A prototype effect and categorization of artificial polymorphous stimuli in pigeons.

In a go-no-go discrimination task, pigeons were trained to discriminate artificial polymorphous stimuli differing along 3 six-valued features. Exemplars of each category were generated by systematic transformations of the features from a single stimulus, a base pattern (prototype). They were then tested for transfer to novel stimuli including the base patterns. The most pronounced discrimination occurred between the base patterns rather than between the extreme positive and negative stimuli. A distance-from-prototype rule and an additive integration of feature utilities or of feature frequencies are suggested to account for the prototype effect. In Experiment 2, pigeons were trained on a 2-key choice discrimination task, with the base patterns as conditional stimuli. Subsequent testing with distorted stimuli suggested that a distance-from-prototype model best explains the present findings.