The Ebbinghaus illusion in the gray bamboo shark (Chiloscyllium griseum) in comparison to the teleost damselfish (Chromis chromis)

This is the first study to comparatively assess the perception of the Ebbinghaus-Titchener circles and variations of the Delboeuf illusion in four juvenile bamboo sharks (Chiloscyllium griseum) and five damselfish (Chromis chromis) using identical training paradigms. We aimed to investigate whether these two species show similarities in the perceptual integration of local elements into the global context. The Ebbinghaus-Titchener circles consist of two equally sized central test circles surrounded by smaller or larger circles of different size, number and/or distance. During training, sharks and damselfish learned to distinguish a large circle from a small circle, regardless (i) of its gray level and (ii) of the presence of surrounding circles arranged along an outer semi-circle. During the subsequent transfer period, individuals were presented with variations of the Ebbinghaus-Titchener circles and the Delboeuf illusion. Similar to adult humans, dolphins, or some birds, damselfish tended to judge the test circle surrounded by smaller inducers as larger than the one surrounded by larger inducers (contrast effect). However, sharks significantly preferred the overall larger figure or chose indifferently between both alternatives (assimilation effect). These contrasting responses point towards potential differences in perceptual processing mechanisms, such as 'filling-in' or '(a)modal completion', 'perceptual grouping', and 'local' or 'global' visual perception. The present study provides intriguing insights into the perceptual abilities of phylogenetically distant taxa separated in evolutionary time by 200 million years.