The brain creates illusions not just for us: sharks (Chiloscyllium griseum) can “see the magic” as well

Abstract

Bamboo sharks (Chiloscyllium griseum) were tested for their ability to perceive subjective and illusionary contours as well as line length illusions. Individuals were first trained to differentiate between squares, triangles, and rhomboids in a series of two alternative forced-choice experiments. Transfer tests then elucidated whether Kanizsa squares and triangles, grating gaps and phase shifted abutting gratings were also perceived and distinguished. The visual systems of most vertebrates and even invertebrates perceive illusionary contours despite the absence of physical luminance, color or textural differences. Sharks are no exception to the rule; all tasks were successfully mastered within 3–24 training sessions, with sharks discriminating between various sets of Kanizsa figures and alternative stimuli, as well as between subjective contours in >75% of all tests. However, in contrast to Kanizsa figures and subjective contours, sharks were not deceived by Müller-Lyer (ML) illusions. Here, two center lines of equal length are comparatively set between two arrowheads or –tails, in which case the line featuring the two arrow tails appears to be longer to most humans, primates and birds. In preparation for this experiment, lines of varying length, and lines of unequal length randomly featuring either two arrowheads or -tails on their ends, were presented first. Both sets of lines were successfully distinguished by most sharks. However, during presentation of the ML illusions sharks failed to succeed and succumbed either to side preferences or chose according to chance.

Why do animals differ in their susceptibility to geometrical illusions?

“…Of the species tested, only sharks did not appear to be susceptible to the Müller-Lyer illusion (Fuss et al, 2014). All other species tested experienced the illusion in the same way as humans, perceiving the target line to be longer when Nakamura et al (2014) 3 Gallus gallus (bantam chickens); 6 months old, 2 male, 1 female…”

Visual Discrimination of Geometric and Complex 2D Shapes in Goldfish (Carassius auratus)

“…Fish perceive a wide variety of illusions (Agrillo, Miletto Petrazzini, & Dadda, 2013;Rosa Salva et al, 2014). Goldfish, redtail splitfins (Xenotoca eiseni), and bamboo sharks (Chiloscyllium griseum) perceive illusory contours in Kanizsa figures (Fuss, Bleckmann, & Schluessel, 2014a;Sovrano & Bisazza, 2009;Wyzisk & Neumeyer, 2007). Although no Kanizsa figures were used in the current study, the fish may have perceived an illusory figure of a circle while looking at the negative stimuli for stimulus pairs 4 and 5.…”

“…Redtail splitfin (Xenotoca eiseni), goldfish (Carassius auratus), and bamboo sharks (Chiloscyllium griseum) fall prey to visual illusions in the same manner as humans when they perceive illusory contours within Kanizsa figures (Fuss, Bleckmann, & Schluessel, 2014a;Sovrano & Bisazza, 2009;Wyzisk & Neumeyer, 2007). Redtail splitfin have shown the ability to visually complete partly occluded objects, known as amodal completion (Sovrano & Bisazza, 2008).…”

World in Motion: Perception and Discrimination of Movement in Juvenile Grey Bamboo Sharks (Chiloscyllium griseum)

“…Previous studies on juvenile grey bamboo sharks using different optical illusions revealed that sharks most likely applied complementing visual mechanisms such as perceptual grouping and local focal attention of a few features of a scene (Fuss, Bleckmann & Schluessel, 2014c;Fuss & Schluessel, in press). It is likely, that sharks (preattentively) identified the basic features of the presented stimuli ('black circles' and 'movement'), but -speculatively -failed to associate both elements, as distinction of the presented stimuli (or single features of these stimuli) was not strong enough when presented without a frame of reference.…”