The Muller-Lyer Illusion in the Pigeon: a Methodological Study

Abstract

A methodological study was performed in order to derive a suitable procedure for the study of the Muller—Lyer illusion in the pigeon. Pigeons were trained to peck at one key when the middle wing in a Muller-Lyer figure was displaced towards one of the outside wings, and at the other key when it was displaced towards the other outside wing. The subjects were then presented with varied settings of the middle wing in order to determine the setting at which they would switch from one key to the other. It was found that, in a two-key situation, the pigeon developed a "preference" for one key over the other and that this tendency, carried over into the testing situation, influenced the "switchover point in a direction determined by the key preference. This evidence enabled the derivation of a more suitable method, which is suggest for use in further research on the Muller-Lyer illusion in the pigeon.