Perception of object unity by fish

Evidence for interpolation processes in fish vision is reported. Redtail splitfins (Xenotoca eiseni) were trained to discriminate between a complete and an amputated disk. Thereafter, the fish performed test trials in which hexagonal polygons were either exactly juxtaposed or only placed close to the missing sectors of the disk in order to produce or not the impression (to a human observer) of an occlusion of the missing sectors of the disk by the polygon. In another experiment, fish were first trained to discriminate between hexagonal polygons that were either exactly juxtaposed or only placed close to the missing sectors of a disk, and then tested for choice between a complete and an amputated disk. In both experiments fish behaved as if they were experiencing visual completion of the partly occluded stimuli. The ability of fish to perceive subjective contours, i.e. contours that lack a physical counterpart in terms of luminance contrast gradients, was also investigated. Fish were trained to discriminate between a geometric figure (triangle or square) located on various backgrounds and a background without any figure. Thereafter, the fish performed test trials in which illusory square- or triangle-shaped figures were obtained by interruptions or phase-shifting of a background of diagonal lines or circular sectors spatially arranged to induce perception of Kanizsa’s subjective surfaces. In all three conditions fish seemed to generalize their responses to stimuli perceived as subjective contours in humans. In another experiment fish were trained to discriminate between a vertical and a horizontal bar with luminance contrast gradients, and then tested with vertical- and horizontal-oriented illusory bars, created either through interruption or spatial phase-shift of inducing diagonal lines. Fish appeared to be able to generalize the orientation discrimination to illusory contours. These findings suggest that the ability to visually complete partly occluded objects and to perceive illusory contours may be widespread among vertebrates, possibly inherited from early vertebrate ancestors.