Hierarchical stimuli in fish perception

Using hierarchical visual patterns, i.e. large letters formed by smaller ones, Navon (1977) first observed that humans identify faster the global letters than the local letters. He hypothesized that visual perception of objects proceeds from an initial code of global shape to a subsequent analysis of local details. Comparative research on global/local visual processing has been focused on birds and primates. In contrast to humans, who discriminate the global shape better than the local elements, pigeons, monkeys and chimpanzees seem to discriminate the local elements better than the global shape. We investigated visual processing of global and local dimensions of hierarchical stimuli in fish (Xenotoca eiseni). In the first experiment fish were first trained with consistent (same local and global information) or inconsistent (different global/local shape) hierarchical stimuli, and then tested for their local/global encoding preferences. Following a re-training with consistent stimuli fish were also tested for their ability to discriminate only the local constituent stimuli. In the second and in the third experiment the density and the size of the local elements of the hierarchical stimuli were manipulated in order to check whether this would affect fish local or global strategy of choice. The results showed that fish, like humans, chose the global rather than the local level of stimulus representation, irrespective of the density and the size of the stimuli. We argued that differences among vertebrates in the processing of hierarchical stimuli could be due to different ecological and evolutionary adaptations.