Spatial orientation by geometric and non-geometric properties of an environment in fish (Xenotoca eiseni)

When disoriented in a novel environment, rats and young children reoriented themselves in accord with the large-scale shape of the environment, but not in accord with non-geometric properties of the environment such as the colour of a wall or the pattern of a distinct featural panel placed in a corner. In contrast to young children and mature rats, human adults conjoined geometric and non-geometric information. It has been claimed that language could be necessary to human beings for combining geometric and non-geometric information. We tested fish (Xenotoca eiseni) in a rectangular environment constructed to eliminate external orientation cues. Ways to escapes from the environment were located in some corners that fish had to find out. When tested in an all-white tank, fish proved able to reorient themselves using purely geometric information: in accord with the shape of the environment, they searched the correct and the rotationally equivalent corners equally often. On the other hand, when tested in a tank with one blue wall, fish searched only at the correct corner thus demonstrating that they also encoded non-geometric information. Moreover, when trained with four distinct panels located at the corners and then tested in the absence of the panels, chicks chose the two geometrically correct corners, thus indicating that they encoded geometric information even when featural information alone would suffice for orientation. These results suggest that spatial orientation by a "geometric module" is a phylogenetic ancient tract in vertebrates and that the ability to conjoin geometric and non-geometric information is not unique to the human species and could be achieved in species that lack any ability for verbal language.