Separate encoding of vertical and horizontal components of space during orientation in fish

The ability to orient through familiar areas is key to the success of many animal groups. To date, research has focused on how animals orient horizontally and very little work has considered three-dimensional environments, particularly the volumetric surroundings inhabited by many birds, flying insects and fish. These animals do not have their movement constrained by surfaces, and can move with three degrees of freedom. This extra freedom of movement increases the amount of navigational information potentially available, which could lead to an information-processing problem. We studied how a fish, the sighted banded tetra, Astyanax fasciatus, copes with this problem by testing how it learns and stores information from its volumetric surroundings. Using a novel assay based on associative learning of the vertical (up/down) and horizontal (left/right) components of a rotating Y-maze, we found that banded tetras learned and remembered information from the vertical and horizontal components when they were presented either separately or as an integrated three-dimensional unit. Furthermore, when information from the two components conflicted, the fish used the previously learned vertical information in preference to the horizontal, possibly because the vertical axis contained an extra, global, cue: hydrostatic pressure. We propose that animals that navigate through volumes may simplify the information storage problem by encoding the horizontal and vertical components separately. These components could then be used together for rapid, efficient orientation.