Perception of Ebbinghaus-Titchener stimuli in starlings (Sturnus vulgaris)

Whether animals experience visual illusions is a fertile area of study for examining the evolution and operation of visual cognition across different species. Here, five starlings were tested to examine whether they experienced the Ebbinghaus-Titchener illusion. Across two experiments using an absolute target circle size discrimination, the size, similarity, distance, and number of the surrounding flankers were manipulated. The results suggest that this passerine species exhibits behavior inconsistent with the perception of the illusion, neither in a human-like fashion nor, as suggested by the first experiment, a reversed illusion. Instead, the typical training used to investigate this illusion caused the starlings to learn to integrate the irrelevant flankers into their decision process in a manner that precludes the study of illusory perception. The resulting discriminative behavior might best be described using a template-matching account. While illusion perception by animals remains an important comparative question, it requires additional validation to confirm the exact mechanisms of any illusory reports.