The geometry in spatial learning by zebrafish (Danio rerio)

Geometric reorientation is a special phenomenon within the research field of spatial cognition in animals and consists in using large-scale environmental attributes to locate a salient place after disorientation. To reproduce in controlled settings such a situation, geometrically typified enclosures have been for long employed: that is the case of rectangular arenas, where attributes of length, distance, and angular magnitude get in touch with left-right directions. The susceptibility to large-scale geometry was suggested after observing that animals consistently confused two goal-positions sharing the same spatial relationships (e.g., the two diagonal corners with a short/long surface on the left/right), both in spontaneous and rewarded reorientation tasks. We were interested at inspecting whether zebrafish (Danio rerio) could learn to reorient through spatial geometry, thereby encoding all the informative attributes provided by a rectangular layout of surfaces. We first tested reorientation behavior of fish within an all-white rectangular arena, to then explore the use of distance, corners, and length, one at a time. To set these attributes apart, reorientation behavior of fish was observed within transparent arenas, where we specially probed the impact of each by handling the global shape of the experimental space (rectangular or square). Along the perimeter: four white surfaces equal in length were used to test the distance attribute; four white C-shape surfaces were used to test the corner attribute; four white surfaces of 2:1 ratio in length were used to test the length attribute. Zebrafish adult males were engaged in a daily training targeted to reward them for swimming across a “corridor of choice” placed at levels of the two corners on the correct-geometry diagonal, until a learning criterion ≥70% per two consecutive sessions (one per day: learning and validation). Results showed that zebrafish learned to approach the correct-geometry diagonal instead of the incorrect-geometry one, thus reorienting through the large-scale geometry provided by the all-white rectangular arena. When dissecting the geometric attributes in fragmented layouts, results showed that zebrafish learned to encode all the geometrically informative attributes, in terms of distance, corners, and length in association with left-right directions (e.g., the two corners on the correct-geometry diagonal had a close/far surface on the left/right). Altogether, our findings: (1) emphasize the role of behavioral protocols (spontaneous vs. over-trained choice) on zebrafish reoriented navigation; (2) stress the ecological relevance of geometry as a source of spatial knowledge; (3) allows targeted comparisons due to behavioral and cognitive similarities among zebrafish and mammals.