec clearly supported the coding model.