Processing of visual hierarchical stimuli by fish (Xenotoca eiseni).

Three experiments were designed to investigate visual processing of global and local dimensions of hierarchical stimuli in fish (Xenotoca eiseni). In the first experiment, fish were trained to discriminate between a circle made of circle elements and a cross made of cross elements (consistent stimuli), and tested with a circle made of crosses and a cross made of circles (inconsistent stimuli) to asses their global/local encoding preferences. Fish were also tested for their ability to discriminate single-element shapes. The second and the third experiments manipulated the density of the local elements (Experiment 2) and the size of the global and local shapes of the stimuli (Experiment 3) to assess whether these variables could affect global or local perception of hierarchical visual patterns in fish. In all the experiments, fish showed a global preference irrespective of the density and the size of the stimuli. This preference was not because of an inability to perceive the local constituents of the stimulus, since both fish trained with consistent and fish trained with inconsistent figures showed a clear capacity to discriminate between single-element shapes. Overall, these results suggest that a global preference is not a unique trait of human beings and that differences among different vertebrate species in the global/local strategies of stimulus encoding may be because of different ecological adaptations making initial elaboration of a visual scene in a global or local way more likely.