A small step for rats alters spatial behavior: rats on a bi-level arena explore each level separately.

We tested rats on a 'bi-level open-field' whose two halves were separated vertically by an 8-cm step that the rats could easily ascend/descend. We sought to determine what might be the factors that shape traveling in three-dimensional environments; what makes an environment perceived as multileveled; and how are multileveled environments explored compared to two-dimensional environments? We found that rats on the bi-level open-field traveled a greater distance on the lower level compared to the upper one. They also spent a long time at the foot of the step before ascending to the upper level. They established a home-base on one level and a local base on the other one, and explored each level separately. We could not find a particular factor that accounted for the preference for the lower level. We suggest that the momentary egocentric sensation of moving vertically, together with an overall area large enough for exploration, result in perceiving an environment as multilevel. Exploration of such environments is fragmented, and each level is explored relatively independently, as has also been shown in other studies. Regarding the unanswered question of earlier studies concerning what integrates fragmented representations, this is the first study that suggests that in rats, and perhaps also in other rodent species, such integration is achieved by means of home-base behavior, resulting in the establishment of a single comprehensive representation of the multilevel environment.