Is the Susceptibility to Visual Illusions Related to the Relative Brain Size? Insights from Small-Brained Species

Visual illusions are powerful tools to understand similarities and differences in the perceptual mechanisms of human and nonhuman animals. Such investigation is particularly important in the presence of animal species whose brains largely differ from ours, because it can reveal whether perceptual laws described in humans are strictly related to the peculiarity of large brains, as the case of mammals and birds. Here we review the literature on visual illusions in species with a much smaller relative brain size. Most works on this subject have investigated fish, whereas only a few studies have been conducted on amphibians and reptiles. Taken together, the existing literature found more similarities than differences in the perceptual mechanisms underlying size, numerosity, brightness, motion, and subjective contours among vertebrates, regardless of the high variability in the relative brain size of the species.