Illusory patterns are fishy for fish, too

Abstract

It has been widely recognized that size,shape, and distance perception are not themere translation of images in the eyes, asretinal images are inherently ambiguous.Some form of knowledge and/or assump-tions by unconscious inductive inferenceseems to be necessary (Gregory, 1997).With respect to this topic, visual illusionsare a valuable tool for understanding theneuro-cognitive systems underlying visualperception by indirectly revealing the hid-den constraints of the perceptual systemin a way that normal perception cannot.In humans, such constraints have beenoftensummarizedastheso-called“Gestaltprinciples,” which can be briefly describedby the motto “the whole is greater thanthe sum of its parts” (Wertheimer, 1938).Almost a century of experimental inves-tigation on visual illusions has broad-ened ourcomprehensionofthe perceptualmechanisms that enableus to perceive fig-ures and forms instead of just a collectionof lines and curves. Such mechanisms arehighly adaptive, as they allow for a quickand stable picture of the environment,enablinganappropriatemotorresponseinevery context (Ikinand Turner, 1972).Given their high ecological value, thereislittlereasontobelievethatselective pres-surestodevelopavisualsystemthatisableto segregate objects from the backgroundhaveacted onlyonhominids. Indeed, overthe lastdecade,research hasdemonstratedthat both apes and monkeys are deceivedbyillusorypatterns. Forinstance, baboonsperceive the Zollner illusion (Benhar andSamuel, 1982), capuchin monkeys per-ceive the Muller-Lyer illusion (Suganumaet al., 2007), and rhesus monkeys per-ceive numerosity illusion (Beran, 2006;Beran and Parrish, 2013), thus showingthattheorganizationofvisualinformationissimilarbetweenhumanandnon-humanprimates.Despite the existence of a large num-ber of studies, it is still unclear to whatextent previous experience plays a role inhow the brain/mind interprets and recon-structs physical reality (Hebb, 1949; Bod,2002; Quinn and Bhatt, 2006). For prac-tical and ethical reasons, it is very difficulttomanipulateexperiences duringdevelop-mental periodsinhumanandnon-humanprimates. Furthermore, as primates lackindependence at birth, different proce-dures are used for studying newborns,juveniles, or adults, presenting one of themajordrawbackswhenstudyingthedevel-opment of visual perception in primates,i.e., the difficulty of devising experimen-tal paradigms applicable to different ages(Bisazza et al., 2010). The recent dis-covery that even relatively simple organ-isms like fish, whose divergence seeminglyoccurred approximately 450 million yearsago (Kumar and Hedges, 1998), also per-ceive visual illusions, as humans do, pavesthe way for the use of new animal modelsto investigate the relative contribution ofgenes and experience.Redtailsplitfin,forinstance,wasshownto be able to perceive illusory contours(Sovrano and Bisazza, 2009). Fish wererequired to discriminate between a squareor a triangle and the corresponding back-ground. After reaching a learning crite-rion, subjects performed test trials in thepresence of two stimuli: one consisted ofasubjectivefigure(triangleorsquare)induced by interruption or spatial phase-shift of diagonal lines; the other consistedof a series of diagonal lines only. In a sub-sequent test, two figures were presented:one in which pacmen were positionedin order to reproduce the Kanizsa tri-angle or square, and one in which thesame pacmen were scrambled in differentpositions so as to prevent an impressionof a subjective figure. Discrimination oforientation, rather than discrimination ofshape, was also tested in a second exper-iment. Subjects were initially trained todiscriminatebetween averticalandahori-zontal line with real physical contours. Intest trials vertically and horizontally ori-ented illusory lines were presented, cre-ated either through interruption or spatialphase-shift of diagonal lines (see