Visual Illusions and Numerical Perception: The Case of the Regular-Random Illusion in Kryptopterus bicirrhis and Poecilia reticulata

The present thesis investigates whether two fish species, Kryptopterus bicirrhis and Poecilia reticulata, are susceptible to the regular-random numerosity illusion (RRNI), a visual illusion in which regularly arranged elements appear more numerous than randomly arranged ones. This phenomenon has previously been observed in humans, chimpanzees, and rhesus macaques. Building on prior research that emphasizes the influence of spatial organization on quantity estimation, this study explores whether similar perceptual mechanisms are present in fish. Subjects were first trained on numerical discrimination tasks using 0.5 ratio comparisons over 40 trials, followed by 0.75 ratio comparisons until they reached a predefined performance criterion. After the training phase, fish underwent a test phase in which they were presented with pairs of illusory stimuli. Each pair consisted of two arrays with the same number of elements—one arranged regularly and the other randomly. Evidence of a consistent preference for regular over random configurations during illusory trials would indicate that fish, like primates, rely on comparable perceptual strategies for numerosity estimation. Such findings would support the hypothesis of an evolutionarily conserved mechanism for numerical perception across vertebrate species.