Further Analysis of Perception of Reversed Müller-Lyer Figures for Pigeons ( Columba Livia )

Abstract

Nakamura, et al. recently showed that pigeons experience the standard Müller-Lyer illusion but not the reversed illusion induced by detaching the arrowheads from the target line. This study re-examined pigeons' perception of this reversed figure by using the stimuli known to induce the maximal contrast effect in humans (Fellows, 1967). Pigeons were retrained to classify six lengths of target lines into “long” and “short” categories by pecking two keys on the monitor, ignoring the two brackets so placed that these would not induce an illusion. In the test that followed, two birds' responses were not affected by directions of arrowheads, as shown in the previous study. The third pigeon significantly chose “long” for inward-pointing brackets figures (> Z<) more frequently than for outward-pointing (< >), that is, the direction of illusion was reversed from what is expected in humans. These results suggest that pigeons may not experience illusions induced by contrast with the surrounding stimuli.