Discriminazione visiva e completamento amodale dello zebrafish (Danio rerio)

Teleost fish possess various complex cognitive functions similar to those of many mammalian and avian species. Although zebrafish (Danio rerio) is emerging as a useful organism for research on the genetics and neurobiology of cognition, including neurological disorders, most of these abilities have not yet been demonstrated in this cyprinid. For this purpose, the present work aimed to investigate the shape visual discrimination learning ability in this species, understand if the performance of the subjects could change in relation to the type of experimental apparatus used and, after analysing the influence of the proposed stimuli on the success of the task, study the perceptual phenomenon of amodal completion (i.e. the ability to visually recognize partly occluded objects). By means of an operant conditioning procedure, fish were first trained to discriminate between a circle and a cross within a small square and opaque arena. As most animals failed to learn the task in this condition, the same two stimuli were then used in a completely different apparatus, a long and opaque corridor. Since most animals did not reach the learning criterion again, two new stimuli (a full circle and an amputated circle) to discriminate in the aforementioned small square and opaque arena were chosen based on previous successful evidence with these two figures in the long and opaque corridor. After reaching the learning criterion, the ability to amodally complete the full circle partially hidden by a hexagonal polygon was finally investigated. Fish behaved as if they were experiencing visual completion of the partially occluded stimuli. These findings suggest further evidence that amodal completion ability may be widespread among vertebrates, possibly inherited from early vertebrate ancestors.