Effects of the inclination of context lines on perception of the Ponzo illusion by pigeons

Abstract

Pigeons discriminated the length of a bar located between two context lines. Responses to one key were reinforced when the bar was longer than a predetermined length, and those to the other key were reinforced when the bar was shorter. The inclination of the context lines was systematically varied from 54.6° (converging upward) to 125.4° (converging downward). Five out of 6 subjects tended to report “long” when the bars were located near the apex of the context lines, regardless of whether the context lines were oriented upward or downward. The magnitude of the illusion varied almost linearly with the ratio of the length of the stimulus bar to the gap between the bar and the context lines. This relationship held equally for upward- and downward-converging context lines.