Further Analysis of Perception of the Standard Muller-Lyer Figures in Pigeons (Columba livia) and Humans (Homo sapiens): Effects of Length of Brackets

Nakamura, Fujita, Ushitani, & Miyata (2006) have shown that pigeons perceive the standard Muller-Lyer illusion. In this report, the authors examined effects of bracket sizes on perception of this illusion in pigeons (Columba livia) and humans (Homo sapiens). In Experiment 1, three pigeons were retrained to classify six lengths of target lines into long and short by pecking two keys on the monitor, ignoring the two brackets oriented toward the same direction. In the tests that followed, the standard Muller-Lyer figures of different bracket sizes were presented. All birds chose long more frequently for the figures having inward-pointing brackets (><) than for those having outward-pointing brackets (<>), regardless of bracket sizes. The overestimation of the target lines of inward-pointing figures continued to increase in pigeons, whereas it decreased as the bracket size became longer in humans (Experiment 2). The results suggest that these two species perceive the standard Muller-Lyer illusion with long brackets in different ways. Perhaps pigeons might not perceive illusions induced by contrast with the surrounding stimuli.